Origin of atmospheric deposition and canopy buffering capacity in stands of radiata pine and...

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Forest Ecology and Management 229 (2006) 268–284

Origin of atmospheric deposition and canopy buffering capacity in

stands of radiata pine and pedunculate oak in the Basque Country

Ander Gonzalez-Arias a,*, Inazio Martınez de Arano b, Marıa Jesus Barcena-Ruız c,Gerardo Besga b, Miren Onaindia a

a Landare-Biologia eta Ekologia Saila, Euskal Herriko Unibertsitatea, P.O. Box 644, 48080 Bilbao, Bizkaia, Spainb NEIKER-Nekazal Ikerketa eta Garapenerako Euskal Erakundea. Berreaga, 1 48160 Derio, Bizkaia, Spain

c Ekonomia aplikatua, Ekonometria eta Estatistika Saila, Euskal Herriko Unibertsitatea, Agirre Lehendakaria, 83, 48015 Bilbao, Bizkaia, Spain

Received 16 June 2005; received in revised form 24 March 2006; accepted 3 April 2006

Abstract

Total atmospheric deposition to four mature forest sites in the Basque Country was estimated using the canopy flux method and a generalised

deposition model (average deposition velocities multiplied by average concentration of gases in the atmosphere). Potential acidity under aerobic

conditions and acidity flux from atmospheric deposition entering the forest ecosystems were also estimated. The sites were close to a number of

opencast limestone quarries and were located at different distances from a livestock farm, which was considered to be a source of emission of

ammonia. Two forest stands, a Pinus radiata plantation and a semi-natural Quercus robur woodland (‘‘Dur pine’’ and ‘‘Dur oak’’, respectively),

were selected in the vicinity of the livestock farm where approximately 300 head of cattle are grazed. Another radiata pine plantation and a

pedunculate oak plantation at a distance of approximately 15 km from the farm were also selected for study: ‘‘Urk pine’’ and ‘‘Urk oak’’,

respectively. Bulk precipitation and throughfall were sampled and the concentration of different constituents analysed. Concentrations of gases in

the atmosphere close to the field stations were provided by Basque Government’s Environmental Protection Agency and best estimates of

deposition velocities were based on data in the available literature. Calcium was the most abundant chemical species in bulk precipitation and

throughfall at every site, whereas sulphate was the most abundant anion in these precipitation fractions. Precipitation was not acidic and the pH

values at the ‘‘Dur’’ and the ‘‘Urk’’ site were 5.8 and 5.5, respectively. The results suggest that deposition of calcium carbonate from nearby

quarries may be an important factor at both sites. A high proportion of the sodium collected in bulk precipitation at both sites was found to be of

non-marine origin. Ammonium deposition in bulk precipitation and throughfall was higher at the ‘‘Dur’’ than at the ‘‘Urk’’ site. Statistical analyses

showed that: (a) there exists a strong relationship among base cations (including sodium) at both sites indicating a possible common source for

them; (b) co-deposition of SOx and NOy along with NHx occurs at both sites, but chloride was not co-deposited with ammonium, and (c) buffering

of total deposition and ammonia uptake in forest canopies may be brought about by cation exchange. Total deposition of NHx and the acidity flux of

total deposition were higher at the ‘‘Dur’’ than at the ‘‘Urk’’ site (approximately 30 kg ha�1 year�1 of N.NHx and from 270 to

290 mmolc m�2 year�1, respectively, at the former site and less than 10 kg ha�1 year�1 of N.NHx and from 65 to 85 mmolc m�2 year�1,

respectively, at the latter site). At both sites the acidity flux entering the forest soil along with throughfall was higher under pine canopies than under

oak (approximately 115% at the ‘‘Dur’’ site and almost 200% at the ‘‘Urk’’ site). Nevertheless, acidity entering the forest ecosystems along with the

total deposition was very similar, regardless of the tree dominant species. The estimated nitrogen loads (NOy + NHx) to the sites were

approximately 35 kg ha�1 year�1 at the ‘‘Urk’’ site and 55 kg ha�1 year�1 at the ‘‘Dur’’ site. These values are higher than the critical threshold

values, suggesting potential nitrogen saturation of these forest ecosystems.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Ammonia; Canopy flux method; Forests; Nitrogen; Pinus radiata; Quercus robur; Total deposition

* Corresponding author. Present address: Forestry Unit, NEIKER-Nekazal

Ikerketa eta Garapenerako Euskal Erakundea, Berreaga, 1 48160 Derio, Biz-

kaia, Spain. Tel.: +34 944034328; fax: +34 944034310.

E-mail address: [email protected] (A. Gonzalez-Arias).

0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2006.04.006

1. Introduction

In recent years, a number of studies have focused on the effects

of various pollutants on forest systems in Europe (e.g., the

EXMAN and NITREX projects: Wright and Rasmunsen, 1998).

Thanks to these studies, it has been established that there is no

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http://dx.doi.org/10.1016/j.foreco.2006.04.006

A. Gonzalez-Arias et al. / Forest Ecology and Management 229 (2006) 268–284 269

single factor responsible for environmental stress in forests and

their subsequent decline and that damage to forests is caused by a

number of biological and non-biological factors acting in synergy

(Tomlinson, 1990; Kimmins, 1997; McLaughlin and Percy,

1999). Other studies have been conducted to assess the

relationships between forest vitality and natural or anthropogenic

stresses on a Pan-European scale (De Vries et al., 2000a; Van

Leeuwen et al., 2000; De Vries et al., 2003a,b); critical deposition

levels for atmospheric deposition have been estimated (De Vries

et al., 2000b) and statistically significant relationships between

defoliation and stresses proposed (Klap et al., 2000). It has been

concluded that amongst thestresses thateffect forestvitality in the

Iberian Peninsula, drought may be the most important (Klap et al.,

2000). However, water stress is probably not an important factor

threatening forest vitality in the northern part of the Atlantic

Iberian Peninsula, where precipitation is frequent (from 1200 to

more than 2000 mm year�1) and distributed more or less evenly

throughout the year (Euskalmet, 2004).

The province of Bizkaia is the most densely populated

province in the Basque Country (a population of 1.125 million

people in an area of 2200 km2). The province is also the most

industrialised, despite there being constant change towards

service economic sectors (EUSTAT, 2004). Atmospheric

pollution in Bizkaia has been shown to be high, due to the

high industrialisation (Eusko Jaurlaritza, 1992, 1999). Radiata

pine (Pinus radiata D. Don) is the most important forest species

in the Basque Country, accounting for almost 60% of the

forested area in Bizkaia. This tree species is considered to be an

important economic resource used for timber production

(Eusko Jaurlaritza, 1997). Although Quercus robur L. was once

the most important forest species in the Basque Country, its

coverage is now limited to less than 5% of the forest area in

Bizkaia. Deposition of pollutants to these forests may threaten

their future vitality and therefore some of the goods and

services that they provide to society.

The main aim of the present study was to estimate the total

atmospheric deposition to the forest ecosystems, as well as the

origin of its constituents in order to understand the effects that

atmospheric deposition may have on these cycles. A further aim

was to estimate the interactions of different constituents and the

buffering capacity of the canopies of the forest species under

study. Another aim of the study was to estimate the annual

potential acid load at different forest sites, as well as the acidity

flux derived from atmospheric deposition and the degree of

acidity.

2. Materials and methods

2.1. Study sites

Two different sites with adjacent forest stands (one of P.

radiata D. Don and the other Q. robur L.) and at different

distances from a source of emission of ammonia were chosen.

One of the study sites, designated ‘‘Dur’’, was in the same

property as a cattle farm is. The other site, designated ‘‘Urk’’,

was farther from the farm. The main characteristics of the

studied sites are presented in Table 1.

The ‘‘Dur’’ site was located on private property of a total

area of 1.5 km2. Farming at the site began in the 1970s, in an

area that had previously been abandoned, and there are

currently around 300 head of cattle on the farm. Pastures on the

farm in question are fertilised annually with livestock manure;

following open-air fermentation, the manure is mixed with ash

from the neighbouring paper mill and the mixture is then spread

uniformly over the land as a fertiliser. Land in the ‘‘Dur’’ area

also includes several copses of Q. robur and areas of more

isolated, thinly spread oak. There are also two areas of

woodland, where the study sites were located (Table 1). At the

south-eastern edge of this area the ‘‘Dur oak’’ site was

established and ‘‘Dur pine’’ was located approximately 750 m

to the northeast of the oak woodland site, separated from the

first site by an area of treeless pasture.

The second, ‘‘Urk’’, site is located in the Durangaldea massif

within the area known as Urkulueta (Table 1), which belongs to

the municipality of Manaria (Bizkaia, Basque Country). Land in

the study site is mostly covered by either naturally regenerated

forests or by forest plantations. The ‘‘Urk oak’’ stand was

established in a mixed plantation of Q. robur and Fagus sylvatica

L. The stand had been managed as a coppice and used until the

mid-40s for coal making, although it is no longer managed in this

way. An area within this plantation where oaks were the only

planted species was selected. At the same altitude and

approximately 250 m northwest of the previous stand, radiata

pine plantation was selected for study (‘‘Urk pine’’).

Two major roads – the national N-632 and the A-8 motorway

(Bilbao to Behobia) – run through the valley linking the Greater

Bilbao area with the Irun/Hendaia border. There is therefore a

considerable flow of road traffic through the area all year round.

Nevertheless, the ‘‘Dur’’ site is also located immediately

adjacent to the Smurfit (Smurfit Nervion) paper factory in

Iurreta (Bizkaia).

2.2. Field equipment

Two bulk precipitation collectors were placed within a

10 m� 10 m, fenced area in a treeless area between the two

field stations. At the ‘‘Dur’’ study site this was an area of open

pasture and at the ‘‘Urk’’ site it was a clear-cut area, previously

a radiata pine plantation. Twelve throughfall water collectors

were randomly placed within the confines of each forest stand

site. Both the bulk precipitation collectors and throughfall

collectors consisted of a polyethylene funnel (of diameter

30 cm) that was attached, using silicone putty, to a white

polyethylene bottle (capacity 15 l). Each collector was placed

1.5 m above ground level, in order to prevent water splashing

back from the ground from entering the collectors. The

collection bottles were opaque to minimise light penetration

that could potentially promote the growth of algae. Washed

glass wool (Panreac 211376: chemically pure; HCl-soluble

material < 0.5%) was used as a filter/plug in the mouth of each

funnel to prevent insects and detritus from contaminating water

samples. The fibre plugs were changed after each water sample

was collected. Sample collection took place weekly (Table 1).

Funnels and collection containers were repeatedly rinsed with

A. Gonzalez-Arias et al. / Forest Ecology and Management 229 (2006) 268–284270

Table 1

Main characteristics of the studied sites

Durango Urkulueta

Code ‘‘Dur’’ ‘‘Urk’’

Management Pastures and forests Forests

Distance from cattle farm (km) 0 15

Direction from cattle farm – South

Distance from ocean (km) 15 25

Direction from ocean South South

Distance from limestone quarries (km) 5 2

Direction from quarries North South

Quarry extraction Lower Cretaceous limestonea Lower Cretaceous reef limestonea

Latitude 438110N 438060NLongitude 28400W 28390WAltitude (masl) 100–200 300–800

Orientation North North

Bulk precipitation collection period June 93 to November 98 December 95 to November 98

‘‘Dur oak’’ ‘‘Dur pine’’ ‘‘Urk oak’’ ‘‘Urk pine’’

Size of stand (ha) 4 8 10 20

Size of fenced area (m2) 700 450 500 400

Average slope of stand (8) 15 8 15 28

Main tree species Q. robur P. radiata Q. robur, F. sylvatica P. radiata

Origin of trees Natural regenerationb Plantation Plantation Plantation

Age of trees (years) �100b 23 �150 25

Height of trees (m) 18.5 (3.5) 20 (4) 20 (5) 26 (8)

Diameter of trees

at 1.3 m (cm)

38.5 (19) 25 (10) 43 (8) 30.5 (10)

Stem density (trees ha�1) 190 720 235 545

Soil type Typic Dystrochreptc Typic Dystrochreptc Dystric Cambisolsd Dystric Cambisolsd

Soils developed from Lower Cretaceous

sandstone and argillitea

Lower Cretaceous


Lower Cretaceous


Lower Cretaceous


Throughfall collection

period

June 93 to November 98 June 95 and November 98 December 95 to November 98 December 95 to November 98

Standard deviations of some parameters are also shown in parenthesis.a EVE, 1994.b Amezaga et al., 1997.c Gonzalez-Arias, 2005.d Eusko Jaurlaritza, 1991.

deionised water following each collection. Throughfall samples

were taken from each two collectors, mixed then taken to the

laboratory for analysis. Within 24 h of collection and prior to

filtration (0.45 mm), all solutions were analysed for pH and

alkalinity, after which the remainder was refrigerated (<4 8C).

Once samples were obtained for a complete calendar month, the

samples from each week were combined proportionally to

produce monthly samples and then stored at less than 4 8C and

in complete darkness until chemical analysis (Galloway and

Likens, 1978). Water samples were analysed for the following:

ammonium, sulphate, nitrate, Kjeldhal nitrogen, phosphate,

total phosphorous, manganese, iron, calcium, magnesium,

sodium, potassium and chloride.

The levels of NOy and SO2 were continuously monitored by

an automated environmental quality station belonging to the

Basque Government’s Environmental Protection Agency. This

station is located close to the ‘‘Dur’’ site (4381004000N283803000W) and is also the closest to the ‘‘Urk’’ site (at a

distance of approximately 10 km). Daily averages of atmo-

spheric concentration of the gases were kindly provided by the

aforementioned agency.

2.3. Chemical analysis of water samples

Solutions were analysed for pH by a Corning 140 and a

SCHOTT CG 843 pH meter. Alkalinity was determined by

titration, using as an indicator a solution of bromcresol green

(0.1%) and methyl red (0.02%) (AOAC, 1980). The H+

concentrations were estimated from pH values. Cation

concentrations were determined using a Varian Spectraa 250

plus or a Varian Spectraa 220 FS atomic absorption spectro-

photometer by flame atomic-absorption (APHA, AWWA,

WPCF, 1989). Concentrations of SO42�, N.NO3

�, N.NH4+

and Cl� were determined with a Segmented Flow Analyser

(F.I.A.-S.F.A.) ALPKEM Flow Solution IV (ALPKEM, 1987;

APHA, AWWA, WPCF, 1989).

2.4. Data analysis

In the present study, data corresponding to the period

between November 1996 and October 1998 will be reported, as

this is the period of time when the Air Quality Monitoring

Station data were available.


Partial correlation and principal component analyses were

used to estimate the possible relationships among constituents

in precipitation and throughfall. Relationships among con-

stituents were analysed using values from the entire period of

study in order to achieve robustness of statistical analyses

(Table 1). Regression analyses have been used in a number of

studies in which the role of the tree canopy as a filtering agent

for atmospheric deposition has been investigated (Lovett and

Lindberg, 1986; Cappellato and Peters, 1995; Avila and

Rodrigo, 2004; Beier and Gundersen, 1989; Hansen and

Nielsen, 1998). Lee (1993) used correlation and partial-

correlation techniques to determine statistical associations

between constituents in bulk precipitation, in order to estimate

patterns of deposition. Data were log-transformed: log (con-

stituent flux + 1) prior to statistical analysis (Sokal and Rohlf,

1981) and associations were considered significant at P = 0.1 or

0.05, after using the Bonferroni method for the adjustment of

probabilities (SAS, 1998; SPSS, 1999; Dytham, 1999). They

were carried out using StatView 5.0 (SAS Institute, 1998) and

SPSS 10.0 for the Macintosh (SPSS, 1999). Factors in PCA

analyses were retained to explain at least 70% of the variance of

the original data set (SAS, 1998) and composite variables of the

factors were selected on the basis of the variable loadings being

greater than or equal to 0.5 (Lee, 1993).

2.5. Estimation of total deposition rate and canopy

interactions

Deposition was investigated using the canopy flux balance

method (Mayer and Ulrich, 1978; Ulrich, 1983; Bredemeier,

1988).

Chlorinity ratios (ratio of ions to chloride) were calculated

for a variety of ions, in order to estimate the possible source(s)

of precipitation (i.e., from seawater or other sources);

comparison of ionic ratios in seawater gives information

regarding the relative contribution of sea salts to rainfall

(Eriksson, 1960; Schlesinger, 1997). Enrichment ratios (ER)

were also calculated, and defined as the ratio of ions recovered

in throughfall (TF) to those collected in bulk precipitation (BP).

Total atmospheric deposition (TD) can be expressed as the

sum of BP and interception deposition (ID) (Bredemeier, 1988).

Bulk precipitation considers also the fraction of gases that are

dissolved on inert collection surfaces (Cape and Leith, 2002).

No distinction is made between these two types of deposition in

Table 2

Deposition velocities and SO2 concentrations used to calculate interception deposit

radiata ‘‘Pine’’

[SO2] (mg m�3) growing season [SO2] (mg m�3) dormant seas

Oak 8a 10b

Pine 8a 10b

a Average values for growing seasons (April to November) during the study period

quality sensors. Data supplied by the Basque Government’s Environmental Protecb Average values for dormant seasons (December to March) during the study pec Average deposition velocities were calculated using values reported by Draa

Brueggemann and Spindler (1999), and Erisman et al. (1999).d An average deposition velocity was calculated using data published by Matt a

terms of whether they are dry or wet deposition, BP is

independent of the collecting surface area, and ID depends on

its filtering efficiency (Bredemeier, 1988).

Stemflow was considered to be negligible (Onaindia et al.,

1995; Gonzalez-Arias, 2005). TF was defined as TD plus

leaching (L) minus uptake (U) of nutrients in the canopy.

A higher ER for any given ion, when compared with another,

indicates that: (i) the ID for that ion in the canopy is greater than

for the other; (ii) L of that particular ion is higher, or; (iii) U is

greater for the ion with the lower enrichment ratio.

Many studies have concluded that sulphur exhibits few

canopy interactions and as such SO42� net throughfall (NTF)

estimates have frequently been used to infer sulphur intercep-

tion deposition (Bredemeier, 1988; Granat and Hallgren, 1992;

Joslin and Wolfe, 1992; Draaijers and Erisman, 1993; Hansen

and Nielsen, 1998). Interception deposition (IDgas) of gaseous

sulphur was calculated using a generalised micrometeorolo-

gical model (Hansen and Nielsen, 1998). Average atmospheric

SO2 concentrations during the growing and dormant seasons for

the period between November 1996 and October 1998 (Table 2)

were calculated using the data provided by the Basque

Government’s Environmental Protection Agency. Average

atmospheric SO2 concentrations recorded in this way were

then multiplied by average deposition velocities, using best

estimates of deposition velocities taken from the available

literature (Table 2). The growing and dormant seasons for oaks

were defined, respectively, as the period that oaks presented

leaves in the canopy and as the leafless period (December to

March). For pines, deposition velocities were considered to be

the same all year round, using the average figure for the

growing season. Interception deposition (IDpart) for particulate

S was calculated as the difference between net throughfall of S

and interception deposition of gaseous-phase S.

Exchange of inorganic nitrogen compounds (NHx; NOy) can

occur within the canopy, therefore making it impossible to

distinguish, purely on the basis of throughfall measurements,

between the canopy uptake/leaching of such compounds and

the atmospheric deposition (Andersen and Hovmand, 1999). In

order to estimate the interception deposition of NOy, the

average annual concentration of N.HNO3 vapour during the

study period was multiplied by the maximum deposition

velocity, which was derived from the literature. Forest canopy

is known to be a perfect sink for this gas and its deposition is

only limited by aerodynamic resistance (Duyzer and Fowler,

ion of gaseous sulphur to the forest stands under study: Q. robur ‘‘Oak’’ and P.

on Vd (mm s�1) growing season Vd (mm s�1) dormant season

6c 3d

6c 6c

November 1996 to October 1998, obtained from the automated environmental

tion Agency.

riod obtained from automated environmental quality sensors.

ijers and Erisman (1993), Edgerton et al. (1992), Matt and Meyers (1993)

nd Meyers (1993) and Padro et al. (1993).


1994). The values of N.HNO3 concentration obtained were of

0.4 mg m�3. Maximum deposition velocity was taken to be

70 mm s�1 (Hansen and Nielsen, 1998). Interception deposi-

tion for N.NO2 gas was also calculated using data from the

automated environmental monitoring station and deposition

velocity figures derived from the literature. Average concen-

trations of N.NO2 gas in spring, summer, autumn and winter

were obtained for the study area for the period November 1996

to October 1998 (Table 3).

Unfortunately, the automated environmental monitoring

station used in the present study did not provide figures for

particulate matter or for NH3 concentrations. The Na-method

(Ulrich, 1983) is widely used to calculate interception

deposition for particulate matter. However, Gonzalez-Arias

et al. (2000) reported that a proportion of the Na+ recovered in

throughfall from a plantation of P. radiata located 25 km from

the stands under study, growing on a typic Dystrochrept and

also close to the seashore, may have arisen from leaching of this

cation from the canopy. Therefore, the ID for particulate S was

used to estimate the particulate interception deposition (IDpart)

for other substances, in a similar way to the Na-method.

Despite its status as an essential nutrient, chloride is not

usually involved in biological transformations in forest

ecosystems (Bredemeier et al., 1998; Kreutzer et al., 1998;

Thomas and Buttner, 1998). Gaseous interception deposition

(IDgas) for HCl was estimated considering that L and U were

zero, and the value calculated for IDpart.

Fowler et al. (1999) and Rennenberg and Gessler (1999)

suggested that NH3 absorption in different forest canopies is

principally due to stomatal exchange, and highlighted the more

rapid solubilisation of NH3 in comparison with its absorption in

gaseous form. The NH4+ ions dissolved in water films on the

leaf surface may therefore be exchanged via the stomata into

the water films that are thought to be connected with the

aqueous phase of the apoplast (Rennenberg and Gessler, 1999).

Leaf uptake of NH4+ may bring about the excretion of base

cations (Ca2+, Mg2+ and K+) (Roelofs et al., 1985; Alenas and

Skarby, 1988; Bobbink et al., 1992; Hansen and Nielsen, 1998;

Rennenberg and Gessler, 1999; De Vries et al., 2003a). It has

been proposed that the leaching of cations may also be due to

Table 3

Deposition velocities and N.NO2 concentrations used to calculate the inter-

ception deposition of gaseous N.NOx to the stands under study (‘‘Oak’’ Q.

robur; ‘‘Pine’’ P. radiata)

[N.NO2]

(mg m�3)

Vd (mm s�1)

coniferous

forest: Pine

Vd (mm s�1)

deciduous

forest: Oak

Winter 10.6a 1.0b 0.5b

Spring 12.4a 3.3b 2.9b

Summer 8.4a 3.3b 3.3b

Autumn 11.5c 3.3b 2.2b

a Mean value of N.NO2 present in the atmosphere close to the study sites

during spring, summer and winter of the study period.b Average deposition velocity was calculated using figures reported by

Duyzer and Fowler (1994).c Mean value of N.NO2 concentration present in the atmosphere close to the

study sites during the autumn months of the study period.

neutralisation of acid precipitation occurring in the forest

canopy (Rehfuess, 1981; Lovett et al., 1985; Bredemeier, 1988;

McLaughlin and Percy, 1999; Gonzalez-Arias et al., 2000;

Zeng et al., 2005).

Co-deposition of SO2 and NH3 is expected to be enhanced

due to a positive feedback mechanism (Kreutzer et al., 1998;

Fowler et al., 1999). Deposition of SO2 on leaf surfaces forms

sulphuric acid, which may trap gaseous NH3. Deposition of

NH3 is therefore enhanced by the buffering of protons

generated during the dissolution of SO2, which causes more

deposition of SO2 (Erisman and Wyers, 1993). A similar pattern

of deposition is also expected for nitric acid and ammonia

(Bytnerowicz et al., 1992). In this study, it was assumed that

amongst the neutralising reactions that take place in the forest

canopy, part of the deposited NH4+ and H+ was taken up by

foliage. Furthermore, it was considered that calcium, magne-

sium, potassium, manganese and sodium (Gonzalez-Arias

et al., 2000) were exchanged and leached from the canopy in

return for the aforementioned cations:

UðN:NHxÞ þ IDðHþÞ � NTFðHþÞ

¼ LðCa2þ þMg2þ þ Kþ þ Naþ þMn2þÞ (1)

UðN:NHxÞ ¼ LðCa2þ þMg2þ þ Kþ þ Naþ þMn2þÞ

� IDðHþÞ þ NTFðHþÞ (1a)

2.6. Estimation of pH, degree of acidity and acidity flux of

deposition

For both forest study sites, the acidity degree was calculated

for bulk precipitation, throughfall and estimated total deposi-

tion using the method employed by Kreutzer et al. (1998).

On the other hand, potential acidification due to deposition

of different acidifying nitrogen and sulphur bearing compounds

was estimated for soil under aerobic conditions. Potential

minimum and maximum H+ production for soils in both forest

stand sites were estimated in accordance with Hansen and

Nielsen (1998).

Minimum potential soil acidification occurs under the

following conditions:

(i) H
+ ions from atmospheric deposition are leached out of the
root zone so that there is no increase in soil H+

(ii) N
Hx uptake is balanced by mineralisation or leaching of
NHx and there is therefore no increase in soil H+

(iii) N
Oy is taken up by and stored in vegetation. In this case, a
decrease of 1 H+ equivalent is produced for every nitrogen

equivalent absorbed by vegetation

(iv) S
Ox is deposited as SO42� and the latter accumulates in the
ecosystem. For each equivalent of SO42� deposited under

these conditions there is a decrease of two equivalents in

soil H+

In contrast, maximum potential soil acidification occurs

when the following conditions are met:


(i) H

Fig. 1

throu

also s

+ ions deposited are adsorbed in the soil, resulting in an

increase in soil H+ proportional to the magnitude of H+

deposition

(ii) N
Hx is nitrified. For each equivalent of NHx that is
oxidised to NO3�, two equivalents of H+ are produced in

the soil

(iii) N
Oy that is deposited on the ecosystem is oxidised to
NO3�. This way, for each equivalent of oxidised NOx one

equivalent of H+ will be produced

(iv) S
Ox is oxidised to SO42�, or SO4
2� is mineralised or

desorped. Under these conditions, for each SO42�

equivalent produced, two H+ equivalents are produced

in the forest soil

3. Results

3.1. Bulk precipitation fluxes

When the ionic content (mmolc m�2) of bulk precipitation

was taken into account, calcium was found to be the most

abundant chemical species at both sites (Fig. 1). A number of

factors may have led to the high level of Ca2+ such as the

proximity of the field sites to a calcareous rock massif that is

industrially exploited in several nearby opencast quarries; or

particulate emission of calcium-containing compounds from

local industrial plants.

. Mean annual deposition (mmolc m�2 year�1) of different constituents coll

ghfall: Pine, P. radiata throughfall), at both study sites during the period Nove

hown.

The high contents of Cl� at both sites suggest that the

majority of precipitation events that occurred during the study

period may have been of maritime origin. However, the

Na+:Cl� ratio (Table 4) was higher than that usually recorded in

seawater. This indicates that Na+ deposition was greater than

expected if all Na+ ions were indeed derived from seawater.

Similarly, the ratios of SO42�:Cl� and Mg2+:Cl� (Table 4)

indicate that factors other than sea water-derived precipitation

accounted for the levels of atmospheric deposition of major

ions in this study. If all Cl� ions recovered in bulk precipitation

were of marine origin, it can be assumed that 69%, 23% and 7%

of Na+, Mg2+ and SO42� ions, respectively, were also from this

source at the ‘‘Dur’’ site and 33%, 13% and 6% of the same

constituents, respectively, at the ‘‘Urk’’ site.

The NH4+ content of bulk precipitation at the ‘‘Dur’’ site was

twice that at the ‘‘Urk’’ site, indicating that livestock is a source

of this ion, and that it may be deposited close to the emission

area even in bulk precipitation (Fig. 1).

The pH values for precipitation were relatively high and

close to the pH of water in equilibrium with atmospheric CO2:

pH 5.8 at the ‘‘Dur’’ site and pH 5.5 at the ‘‘Urk’’ site. The

volume weighted monthly alkalinity was 208.8 (standard

deviation 29.6) and 290.0 (standard deviation 55.6) mmol l�1at

the ‘‘Dur’’ and the ‘‘Urk’’ sites, respectively, showing that

carbonates deposited along with base cations (probably from

local limestone) may be an important source of alkalinity at

both sites.

ected in bulk precipitation and throughfall in the forest stands (Oak, Q. robur

mber 1996 to October 1998. Standard deviations for annual throughfall flux are


Table 6

Principal component analysis of standardised residual fluxes (residuals obtained

after regressing log transformed (log x + 1) fluxes in mmolc m�2 against log-

transformed (log x + 1) precipitation volume in mm) of different chemical

species in bulk precipitation collected at the ‘‘Dur’’ site

Factor Eigenvalue % Variance Cumulative %

1 2.768 27.7 27.7

2 2.139 21.4 49.1

3 1.239 12.4 61.5

4 1.047 10.5 72.0

Table 4

Chlorinity ratios: relationship (in mmolc) of chemical species to chloride in

precipitation fractions analysed (bulk: bulk precipitation; Oak: Oak throughfall;

Pine: Pine throughfall; ‘‘Dur’’ close to livestock farm; ‘‘Urk’’ farther from the

farm) and the relationship of these species in sea water (seawater chlorinity)

Bulk

Dur

Oak

Dur

Pine

Dur

Bulk

Urk

Oak

Urk

Pine

Urk

Sea water

chlorinitya

Na+:Cl� 1.6 1.3 1.4 2.6 2.3 2.1 0.86

Mg2+:Cl� 1.0 0.7 0.7 1.4 1.2 1.0 0.19

SO42�:Cl� 1.6 1.3 1.4 1.7 1.8 1.7 0.10

Ca2+:Cl� 2.3 1.7 1.7 3.3 2.9 2.7 0.04

K+:Cl� 0.2 0.5 0.4 0.2 0.4 0.4 0.02

a Data taken from Schlesinger (1997).

3.2. Throughfall fluxes

As with bulk precipitation, calcium was the most abundant

constituent (in mmolc m�2) in the throughfall flux collected at

each field station (Fig. 1). The second most abundant chemical

species was SO42� at the ‘‘Dur’’ site, whereas it was still Na+ at

the ‘‘Urk’’ site, suggesting the importance of atmospheric

deposition for these elements at both sites.

The throughfall fluxes at both of the ‘‘Dur’’ sites contained

more NH4+ than those collected at the ‘‘Urk’’ site. Potassium

was more abundant than nitrogen bearing compounds at the

‘‘Urk’’ site.

Average monthly pH and volume weighted alkalinity at the

field stations were as follows: 6.2 and 242.4 (51.9) mmol l�1 for

‘‘Dur oak’’; 6.2 and 371.3 (59.3) mmol l�1 for ‘‘Urk oak’’ 5.5

and 331.6 (48.6) mmol l�1 for ‘‘Dur’’ pine and 6.3 and 434.9

(87.3) mmol l�1 for ‘‘Urk’’ pine.

Chlorinity ratios for the majority of throughfall constituents

were lower than those for bulk precipitation (Table 4). Only K+

showed a higher chlorinity ratio in throughfall than in bulk

precipitation, indicating that this was the only ion to have

increased in concentration in throughfall relative to Cl�.

In both ‘‘Dur’’ forest stands, the only ions with higher

enrichment ratios than Cl�were NH4+, NO3

� and K+, and at the

‘‘Urk’’ site the same was true for K+ (Table 5). If it is assumed

that Cl� is not involved in biological transformations in forest

ecosystems, the interception deposition for Cl� can be assumed

to be greater than for other ions, indicating that some of the

chloride recovered in throughfall may have come from gaseous

sources. Comparison of ER for throughfall collected in the four

stands (Table 5) revealed them to be similar for most ions. The

only exceptions were nitrogen-bearing compounds at the

‘‘Urk’’ site.

Table 5

Annual enrichment ratios (TF/BP) for constituent fluxes in throughfall

(mmolc m�2) divided by constituent fluxes collected in bulk precipitation

(mmolc m�2) at both study sites

SO42� Cl� NO3

� NH4+ Ca2+ Mg2+ Na+ K+ H+

Dur Oak 1.4 1.7 1.9 2.9 1.3 1.2 1.2 4.7 0.4

Dur Pine 1.5 1.7 1.9 3.3 1.2 1.2 1.3 3.4 1.8

Urk Oak 1.6 1.6 1.3 0.8 1.4 1.3 1.4 3.7 0.2

Urk Pine 1.7 1.8 1.7 1.7 1.5 1.3 1.4 3.6 0.1

3.3. Relationships between chemical species in bulk

precipitation

Fluxes of chemical species in bulk precipitation were highly

correlated (data not shown). There were 35 and 30 significant

correlation coefficients (from a total of 55 comparisons)

corresponding to the ‘‘Dur’’ and ‘‘Urk’’ sites, respectively.

Multicolinearity presents a problem when interpreting the

coefficients. The high intercorrelation, and the fact that there

was a high frequency of correlations between fluxes and

amount of precipitation may be explained by the great effect

that precipitation volume has on flux determination. Another

correlation analysis (data not presented), revealed a high

frequency of direct correlations among constituent concentra-

tions. It also showed that there was a high frequency of inverse

correlations between concentrations of chemical species and

precipitation depth. To minimise the effects that precipitation

volume has on the flux determination, log-transformed

constituent fluxes were regressed against log-transformed

precipitation volumes, and the standardised residuals (residual

fluxes) from this model were used to carry out principal

component analysis (Tables 6 and 7).

PCA (bearing in mind that the influence of precipitation

volume had been removed) indicated a possible common

source for base cations and accounted for much of the variation

in the data at both sites. Also significant were the relationships

between both SO42� and NO3

�, and NH4+, indicating possible

co-deposition of these constituents at both sites. The same trend

was not observed for Cl�, indicating that co-deposition of NH4+

and this anion was unlikely (Tables 6 and 7).

3.4. Canopy interactions

Regression analyses were carried out on the log-transformed

flux for each constituent recovered in throughfall (mmolc m�2)

Factor 1 Factor 2 Factor 3 Factor 4

res SO42� 0.741

res Cl� 0.871

res N.NO3� 0.617

res N.NH4+ 0.905

res Ca2+ 0.889

res Mg2+ 0.919

res Na+ 0.711

res K+ 0.776

res Fe3+ �0.547

res H+ �0.565


Table 7


after regressing log-transformed (log x + 1) fluxes in mmolc m�2 against log-

transformed (log x + 1) precipitation volume in mm) of different chemical

species in bulk precipitation collected at the ‘‘Urk’’ site


1 2.950 29.5 29.5

2 1.980 19.8 49.3

3 1.602 16.0 65.3

4 1.311 13.1 78.4


res SO42� 0.623 0.501

res Cl� 0.624

res N.NO3� 0.725

res N.NH4+ 0.660 0.554

res Ca2+ 0.817

res Mg2+ 0.932

res Na+ 0.684 �0.526

res K+ 0.802

res Fe3+ 0.559 0.587

res H+ �0.643

Table 9


after regressing log-transformed (log x + 1) fluxes in mmolc m�2 in throughfall

collected in the Q. robur stand at the ‘‘Dur’’ site against log-transformed

(log x + 1) fluxes in mmolc m�2 in bulk precipitation) of different chemical

species


1 4.229 39.1 39.1

2 1.787 16.2 55.3

3 1.294 11.8 67.1

4 1.017 9.2 73.3


res SO42� 0.734

res Cl� 0.710

res N.NO3� 0.742

res N.NH4+ 0.864

res Ca2+ 0.877

res Mg2+ 0.856

res Na+ 0.693

res K+ 0.790

res Mn2+ 0.667

res Fe3+ 0.606

res H+ 0.657

Table 10



collected in the Q. robur stand at the ‘‘Urk’’ site against log-transformed�2

over the log-transformed flux of the same component in bulk

precipitation (Table 8). These calculations were also meant to

remove the variations in throughfall fluxes resulting from inputs

from bulk precipitation. There was a close relationship between

bulk precipitation and throughfall fluxes for most of the

constituents analysed, indicating that much of the variation in

the latter fluxes could be accounted for by the inputs of

constituents along with bulk precipitation (Table 8). Standar-

dised residuals from the regression analyses were used as input

data for further PCA. As Mn2+ was below the limit of detection

in bulk precipitation, log-transformed Mn2+ flux was regressed

against the log-transformed throughfall volume.

The relationships between chemical species suggest on one

hand that there was co-deposition of anions (SO42� and NO3

�)

and NH4+ in bulk precipitation but that this relationship did not

exist in the fraction crossing the canopy (Tables 9–12). However,

there was a clear relationship between residual flux of SO42� and

Table 8

Coefficients of determination (R2) between log-transformed (log x + 1) monthly

deposition fluxes (mmolc m�2) of constituents present in throughfall for both

field stations and log-transformed (log x + 1) monthly fluxes (mmolc m�2) of

constituents in bulk precipitation during the whole study period

Oak Dur

(n = 65)

Pine Dur

(n = 42)

Oak Urk

(n = 36)

Pine Urk

(n = 36)

SO42�TF:SO4

2�BP 0.55 0.85 0.91 0.85

Cl�TF:Cl�BP 0.81 0.79 0.90 0.70

N.NO3�TF:N.NO3

�BP 0.24 0.44 0.34 0.45

N.NH4+TF:N.NH4

+BP 0.13 0.11 0.37 0.35

Ca2+TF:Ca2+BP 0.72 0.82 0.91 0.78

Mg2+TF:Mg2+BP 0.84 0.90 0.93 0.91

Na+TF:Na+BP 0.84 0.90 0.91 0.88

K+TF:K+BP 0.38 0.40 0.54 0.34

Mn2+TF:throughfall

volume (mm)

0.07 0.08 0.03 0.08

Fe3+TF:Fe3+BP 0.51 0.49 0.90 0.83

H+ TF:H+BP 0.66 0.64 0.12 0.18

residual fluxes of base cations. The result suggests that there was

an exchange of cations in the canopies, in return for a proportion

of the deposited NHx. This finding supports the hypothesis stated

in the methods section. On the other hand, Cl� flux collected in

bulk precipitation was not related to NH4+ flux when the effect of

precipitation volume had been removed, suggesting that co-

deposition of these two constituents did not occur. Taking this

into account it was considered that at the sites under study

gaseous HCl deposition was the main source of dry deposited H+.

Chloride residual flux in throughfall was also related to base

(log x + 1) fluxes in mmolc m in bulk precipitation) of different chemical

species


1 2.754 25.0 25.0

2 2.002 18.2 43.2

3 1.403 12.8 56.0

4 1.182 10.7 66.7

5 1.020 9.3 76.0

Factor 1 Factor 2 Factor 3 Factor 4 Factor 5

res SO42� 0.560

res Cl� 0.690

res N.NO3� 0.631

res N.NH4+ 0.711

res Ca2+ 0.555 0.570

res Mg2+ 0.645 0.641

res Na+ 0.580

res K+ 0.644

res Mn2+ 0.622 0.547

res Fe3+ 0.684

res H+ 0.538


Table 11



collected in the P. radiata stand at the ‘‘Dur’’ site against log-transformed


species


1 4.598 41.8 41.8

2 1.455 13.2 55.0

3 1.137 10.3 65.3

4 0.988 9.0 74.3


res SO42� 0.681

res Cl� 0.826

res N.NO3� 0.672

res N.NH4+ 0.560 �0.573

res Ca2+ 0.792

res Mg2+ 0.795

res Na+ 0.855

res K+ 0.809

res Mn2+ 0.811

res Fe3+ 0.525

res H+ 0.545 0.523

cation flux, which supports the hypothesis that dry deposited H+

is also buffered in forest canopy by cation leaching.

3.5. Total deposition

Estimated S.SO42� deposition was similar at both forest

stations and around 35 kg ha�1 year�1 in the ‘‘Dur’’ site

(Table 13). Total S.SO42� deposition at the ‘‘Urk’’ site was high

(35–40 kg ha�1 year�1) with no major differences between the

two forest stands. Quantities recovered in ‘‘Dur’’ BP were also

high and accounted for around 70% of total deposition. A large

Table 12



collected in the P. radiata stand at the ‘‘Urk’’ site against log-transformed


species


1 4.127 37.5 37.5

2 1.366 12.4 49.9

3 1.270 11.5 61.4

4 1.205 11.0 72.4


res SO42� 0.529

res Cl� 0.850

res N.NO3� 0.502 �0.521

res N.NH4+ 0.513 0.730

res Ca2+ 0.733

res Mg2+ 0.786

res Na+ 0.851

res K+ 0.771

res Mn2+ 0.818

res Fe3+ 0.722

res H+

part of the S may have been dissolved in precipitation, and/or

deposited gravimetrically, and/or SO2 may have also been

dissolved at the water layers present in the collectors when they

were wet. It was estimated that the amount of particulate SO42�

recovered in the Oak stand was around 70% of that recovered in

the pine stand at the ‘‘Dur’’ site. This small variation may have

occurred because oaks typically remain leafless for just 4

months each year. Furthermore, part of the particulate fraction

may have been deposited directly on the bare branches. The

sulphate collected as bulk precipitation at the ‘‘Urk’’ site was

approximately 60% of total deposition and estimated particu-

late deposition was almost 25% of the total. Particulate

deposition of this constituent in the oak stand was almost 90%

of that in the pine stand (Table 13).

Total Cl� deposition was also high and similar in both

stands, around 60 kg ha�1 year�1. The estimated gaseous HCl

deposition was high, accounting for a third of the estimated

total Cl� deposition.

Base cation leaching from the canopy of the ‘‘Dur oak’’ stand

was calculated to be 127 mmolc m�2 year�1 and from the ‘‘Dur

Pine’’stand, 77 mmolc m�2 year�1. In both forest stands, K+ was

the predominant cation leached, accounting for 45–50% of total

base cations leached from the canopy. Cation leaching was

47 mmolc m�2 in the ‘‘Urk oak’’ stand and 52 mmolc m�2 in the

‘‘Urk pine’’ stand. Magnesium flux was slightly lower in

throughfall fluxes than the estimated total deposition for both

forest stands, suggesting canopy uptake of this cation in the

‘‘Urk’’ site. It was assumed that canopy uptake of Mg2+ was

occurring, and that cation exchange was the possible mechanism

for this uptake. Base cation inputs in bulk precipitation to the

‘‘Urk’’ site were higher than at the ‘‘Dur’’ site. The same pattern

was found in throughfall fluxes. Nevertheless, the estimated

cation leaching at the former site was lower than at the latter. This

finding is also consistent with the hypothesised cation exchange

for NHx uptake and proton buffering in forest canopies.

In both stands in the ‘‘Dur’’ site total inorganic nitrogen

deposition was approximately 55 kg ha�1 year�1. The con-

tribution that N.NHx compounds made to total inorganic

nitrogen was similar at both stands, approximately 55–60%. In

the ‘‘Urk’’ stands inorganic nitrogen deposition was also high,

approximately 30 kg ha�1 year�1. The contribution of the

estimated N.NHx total deposition to this figure was ca. 20%.

The greatest difference between sites was the estimated NHx

nitrogen deposition (Table 13). N.NH4+ collected in throughfall

at the ‘‘Dur’’ site was seven times higher than at the ‘‘Urk’’ one

for oaks and four times higher for pines. N.NH4+ flux in the

thoughfall collected under pines were consistently higher than

under oaks, suggesting a smaller affinity for uptake of this

compound in the evergreen canopy.

3.6. Parameters of acidity

Acidity entering the ecosystem as bulk precipitation in the

‘‘Dur’’ site was almost twice that at the ‘‘Urk’’ site, but at both

sites, it was mainly due to the deposition of NH4+. When

throughfall fluxes are taken into account, acidity flux in both of

the ‘‘Urk’’ stands was around 20% of the acidity flux at the


Table 13

Mean annual deposition (kg ha�1 year�1) for bulk precipitation (BP) and throughfall in the stands under study (Oak, Q. robur throughfall: Pine, P. radiata

throughfall), and calculated deposition rates at both study sites during the period November 1996 to October 1998

BP Dur Oak Dur Pine Dur BP Urk Oak Urk Pine Urk

Volume (mm) 1388 (65) 945 (20) 1015 (7) 1475 (87) 1228 (91) 999 (96)

S.SO42� 25.3 (3.6) 35.0 (4.0) 37.3 (1.0) 22.6 (1.4) 37.4 (3.0) 39.6 (1.2)

IDgas (S.SO2 growing) 5 5 5 5

IDgas (S.SO2 dormant) 2 3 2 3

IDpart (S.SO42�) 2.7 4.0 7.8 9.0

Cl� 35.2 (9.3) 59.1 (6.4) 58.8 (7.9) 28.9 (2.8) 44.9 (3.7) 50.8 (9.0)

IDpart (Cl�) 3.7 5.5 10.0 11.5

IDgas (HCl) 20 18 6 10

N.NO3� 5.9 (1.0) 11.4 (2.6) 11.6 (0.1) 4.1 (1.3) 5.4 (2.4) 7.1 (1.5)

IDgas (N.HNO3) 9 9 9 9

IDgas (N.NO2 spring) 3 3 3 3

IDgas (N.NO2 summer) 2 2 2 2

IDgas (N.NO2 autumn) 2 3 2 3

IDgas (N.NO2 winter) 0 1 0 1

IDpart (N.NO3�) 0.6 0.9 1.4 1.6

ID (N.NOy) 17 19 17 20

TD (N.NOy) 23 25 22 24

Ca2+ 45.9 (2.5) 57.9 (1.2) 55.3 (4.6) 53.0 (3.2) 73.3 (0.1) 77.2 (5.2)

IDpart (Ca2+) 4.9 7.2 18.4 21.2

L (Ca2+) 7.2 2.2 1.9 2.9

Mg2+ 12.4 (0.8) 14.9 (1.0) 14.9 (1.2) 14.6 (0.1) 19.2 (0.8) 18.4 (0.2)

IDpart (Mg2+) 1.3 1.9 5.1 5.8

L (Mg2+) 1.2 0.6 �0.4 �2.0

Na+ 37.2 (1.9) 46.4 (2.4) 48.6 (0.6) 48.5 (0.1) 65.9 (2.0) 69.0 (4.1)

IDpart (Na+) 3.9 5.8 16.8 19.4

L (Na+) 5.3 5.6 0.7 1.2

K+ 6.4 (1.0) 30.1 (3.0) 22.1 (1.0) 5.7 (0.2) 21.1 (3.4) 20.4 (4.2)

IDpart (K+) 0.7 1.0 2.0 2.3

L (K+) 22.9 14.6 13.4 12.4

Mn2+ 0.00 (0.00) 0.17 (0.04) 0.13 (0.00) 0.00 (0.00) 0.02 (0.01) 0.03 (0.04)

IDpart (Mn2+) 0.0 0.0 0.0 0.0

L (Mn2+) 0.2 0.1 0.0 0.0

Fe3+ 0.29 (0.11) 0.32 (0.02) 0.36 (0.05) 0.42 (0.17) 0.39 (0.23) 0.36 (0.14)

IDpart (Fe3+) 0.0 0.0 0.2 0.2

L (Fe3+) 0.0 0.0 �0.2 �0.2

H+* 0.03 (0.01) 0.01 (0.00) 0.04 (0.03) 0.04 (0.06) 0.01 (0.00) 0.00 (0.00)

IDpart (H+)* 0.00 0.00 0.02 0.02

IDgas (H+)* 0.55 0.49 0.16 0.29

NTF (H+)* �0.01 0.02 �0.04 �0.04

N.NH4+ 7.9 (2.7) 22.6 (3.6) 26.0 (5.9) 3.7 (1.3) 2.8 (0.4) 6.3 (2.2)

U (N.NHx) 10 4 3 0

TD (N.NHx) 33 30 6 7

Standard deviations for annual throughfall flux and bulk precipitation flux are shown in brackets. Amounts of precipitation are given in millimeter (standard

deviation). IDpart: particulate interception deposition; IDgas: gaseous interception deposition; L: leaching; NTF: net flux; U: Uptake; TD: total deposition.* kmol ha�1 year�1.

‘‘Dur’’ site. Once again at all sites NH4+ was the constituent that

had the greatest effect on this parameter. Nevertheless, the

degree of acidity was low at each TF site (<25%). The high

input of base cations at both sites explains the low degree of

acidity found in bulk precipitation and throughfall.

Total acid fluxes showed that they were higher than the

levels predicted using values from throughfall (Table 14). This

was principally due to the high estimated values for NHx ID for

canopies in the ‘‘Dur’’ site, as well as to the estimated dry

deposition of protons at both sites. In this way, acidity flux in

throughfall in the oak stand was 85% of the acidity flux in the

pine stand at the ‘‘Dur’’ site. However, acidity fluxes were

similar in both stands when estimated total deposition was used

to calculate the flux. At the ‘‘Urk’’ site, acidity flux in the

throughfall collected under oaks was 45% of that under pines.

Nevertheless, when estimated total deposition acidity flux was

taken into account, acidity in the oak stand was 80% of that in

the pine stand.

Estimated maximum potential acidity due to protons and S

and N-containing compounds was high for both sites (Table 15).

At the ‘‘Dur’’ site the maximum potential acidification under

aerobic conditions was similar for both forest stands, whereas at


Table 14

Mean annual values for acidity parameters in bulk precipitation (BP), throughfall (TF) and estimated total atmospheric deposition (TD) at both study sites

Dur

BP

Urk

BP

Oak

Dur TF

Oak

Urk TF

Pine

Dur TF

Pine

Urk TF

Oak

Dur TD

Oak

Urk TD

Pine

Dur TD

Pine

Urk TD

Degree of acidity (%) 11 5 19 2 22 5 34 7 31 9

Acid flux

(mmolc m�2 year�1)

60 31 165 21 191 45 292 66 269 83

NHx (%) 94 86 98 96 97 99 80 66 80 58

Fe3+ (%) 3 0 1 0 1 0 0 0 0 0

H+ (%) 3 14 1 4 2 1 20 34 19 42

pH 5.8 5.5 6.2 6.2 5.5 6.3

Percentages of contribution of NHx, Fe3+ and H+ to the acidity flux in the deposition fractions are also shown.

Table 15

Potential annual acid deposition (mmolc m�2 year�1) due to H+ sulphur and

nitrogen-containing compounds, expressed as the equivalent maximum and

minimum H+ production under aerobic conditions at both sites

H+ NHx NOy SOx Sum

Min H+ production 0 0 �1 �2

Max H+ production +1 +2 +1 +2

Q. robur Dur

Total deposition 57 233 161 219 671

Equi. min H+ 0 0 �161 �437 �599

Equi. max H+ 57 467 161 437 1123

Q. robur Urk


Equi. min H+ 0 0 �154 �464 �618

Equi. max H+ 126 86 154 464 726

P. radiata Dur


Equi. min H+ 0 0 �178 �466 �644

Equi. max H+ 52 467 178 466 1128

P. radiata Urk


Equi. min H+ 0 0 �170 �494 �663

Equi. max H+ 35 95 170 494 793

the ‘‘Urk’’ site, maximum potential acidification in the oak stand

was around 90% of that in the pine stand. Nitrogen bearing

compounds accounted for over 55% of maximum potential

acidification at the ‘‘Dur’’ site, whilst they accounted for 30% of

maximum potential acidification at the ‘‘Urk’’ site.

4. Discussion

4.1. Bulk precipitation and throughfall fluxes

Precipitation in the Basque Country is principally of

maritime origin (Eusko Jaurlaritza, 1992). However, regression

analysis of the fluxes constituents against precipitation volume,

i.e., removing the effect of precipitation volume (SPSS, 1999),

revealed that there were clear relationships between the residual

flux of Na+ and those of Ca2+ and Mg2+. The concentration of

dissolved constituents found in bulk precipitation are inversely

related to the rate of precipitation (Gatz and Dingle, 1971) and

total inputs of these chemical species are directly related to the

total volume collected (Likens et al., 1984).

It was estimated that 69% of sodium recovered in bulk

precipitation at the ‘‘Dur’’ site was from a marine source

(Table 4). However, Lee (1993) states that HCl is highly soluble

and will rapidly dissolve in cloud or rain and that HCl may be

efficiently removed from pollution plumes close to their

emission sources. As the amount of Cl� collected at the ‘‘Urk’’

site was 82% of that collected at the ‘‘Dur’’ site (Fig. 1;

Table 13) it is possible that a proportion of the Cl� recovered in

bulk precipitation was from a non-marine source, and therefore

the percentage of marine Na+ may differ from that estimated for

the ‘‘Dur’’ site and be closer to that estimated for the ‘‘Urk’’

site. Previous studies carried out in the Basque Country have

shown that Na+:Cl� ratios that are higher than those detected in

sea water are often observed (Ezcurra et al., 1988; Casado et al.,

1989; Eusko Jaurlaritza, 1992; Mesanza and Casado, 1994;

Onaindia et al., 1994, 1995; Amezaga et al., 1997; Gonzalez-

Arias et al., 2000). Eusko Jaurlaritza (1992) proposed that a

proportion of the Cl� deposited in bulk precipitation may

undergo chemical reactions inside the collectors, with

subsequent losses of Cl� by evaporation in the form of HCl

or NH4Cl. However, the residual fluxes of Na+, Mg2+ and Ca2+

were all strongly correlated with each other (Tables 6 and 7)

suggesting that these cations were derived from a common

source.

Some of the sodium content of throughfall was related to

canopy leaching. Farrell (1995) proposed that high intensity

storms of marine origin might lead to high inputs or pulses of

sea salts in terrestrial ecosystems close to maritime environ-

ments. The pulses might result in the displacement of H+ and

Al3+ from the soil exchange complex, by Na+ and possibly

Mg2+. This may also occur in the northern part of the peninsular

Basque Country, which experiences a temperate climate, with a

particularly marked maritime influence (Eusko Jaurlaritza,

1999). Will (1959) reported that canopy leaching of this

constituent can occur in radiata pine plantations, even when the

topsoil contains relatively low quantities of exchangeable

sodium. Cappellato et al. (1993) also showed sodium canopy

leaching in coniferous and deciduous forests, even during the

dormant season.

Bulk precipitation was rich in base cations at both sites

under study here. This has also been found in other studies

carried out in the Basque Country (Ezcurra et al., 1988; Casado

et al., 1989; Eusko Jaurlaritza, 1992; Mesanza and Casado,

1994; Onaindia et al., 1994, 1995; Amezaga et al., 1997;


Gonzalez-Arias et al., 2000). This suggests that the enrichment

of bulk precipitation with cations may have been due to the

proximity of the sites to the sea and/or the influence of nearby

mountains composed of base-rich rock. Emissions from nearby

industries may well have contributed to basic cation load

(Kratz, 1991; Berger et al., 1996). For the reasons suggested

above, it was difficult to propose a specific cause for the high

inputs of basic cations detected in this study.

The values of chlorinity and enrichment ratios (Tables 4 and

5) show that the relative input of Cl� in throughfall was higher

than that of other constituents. This may have been due to a

number of factors, including: errors in measurement; lack of

stemflow estimates; leaching of Cl� from the canopy (Moreno

and Gallardo, 2002; Berger et al., 2001); higher efficiency of

the canopy in capturing aerosols and the particulate fraction

containing Cl� than those containing other ions, or increased

deposition of HCl. Part of the Cl� recovered in throughfall at

both sites may also have been of a marine origin other than sea

spray, e.g., organically bound Cl, such as methychloride, or

gaseous HCl formed in chemical reactions in the atmosphere, in

which aerosol NaCl particles combine with NO2 or SO2

(Kreutzer et al., 1998). It is also possible that a proportion of the

Cl� recovered was derived from human activities (e.g., paper

bleaching, waste incineration, combustion of coal, etc.) (Lee,

1993) and that this was subsequently recovered in the forest

canopy as HCl. The relationship between residual fluxes of

SO42� and NO3

� was significant, possibly because of common

emission sources and transport processes.

On the other hand, acid cations recovered at the two sites

were of different origin (Table 13). The Mn2+ recovered in

throughfall mainly originated from leaching from the tree

canopy (Bredemeier, 1988; Kreutzer et al., 1998), whereas it

was considered that all the Fe3+ recovered originated from

outside the forest stands, i.e., there was no canopy leaching of

Fe3+. Kreutzer et al. (1998) concluded that acidic cations

originated from similar sources to those described above; the

Mn2+ had been leached from the canopy, i.e., internal cycling,

whereas Al3+ was found to be from an external source.

4.2. Relationships between chemical species in bulk

precipitation and throughfall

There have already been attempts to use regression analysis

in order to estimate relationships between constituents present

in bulk precipitation and in throughfall (Hansen and Nielsen,

1998), as well as between elements in dry deposition collected

in a filter gauge, in throughfall or that recovered via a standard

rain gauge (Lakhani and Miller, 1980). However, some of the

relationships between different constituents may be due to the

effect that precipitation volume has on the calculation of fluxes

(Tables 6 and 7).

The residuals from the regression analysis represent the

portion of the data that is not explained by the model and can be

a valid way of estimating the deviation of each of the data from

the regression model used (SAS, 1998). Residuals from the

regression model ‘constituent flux in throughfall:constituent

flux in bulk precipitation’ were used to investigate the effect of

forest canopy on variations in throughfall flux. It might have

been more usual to use net throughfall (NTF) rather than the

residuals from the regression model for this purpose. However,

NTF is no more than the residuals from the regression

‘constituent flux in throughfall: constituent flux in bulk

precipitation’, assuming that the intercept of this model is

zero and the regression coefficient (slope) is 1. However, it does

not seem reasonable to assume that the intercept of the

atmosphere–canopy interaction model is zero, given that a zero

intercept would be unlikely for species deposited to the canopy

in gaseous form, for example.

Nitrogen in throughfall at both study sites is very different,

showing high deposition of NH3 close to the emission point

(Table 13) (Sutton et al., 1994; Spangenberg and Kolling, 2004)

and that it can be efficiently dissolved in the water layers

present in the canopies and, perhaps also in bulk precipitation

collectors (Cape and Leith, 2002). It can be assumed that

differences in the NH4+ collected with the different fraction of

precipitation may be due to NH3 rather than to NH4+

transported from further afield.

Nitrate net throughfall in the different stands behaved

differently (Table 13). The NTF recorded in the ‘‘Urk oak’’

stand is around 25% of that recorded in the ‘‘Dur oak’’ stand.

The value corresponding to ‘‘Urk pine’’ is around 50% that of

‘‘Dur pine’’. This may be due to oxidation of the deposited

NH4+ in the canopies or in the collectors at the ‘‘Dur’’ site.

Nevertheless, the strong positive relationship between both

constituents in bulk precipitation (Tables 6 and 7) suggests co-

deposition, rather than chemical transformation of NH4+ to

nitrate (Bytnerowicz et al., 1992). The estimated total

deposition of NOx is similar for the four sites (Table 13),

and is higher than that reported for the throughfall flux,

suggesting canopy uptake for this constituent. Nevertheless, on

the basis of the data from net fluxes, canopy uptake of this

constituent appears to be higher at the ‘‘Urk’’ site than at the

‘‘Dur’’ site. This may be considered as an indicator of nitrogen

saturation in forest ecosystems in the area closest to the farm

(Takemoto et al., 2001). Forti et al. (1990) pointed out that

N.NO3�may be used as an indicator of pollution because there

is little active leaching of this anion, and nitrogen saturation

usually focuses on the inability of an ecosystem to retain N

(Emmett et al., 1998).

4.3. Total deposition

There are some uncertainties associated with the way that

deposition is estimated (Table 13).

Firstly, despite concentrations of gaseous compounds being

measured close to at least one of the sites, deposition velocities

are unknown for the species and sites analysed in the present

study. When estimating deposition velocities of different

chemical species for a given forest stand, it is important to take

into account the different atmospheric conditions and the

surface type that exist in the area under study (Hansen and

Nielsen, 1998; Fowler et al., 1999; Zhang et al., 2003).

Furthermore, it is also known that deposition of gaseous

chemical species can be calculated as the average of the product


of inferred deposition velocities and concentrations, and that

the average concentration multiplied by the average deposition

velocities are not necessarily equal (Andersen and Hovmand,

1999). The deposition velocity and concentration measure-

ments have to be determined on a time scale within which these

parameters do not change significantly. Therefore, dry

deposition models have been widely used, in which para-

meterised dry deposition velocities for gaseous species are used

along with concentrations in the atmosphere (Chang et al.,

1987; Wesely, 1989; Carmichael et al., 1991; Harley et al.,

1993; Byun and Ching, 1999—cited in Zhang et al., 2003).

However, Duyzer et al. (1994) proposed that although

atmospheric conditions are taken into account, calculations

of chemical fluxes may also contain large errors due to

variations in turbulence intensity and chemical analysis

performance.

The average deposition velocities used to calculate total

deposition in the present study (Tables 2 and 3) fall in the range of

the parameterised values recently reported by Zhang et al. (2003)

and corresponding to different categories of land use (evergreen

needle-leaved trees; deciduous broadleaved trees). Nevertheless,

dry deposition velocity for HNO3 parameterised by these same

authors is somewhat lower than that reported here.

Secondly, Cappellato et al. (1993) and Quilchano et al.

(2002) reported net canopy uptake of sulphate, and Van Ek and

Draaijers (1994), canopy leaching. Chloride leaching from the

canopy has also been reported (Moreno and Gallardo, 2002).

Moreover, Berger et al. (2001) found that some Cl� leaching

can occur within the canopy and that the proportion leached

increased with increasing Cl� content of green foliage.

However, Joslin and Wolfe (1992) and Granat and Hallgren

(1992) found a high correspondence between estimates of dry

plus cloudwater S and independent measures of net throughfall

of SO42�. Furthermore, other authors (Lindberg and Garten,

1988; Garten et al., 1988; Garten, 1990) demonstrated that

canopy leaching of soil-derived S was minimal, and thus that

throughfall sulphate measurements are a more direct alternative

to the measurement of total S deposition. On the other hand, dry

deposition of chloride close to its sources has also been reported

(Lightowlers and Cape, 1988; Lee, 1993). The minimal

interaction of Cl� and any of the ecosystem compartments

has led to the use of this constituent as a tracer for water

movement through the ecosystem (Kreutzer et al., 1998; Beier

et al., 1998; Thomas and Buttner, 1998).

Thirdly, particulate deposition has also been estimated from

particulate sulphate deposition. The use of a particle that hardly

interacts with the canopy has been widely used as a tracer for

this IDpart (Ulrich, 1983; Lindberg et al., 1986; Bredemeier,

1988; Farrell, 1995; Hansen and Nielsen, 1998). On the other

hand, Lee (1993) and Fowler et al. (1999) reported values for

particle sizes that showed that sulphate-bearing aerosols are

much smaller than, e.g., aerosols bearing base cations, or cloud

droplets.

However, it must be kept in mind that cloud droplets may

also have high constituent concentrations, and that sulphate is

considered a good tracer when cloud droplets are considered as

a deposition fraction (Shubzda et al., 1995; Fowler et al., 1999).

In the fourth place, neither NH3 concentrations nor

deposition velocities were measured during the course of the

present study. It is assumed that cations are exchanged and

leached in forest canopies during the uptake of dry deposited

NH3, and buffer the acid deposition. It is known that the

metabolism of nitrogen in plants may produce NH3 and as a

result, there is a compensation point. At ambient concentrations

of NH3 above compensation point, NH3 is deposited and vice

versa (Duyzer, 1995; Sutton et al., 1995). Thus, there may be a

large degree of uncertainty when estimating total deposition of

NHx.

Sutton et al. (1995) stated that compensation point may be

substantial for cropland vegetation and highly polluted

ecosystems, but not for semi-natural vegetation at cleaner

sites. They moreover pointed out that the emission of NH3 by

stomata may be recaptured by leaf surfaces and associated

water layers.

Taking into account the statistically significant relationships

among constituents found in bulk precipitation and in

throughfall (Tables 6, 7 and 9–12) and on the basis of the

above-mentioned literature, base cation leaching may be a

plausible method of evaluating total deposition of NHx and H+.

De Vries et al. (2003b) also used the leaching of base cations

from forest canopies to estimate NH3 uptake. The values of NHx

uptake, estimated using the method proposed by De Vries et al.

(2003b) were: 17 kg ha�1 year�1 at ‘‘Dur oak’’;

5 kg ha�1 year�1 at ‘‘Urk oak’’; 10 kg ha�1 year�1 at ‘‘Dur

pine’’; and 5 kg ha�1 year�1 at ‘‘Urk pine’’. These figures were

higher than the previously estimated values (Table 13). Another

statistical analysis was performed to further test the validity of

the method used to estimate NH3 uptake in the forest canopies

under study. The NH4+ flux present in bulk precipitation was

log-transformed and regressed against log-transformed pre-

cipitation volume. The sum of cation fluxes in throughfall was

regressed against the sum of cation fluxes in bulk precipitation.

Principal component analysis followed by varimax rotation was

performed with the standardised residuals from these regres-

sion models, and provided the following parameters: Factor 1

(accounting for 38.7% of the variance) loadings: ‘‘Dur’’ bulk

precipitation res-NH4+: 0.477; ‘‘Dur oak’’ res-cations: 0.720;

‘‘Dur pine’’ res-cations: 0.899. Factor 2 (accounting for 18.4%

of the variance) loadings: ‘‘Urk’’ bulk precipitation resNH4+:

0.887; ‘‘Urk oak’’ res-cations: 0.602. This result suggests that

uptake of NH3 was taking part through cation exchange in the

first three stands and that no uptake of NH3 through cation

exchange was taking part in the forest canopy of pines at the

‘‘Urk’’ site. This is also consistent with the NHx uptake

estimated by the method used in the present study (Table 13).

4.4. Parameters of acidity

It has been suggested that broadleaved forest canopies buffer

acidic precipitation more efficiently than coniferous forest

canopies (Cappellato et al., 1993; Amezaga et al., 1997;

Quilchano et al., 2002). Buffering has been reported to occur as

cation exchange in the canopy and there exist more exchange

sites in broadleaved forests. Nevertheless, Uyttendaele and


Iroume (2002) reported higher cation content in the throughfall

collected under radiata pine plantations than under broadleaved

evergreen forests in Chile. Bredemeier (1988) stated that when

neutralisation occurs in the tree canopy, deposited protons do

not appear as measurable acidity in throughfall, but the total H+

load to the ecosystem is not reduced because when the

neutralising capacity is recharged, protons are liberated in the

rhizosphere. It therefore appears to be more convenient to use

the acidity flux, the degree of acidity and the potential

acidification estimated in total deposition rather than that

measured in throughfall to compare acidity parameters. The

acid flux entering forests ecosystems in the ‘‘Dur’’ site is twice

that entering the ‘‘Urk’’ site (Table 14). The degree of acidity in

estimated total deposition was 2.5 times higher for the ‘‘Dur

oak’’ site than for the ‘‘Urk oak’’ site. Similar figures were

obtained for the potential acidification at both sites. On the

other hand, acidity fluxes, degree of acidity and maximum

potential acidification were very similar under different forest

species in the same site (Tables 14 and 15). This suggests that

acidification caused by atmospheric deposition to forest

ecosystems may be very similar under the same circumstances,

despite the tree species dominating.

Cations leached from canopies can be readily lost from

forest ecosystems, along with anions, in the percolation water.

Nevertheless, the high cation inputs to the forest sites suggest

that soil acidification due to atmospheric deposition may be of

little importance and that cation leaching from these forest

ecosystems will not lead to serious cation deficits (Warfvinge

et al., 1993; Van der Salm and De Vries, 2001). However,

reduced leaching or canopy uptake of magnesium at the ‘‘Urk’’

site might be explained by a deficit of this element. Nitrogen

saturation, on the other hand, may be of great concern at the

‘‘Dur’’ site in the short/medium term and at the ‘‘Urk’’ site in

the long term, as both may be receiving higher loads than

proposed critical loads: 15–30 kg ha�1 year�1 (Wright and

Rasmunsen, 1998; De Vries et al., 2000b; De Vries et al.,

2003a).

5. Conclusions

The canopy flux method was used along with a generalised

deposition model to estimate total atmospheric deposition of

different constituents to four forest ecosystems: two radiata

pine plantations and two mature pedunculate oak forestlands,

both of which were represented at each of two study sites. One

of the sites was located close to a cattle farm whereas the other

one was farther from it. It can be concluded from the present

study that:

� A
mmonia is deposited close to the emission sources;
limestone present in the mountains surrounding the study

sites, probably enhanced by industrial extractions and/or

local industry emissions, is rich source of calcium (probably

as calcium carbonates). These two factors may act as a

buffering agent of precipitation.

� S
tatistical analyses revealed relationships between SO42�
and NO3� deposition and between deposition of both anions

and NH4+ in the bulk precipitation, suggesting co-deposition

of SOx and NOy with NHx at both sites. There was no

relationship between Cl� and NH4+ suggesting that these two

constituents were not co-deposited at the study sites.

� T
he relationships between Cl� and SO42� and base cations
measured in throughfall suggest that buffering of precipita-

tion and NHx uptake in the forest canopy was brought about

through cation leaching.

� A
cidity derived from total atmospheric deposition entering
both ecosystems at each study site was similar, regardless of

whether the dominant species was P. radiata or Q. robur,

although throughfall acidity fluxes were higher in radiata pine

plantations than in oaks stands.

� T
he high fluxes of base cations in deposition suggest that soil
acidification and/or cation deficits at the study sites will

probably not occur. However, the canopy uptake of

magnesium at the ‘‘Urk’’ site may be an indicator of deficit

of Mg2+. Nevertheless, the high inputs of inorganic nitrogen

to the forest stands suggest that nitrogen saturation may be an

important stress factor in the future.

Acknowledgements

The authors would like to thank the following: Julian

Agirrezabal for allowing us using his property for carrying out

the present study; The Basque Government’s Environmental

Protection Agency (Department of Territorial Planning and

Environment) for providing the air concentration data; Ibone

Amezaga, Isabel Albizu, Sorkunde Mendarte, Belen Gonzalez

and Robert Parker for helping with the sampling; Robert F.

Parker and Christine Francis for helping to improve the

English; Arsenio Echeandıa and Yolanda Uriondo for helping

with the chemical analyses, Professor Fernando Tussel Palmer

(Econometrics and Statistics Department UPV/EHU) for his

advice on the statistical analyses and two anonymous reviewers

for their valuable comments and criticisms. We also thank the

Inter-ministerial Commission of Science and Technology of

Spain (CICYT) project Ref.: REN 2000 0196 P4 03 and the

Department of Agriculture and Fisheries of the Basque

Government for their financial support.

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