Natural abundance of 15N in forests across a nitrogen deposition gradient

ELSEVIER Forest Ecology and Management 101 (1998) 9-18

Pores;f;ology

Management

Natural abundance of 15N in forests across a nitrogen deposition gradient

B.A. Emmett a* * , O.J. Kjenaas b, P. Gundersen ‘, C. Koopmans d, A. Tietema d, D. Sleep e

’ institute @‘Terrestrial Ecology. Bangor Research Unit. UWB.. Deiniol Rd, Bangor, Gwynedd, LL57 2(/P. UK

b Norwegian Forest Research Institute, Hogskoleceien 12, N-1432 AOs, Norway ’ Danish Forest and Landscape Research Institute, Hoersholm Kongecej 1 I, DK-2970 Hoersholm, Denmark

’ Unirersihl of Amsterdam, Physical Geography and Soil Science, Nieuwe Prinsengracht 130. NL-1018 VZ Amsterdam, Netherlands ’ Institute of Terrestrial Ecology. Merlewood Research Station, Grange-orer-Sands, Cumbria, LA 1 I 6JU. UK

Accepted 11 March 1997

Abstract

Chronic atmospheric nitrogen deposition can alter the rate of internal nitrogen cycling and increase the magnitude of N leaching losses in forested ecosystems. As fractionation of nitrogen in favour of the lighter 14N occurs during various transformations associated with N-enrichment and nitrogen loss, it has been proposed that the 15N signal of vegetation may provide a useful tool in evaluating the past and current N status of forested ecosystems. A series of coniferous forests across a European nitrogen deposition gradient within the NITREX project provided an opportunity to test the relationships between nitrogen supply from atmospheric deposition and the relative ‘%enrichment of vegetation to soil, across a large geographical area. Most S15N values for above- and below-ground tree components, soil at four depths, bulk precipitation and/or throughfall water and soil solution or outflow water values were within those observed elsewhere except for a few notable exceptions. There was a significant positive relationship between the 6 “N enrichment of the tree foliage relative to the soil horizons (or the enrichment factor), and nitrogen flux in the throughfall if Aber forest, N. Wales, was excluded from the regression analysis. An unusually high enrichment factor at the Aber site indicated that a the high rate of N cycling at the site was in excess of that predicted from current N deposition. This was attributed to the effect of ploughing and tree planting on the relatively N- and clay-rich mineral horizons at Aber compared to other sites. Highly significant relationships (P < 0.01) between enrichment factors and parameters describing internal rates of N cycling, such as litterfall N flux and nitrification rates in upper soil horizons, supported this conclusion. There appears to be a strong link between the rate of N cycling and the 615N enrichment factor, rather than N deposition or nitrate leaching per se. These results confirm the potential use of the 615N enrichment factor to identify sites influenced by nitrogen deposition. However, consideration should be taken of other site characteristics and land management practises which also influence soil N dynamics and N cycling. 0 1998 Elsevier Science B.V.

Keywords: 8 15N; Forests: Natural abundance; N Deposition: Nitrate; NITREX

* Corresponding author. Tel.: + 44248-37-00-45; fax: + 44.

248-35-53-65; e-mail: [email protected].

0378-l 127/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII s037x-11’7(97)00121-7

1. Introduction

Chronic atmospheric nitrogen deposition can alter the rate of internal nitrogen cycling and increase the magnitude of N leaching losses in forested ecosystems (Aber et al., 1989). As fractionation of nitrogen in favour of the lighter “N occurs during various transformations associated with N-enrichment and nitrogen loss (Shearer et al., 19741, the I5 N signal of vegetation may provide a useful tool in evaluating the past and current N status of forested ecosystems (Garten, 1993; Garten and Van Miegroet, 199.5: Hogberg, 199 1; Hogberg and Johannisson, 1993).

Fractionation is particularly important during nitrification, denitrification and ammonia volatilisation processes (Shearer and Kohl, 1986; Delwiche and Steyn, 1970; Mariotti et al., 1981) and can result in lSN enrichment of the soil N pool and leaching or gaseous loss of the lighter “N (Shearer et al., 1974; Vitousek et al., 1989; Hogberg. 1990). As a consequence, 15N-enrichment of the foliage can occur as nitrogen supply increases following fertiliser applica- tions, as demonstrated in both agricultural (Meints et al.. 1975) and forested systems (Hogberg, 19901, or chronic atmospheric nitrogen pollution (Gebauer and Schulze, 1991). The relative change in the ‘“N abundance of the vegetation is greater than that in the soil, as soils have a large pre-treatment ‘memory’ due to the large, relatively inactive, soil N store. Vegetation “N abundance thus reflects the more active soil nitrogen in soil solution or on ion ex- change sites and therefore, the more recent history of N dynamics at a site (Johannisson and Hogberg, 1994). A positive relationship between N supply and an increase in the relative “N enrichment of vegetation to soil has been observed for several vegetation types (Johannisson and Hogberg, 1994; Garten, 1993: Garten and Van Miegroet, 1995). This has been associated with accelerated nitrification in N-rich sites leading to constant, or increasing uptake, of ‘sN enriched NH,-N by vegetation as “N depleted NO,-N is leached from the system (Garten and Van Miegroet, 1995). However. “N enriched vegetation has also been associated with high nitrate reductase activity and utilisation of “N enriched nitrate by plants (Pate et al., 1993).

A series of coniferous forest stands within the NITREX project provided an opportunity to test the

relationships between nitrogen supply from atmospheric deposition, soil N transformations, and the S “N enrichment of vegetation relative to soil. across a large geographical area. The use of the 15N enrichment factor of vegetation provided means by which initial differences in soil 6 “N values could be nor- malised for effects of land management practices. soil age and climate. The NITREX experimental sites cover a nitrogen pollution gradient of 10 kg N ha-’ yr-’ in southern Sweden to 60 kg N ha i yr-’ in the Netherlands (Wright and Van Brermen, 1995). A companion study involving field-scale li‘N tracer experiments to experimental plots with different N inputs at the different NITREX sites i.\ reported in Tietema et al. (1998). A cross laboratory comparison on “N analysis was conducted to enable cross-comparison of data between sites in both pro- jects.

2. Methods

2.1. Site descriptions

Details of the five sites are presented in Table 1. More detailed information on sites can be found in Emmett et al. (1995), Kjonaas et al. (1998). Van Dijk et al. (19921. Boxman et al. (1995) and Gunder- sen and Rasmussen ( 1995).

2.2. Vegetutim

A range of coniferous tree species was present at the different NITREX sites (Table I). Samples of branches (BR), twigs (TW) and needles were collected from 7th whorl of the trees in the dormant season at Aber. G&dsjiin and KJosterhede and from the upper sun crown at Speuld and Ysselsteyn. All trees in the experimental areas were sampled in the Dutch site whereas at G%rdsjijn. Aber and Kloster- hede. a subsample was selected according to tree size due to the large number of trees present. Needles were divided into current yr (0.51, previous yr (1) and rest (> 2). No needles from the oldest category (> 2) were sampled at the Dutch sites. The needles were quickly rinsed with deionized water to remove dust particles. Twigs, defined as second or higher order branches with needles attached, were bulked;

B.A. Emmett et al. / Forest Ecology and Management 101 (1998) 9-18 11

Table I A summary of site locations, histories, stand densities and nitrogen flux in throughfall and drainage water

Site, country, and code History Density (stems ha- ’ ) Throughfall

(leaching losses) kg N ha-’ yr-’

Gardsjbn, Sweden (GD) Lower part of catchment dominated by 81 yr old Picea abies, 600e IO(l)” podzolic soil, shrubs (Vaccinium spp.) and Dicranum moss.

Klosterhede, Denmark (KH) Picea abies plantation. 2nd generation after heathland. 860 23 (0.2jb Shallow plough before planting in 19 19.

Aber. Wales, UK CAB) Previously unimproved moorland, ploughed and planted in 1960 2000 15 (9JC with Picea sirchensis.

Speuld. the Netherlands (SP) Pseudotsuga menziesii planted in 1962. No ground vegetation. 1200 50 (88jd Previously broadleaved woodland.

Ysselsteyn, the Netherlands (YS) Pinus &estris planted in 1950 following ploughing. 900 58 (65jd

Ground vegetation including Ruhus sp., Dyopreris dilata, Deschampsiajlexuosa. Previously under cultivation.

‘Moldan et al. (1995). bGundersen and Rasmussen (1995). ‘Emmett et al. (1998).

dBoxman et al. (1998). ‘Mean of control and treatment catchments.

Nitrogen leaching losses are shown in parentheses.

and branches were sampled at the bottom 10 cm of a first-order branch. Wood cores (WD) were taken from selected trees at breast height, and the hard- wood and sapwood bulked to obtain sufficient sample for analysis. All samples were dried at 60-80°C and milled using either ball or planetary mills or a liquid-nitrogen freezer mill.

Litter samples (LT) were collected monthly, dried, and bulked for three month periods (Oct.-Dec., Jan.-Mar., Apr.-Jun., Ju1.Sep.J. Data presented are for annual values for needles or needle + twigs.

A/H) and easily visible roots < 2 mm in diameter removed and analysed separately CRT) with the exception of G&dsjiin where roots < 2 mm were included in the organic soil samples. Here a separate root study provided samples for r5N analysis (Clemensson-Lindell and Persson, 1995). Mineral soil was divided into the upper 5 cm and a minimum of two horizons below the top 5 cm (15 cm and 15

Table 2 Selected soil characteristics at the different NITREX sites

Site code

2.3. Soils

Sampling of soils varied between sites due to variability in both scale (plot or catchment) and soil type. Samples were collected randomly according to a sample grid system at Klosterhede, Aber, Speuld and Ysselsteyn. At Girdsjon, a smaller, relatively homogeneous part of the catchment was sampled both for soil and plant material. Here, the soil was sampled in a 5 X 5 m grid system according to diagnostic horizons. Some selected soil characteristics are presented in Table 2.

GD KH= AB” SP YSd

Forest Poor

pH-salt 2.8 2.8 2.7 2.9 2.9

kg OM m-’ 14.4 7.0 5.9 3.1 3.4

gNm-’ 291 105 129 61 15

C/N(g/g) 32 33 22 21 20

Mineral soil (O-30 cm)

pH-salt 4.2 4.3 3.1 4.4 3.0

g OM mm2 20.9 14.4 19.4 16.3 25.8

gNm-’ 376 244 826 304 340 C/N(g/g) 31 30 18 27 38

% clay 8 3 9 2 2

Forest floor was divided according to the different horizons most apparent at the different sites (FF and

‘Site has been ploughed. From Gundersen et al. (1998).

cm + 1. The samples were sieved through a 2 mm sieve prior to drying and grinding.

2.4. Water

Bulk precipitation. throughfall and soil water samples were collected as described in Emmett et al. (19951, Gundersen and Rasmussen ( 199.51, Boxman et al. (1995) and Moldan et al. (1995). The soil water was collected at the outflow or lowest soil horizon of the plot/catchment. Samples were bulked over 3 or 6 months as described for litter samples, in propor- tion to volume collected. The samples were filtered and either stored below 4°C or frozen. prior to bulking and analysis. “N analysis was carried out for ammonium and nitrate separately if sufficient ammonium-N was present. For more details on sampling methods see Kjonaas et al. (1992).

2.5. Analytical methods

The solid samples from Girdsjon were run at Boston University Stable Isotope Laboratory, USA. on a Finnigan Delta-S isotope ratio mass spectrometer with a Heraeus carbon-nitrogen analyser and a Finnigan CT box for cryogenic separation of gasses prior to injection into the mass spectrometer. The gas was calibrated against atmospheric NX and IAEA reference sample N, (Ammonium Sulphate). The N concentrations of plant and soil samples were calcu- lated from a glycine curve run with each set of samples. Solid samples from Speuld and Ysselsteyn were analysed at the Energy Research Laboratory (ECN), the Netherlands, for total N using a Carbo Erba CHN analyser. Nitrogen isotopes were measured on a Finnigan MAT stable isotope mass spectrometer, equipped with a Hereaus elemental analyser. Samples were measured against N,-gas (> 99.99%) which was calibrated against acetanilide, KNO,, and pine needles. Samples from Klosterhede were analysed on an ANCA system by Europa Sci- entific, Crewe, UK. (20-20 stable isotope analyser and ANCA SL combustion module, Europa Scien- tific). Samples were calibrated against internal samples of wheat flour. Solid samples from Aber suffi- ciently high in nitrogen were analysed by the NERC Stable Isotope Facility at ITE Merlewood, UK, using a Roboprep/Tracermass ANCA system (Europa Sci-

entific. Crewe, UK). Ammonium sulphate was used as the working standard after calibration against SN3, Ml36 (ammonium sulphate and gelatine respectively; analyses provided by NIGL. Keyworth, UK), Nl(ammonium sulphate provided by IAEA. Vienna). acetanilide (kindly provided, together with its analysis by G. Gebauer. Bayreuth? and atmospheric nitrogen.

Mineral soils and woody vegetation samples from Aber, Speuld and Ysselsteyn were digested using modified Kjeldahl procedures and steam distilled prior to analysis for “N. The water samples from Girdsjon and Aber were concentrated using ion-ex- change resins. Amberlite IR- 120 for ammonium and Amberlite IRA-458 for nitrate using methods of M. Downs. Woods Hole. IJSA (Pers. Comm.1. After elution with KCl. the eluent was rendered alkaline in the steam distillation apparatus by addition of excess MgO powder. Where reduction of nitrate to ammo nium was desired. an excess of Devarda’s alloy was also added to the MgO/eluent mix. The ammonia released was trapped in a light excess of sulphuric acid, this was then evaporated to a dry ammonium sulphate salt prior to isotopic analysis. The diffusion method of Sorensen and Jensen (1991) was used to prepare water samples from Speuld and Ysselsteyn. No water data is available from Klosterhede.

The 6”N values of the isotopic composition is expressed as parts per thousand differences from a standard:

the samples and the standards. respectively (Peterson and Fry. 1987).

To allow for initial differences on soil and plant S values due to previous land management and soil age. an enrichment factor (E,,,,~) was used, defined as the difference between “N abundance in the cub- strate and product, in this case, between soil and vegetation (Mariotti et al., 19811. Thus, cr , = 6’“N. - a”N tulq‘ .). WI1

2.6. Laborator?, comparisorr

To ensure quality control for the “N analysis, and to enable comparisons between different laboratories. a common set of reference material for 15N analysis

Table 3

B.A. Emmett et al. / Forest Ecologv and Management 101 (1998) 9-18 13

Comparison of variability in 6 “N analysis of 7 standard samples across five laboratories both within and during three or more separate runs greater than two weeks apart

Sample type Laboratory

A B C D E

AB needles 1.77 ( * 0.39) 2.10 (ZtO.07) 2.4 (+O.l) 2.83 (kO.10) 2.01 (rtO.15) KH forest floor - 1.91 (i.O.30) - 1.53 (+0.20) - 1.4 (kO.1) -0.94(+0.13) - 1.74 (*0.16) YS mineral soil - 1.66 (iO.56) - 1.4 (+0.1) - 1.30 (* 0.39) Mineral soil 5.58 ( k 0.73) 6.05 (+ 0.08) Oak leaves - 3.03 (i 0.65) - 3.42 ( + 0.37) -2.37(+0.12) - 2.63 ( f 0.25)

NH,SO, - 2.44 ( f 0.30) -2.lO(f0.07) - 1.52 (fO.06) -2.21 (+0.04) Glycine 9.72 (+0.41) 10.10(~0.10) 10.66 (&0.04) 10.04 ( * 0.05)

Values represent means ( + s.d). Laboratories A, B and D were involved in analysis of NITREX samples.

was used. This consisted of needles, organic soil, two sources of mineral soil, red oak leaves, ammonium sulphate and glycine. All reference samples were freezer-milled prior to distribution. Samples were included in three separate runs on the mass spectrometers at least a week apart, with replicate samples in each run. Participating laboratories included the three laboratories used in this study (A, B and D> and two independent laboratories (C and E).

A consistent bias in 15N abundance values was observed between the laboratories A to D with laboratory A giving the most positive values and laboratory giving the most negative values (Table 3). No consistent trend was apparent in the results from laboratory E. These analytical differences between and among laboratories did not alter any of the ‘between site’ differences or relationships described below.

3. Results

3.1. Nitrogen concentrations

Nitrogen concentrations decreased from foliage to woody components and from upper soil horizons down the soil profile (Fig. 1). The two high nitrogen deposition sites in the Netherlands (Speuld and Ys- selsteyn) had high nitrogen concentrations in all plant components including wood and in the upper soil organic horizons. The nitrogen concentrations in the other three stands diverged most strongly in foliar components with little difference in woody components or soil horizons.

3 2 6’jN abundance . .

3.2.1. Trees The ‘*N enrichment of the current foliage ranged

from -5 to +4 (Fig. 2). All tree components at Aber had higher 6 15N values than for all other sites. At all sites, 6 15N decreased from current (0.5) to l-year old (1) foliage. Litter was similar or lower 6 I5 N values relative to older foliage with the exception of Ysselsteyn which had higher 615N value by 1.5%0. At Ysselsteyn, woody components had lower 615N values relative to foliage at Ysselsteyn and lower or equal to foliage at Klosterhede and Aber. At Girdsjon, twigs had lower values and branches higher

1

0 0.5 1 1.5 2 2.5 3 %N

Fig. 1. Nitrogen concentration (% dry weight) in different tree components and soil horizons at the five NITREX sites. Site codes are; G&dsjijn (GD), Klosterhede (KH), Aber CAB), Speuld (SP)

and Ysselsteyn (YS). See text for description of tree component and soil horizon codes.

14 B.A. Emmett et al./ Forest Eco/og?; and Manapment 101 (19981 Y-16:

I

-12 -a 4 0 4 a 12 6”N (%)

sGD*KH-t-ABeSPa-YS

Fig, 2. Observed fi”N values (%o) of different tree components and soil horizons at the five NITREX sites. Site and ecosystem component codes are the same as Fig. 1. Additional codes; PNO,

and PNH, indicates NO, and NH, mid-point values for bulk precipitation and SNO, and SNH, for mid-point values for soil

solution or runoff.

6 “N values relative to foliage. Fine roots in the forest floor had higher 6 15N values relative to the foliage at G&dsj& but lower values at Aber and Klosterhede.

3.2.2. Soil Upper soil horizons at Aber had higher 6 “N

values relative to all other sites (Fig. 2). The two Dutch sites had the lower 6 “N values at depth

Table 4

Range (minimum and maximum) of S “N abundance (%,) of ammonium-N and nitrate-N in bulk precipitation, throughfall and soilwater or runoff at four NITREX sites

Site Bulk Throughfall Soil solution Precipitation (50 cm)/runoff

NH, NO, NH., NO, NH, NO, -N -N -N -N -N -N

GD min - 1.9 - 3.3 -4.4 - 1.9 I/S I/S max -4.5 f 1.2 - I.1 +2.3

AB” min -5.2 +3.7 N/A N/A I/S + 2.4

max + 1.8 +4.9 SP min - 10.0 - 13.9 - 8.8 -5.3 - 10.2 - 12.0

max + 8.6 +8.7 + 12.9 - 16.9 +8.0 - 4.4 YS min t3.2 - 3.9 +5.7 - 16.6 - 10.9 - 7.6

max + 17.2 - 16.9 + 16.6 +3.2 -4.1 - 4.6

I/S Insufficient nitrogen for analysis. N/A Data not available.

‘Bulk precipitation samples were collected during the winter of 1995 and thus do not coincide with soil sampling in 1992/1993.

( + 3.S%i;,r;) compared to + 6 to + 8%~. at other sites. Within sites, 8 15N values increased by 5 to 125L with soil depth at all sites. A steep increase in 6”N of the soil with decreasing ?C N at depths of greater than 5 cm ( < 0.6% NJ was observed at Klosterhede and the two Dutch sites. A shallower trend wa.‘; observed at Aber and Girdsjon which had higher N concentrations at depth relative to other sites.

3.2.3. Waters The 6 ‘“N of ammonium-N and nitrate-N oi

throughfall and bulk precipitation generally fell between - 5 and + 5%~ with the exception of the two Dutch sites (Table 4). At Ysselsteyn. S I5 N was high in ammonium-N and low in nitrate-N in some samples of both bulk precipitation and throughfall whilst at Speuld, no consistent trend was observed. 6 ‘“N abundances of soil water in the mineral soil at Aber and Ysselsteyn appeared to reflect the “N signature of nitrate-N inputs. There was insufficient nitrogen for analysis in runoff samples from Girdsjon. Midpoints of the range of 6 “N for NO, and NH, recorded for bulk precipitation (PNG, and PNH, respectively) and runoff or soil water (SNO, and SNH, respectively) are presented in Fig. 2.

4. Discussion

4.1. Trends in S”N ~~11ue.s oj’ vegetation, soils arid waters across the sites

The 15 ‘“N values of --- 5 to -t 2%~ for vegetation were within the values observed for other coniferous forests with the exception of Aber at + 4’%0 (H&berg, 1991; Gebauer and Schulze, 1991; Nadelhoffer and Fry, 1994: Gebauer et al., 1994). The positive values at Aber may have been a result of the ploughing prior to afforestation in 1960. This would hav~e resulted in the mixing of soil to about 10 cm depth in the mineral soil, a deepening of the active soil horizons and a stimulation of nitrogen minerallisation and nitrification rates due to both plougbing and the change in soil water dynamics as the stand matured. All of these factors could have contributed to the positive 6 “N values in the upper soil horizons and the formation of a shallow gradient in 6 15N down the soil profile. Similar poorly developed profiles of

B.A. Emmett et al. / Forest Ecology and Management 101 (1998) 9-18 15

S15N are often observed in cultivated soils (Karamonos and Rennie, 1980; Nadelhoffer and Fry, 1994). However, both Ysselsteyn and Klosterhede were also ploughed prior to afforestation without a similar “N enrichment of the whole ecosystem. An additional feature at Aber however, was the relatively high nitrogen and clay content of the Aber soil (Table 2). This large soil nitrogen pool may have been more susceptible to disturbance following ploughing and drying of the soil following tree planting resulting in high N mineralisation rates and accelerated nitrogen cycling at the site. This could have resulted in the unusually high nitrate leaching losses at the site relative to inputs and thus, an 15N enrichment of both soil and vegetation.

The 6 15N abundance of all tree components and the fine roots in the forest floor was similar to those observed in the upper soil horizons. The majority of fine roots occur in these upper horizons except in the high N deposition Dutch sites, (Gundersen et al., in press). Low 6 “N values are maintained in the upper soil horizons by a tight cycle between tree uptake and return of “N depleted foliage and root litter. Thus, upper soil organic horizons are influenced by inputs of “N depleted litter. as experimentally demonstrated by Nadelhoffer and Fry (1988). The low S ‘“N values of vegetation are generally attributed to fractionation during soil nitrogen transformations and the tight cycle of N in forest ecosystems.

At all sites, a decrease in 6 “N values was observed from current to l-year old foliage. This has been associated with greater anabolic activity in younger needles and thus a stronger sink for assimi- lated nitrogen (Gebauer and Schulze, 1991). There was no strong trend in 6 15N values from older needles to litter in all stands except at Ysselsteyn for which an increase of 1.5%0 was observed. This may be due to accelerated decay and catabolic processes prior to abscision in this declining stand. No consistent trend was observed in 6 15N values between woody and foliage components.

Soil 6 I5 N values generally increased with depth. This has been associated with transport of 15N enriched soil humus particles down the soil profile in combination with continued fractionation during the nitrification process, and enrichment of the remain- ing ammonium and soil organic-N pools (e.g. Vi-

tousek et al., 1989; Nadelhoffer and Fry, 1988). The rate of increase down the soil profile was not consistent between sites perhaps reflecting the different soil characteristics, biologically active horizons and hydrology at the sites.

The 6 15N values in the deeper mineral horizons were significantly greater in the Scandinavian and UK site relative to the two Dutch sites, No relationship with nitrogen concentration or soil organic matter content and the S “N values in the lower mineral soil was observed and the same pattern was not observed in the vegetation. Other factors often associated with low soil enrichment are soil fertility (Mariotti et al., 1980) and the age and thus size distribution of soil organic matter and soil development (Vitousek et al., 1989; Nadelhoffer and Fry, 1994; Ledgard et al., 1984). No information on the age of soil organic matter and its relative location in the soil profiles is available, but it is possible that the poorly-stratified, sandy soil at the Dutch sites is less able to retain leached 15N enriched organic matter at depth relative to the other sites. The low organic matter content of the Dutch soils may also decrease the relative importance of immobilisation of ammonium relative to nitrification and result in low S”N values (Shearer et al., 1974).

The bulk precipitation and throughfall 6”N values of Girdsjon and Aber were within the range of - 10 to + 6%0 observed by Nadelhoffer and Fry (19941, Durka et al. (1994) and Garten (1991). A general trend of lower 6 15N in NH,-N than NO,-N has been observed in most of these studies and by Freyer (1978) and Heaton ( 1987). The opposite trend was, however, the case for the two Dutch sites where, particularly at Ysselsteyn, the input of nitrate was strongly depleted while the ammonium had high 6 “N values. This high enrichment of ammonium is probably due to the dominant source of NH,, nearby livestock feed lots. Ammonium from such sources has previously been reported to have high 6 ‘“N values (Koopmans et al., in press). Leaching losses were dominated by nitrate at all sites and reflected the 6 Ifi N signature of incoming NO,-N rather than NH,-N at two of the three sites. This suggests that retention of incoming NO,-N may be lower than that of NH,-N, as demonstrated at Aber (Emmett et al., 1998; Tietema et al., 1998).

The influence of the 6 I5 N of incoming N at the

two Dutch sites on the 6 “N values of the trees and soils was unclear. With the exception of unusually low 615N values at depth in the soil. the two sites exhibit the usual trend of low 6 15N values in vegetation and increasing 6 “N values down the soil profile. The main influence on the relative 6”N values of vegetation and soil appears to be the absolute nitrogen supply rather than its 6 “N value.

4.2. Assessment oj’ the ewichment fuctor

I 2” E -5.

fs ;

UJ -7 /z/ f=3.553&5.9915,

-8 . ~=oe.?m

A significant positive relationship was observed between the enrichment factor in foliage and nitrogen in throughfall was observed if Aber was excluded from the regression analysis (R’ = 0.97, P < 0.05) (Fig. 3). The soil values used for this calculation were means of the 6 “N values of the most active soil layers (Ah, + 5 cm and 5- 15 cm) exclud- ing the forest floor. Aber lies above the line indicating that vegetation is more 15N enriched relative to the soil than would be predicted from throughfall N flux alone. As discussed previously. high rates of N cycling at Aber may be a consequence of increased nitrogen availability in the soil following the ploughing of the soil in 1960 and the continuing effect of mixing of soil horizons and changes in soil moisture status on soil N dynamics. Aber falls nearer the line if various parameters describing the internal rate of N cycling are substituted for throughfall N flux; N flux in litterfall (R” = 0.93, P < 0.01) and the soil nitrification rate (R’ = 0.89, P < 0.01) (e.g. Fig. 41, b). Significant relationships were also observed if C/N ratio of the forest floor (R2 = 0.86. P < 0.01) and annual mean temperature (R’ = 0.77. P < 0.05)

Fig. 3. The relationship between enrichment factor (see text for calculation) and nitrogen flux in throughfall which is used as a aurrogate for deposition. Aber is marked with a box and was not included in regression analysis.

Fig. 3. The relationship between the enrichment factor and: (ai annual soil nitritication rate in forest floor, (b) litterfali IN thIx. and (0) the first principle component which was significantly

correlated with a range of stand variables indicative of the nitn-- gen status of a stand. Aber is marked by a box in all figures.

were used in place of throughfall these various parameters are taken et al. (1998).

flux. Values for from Gundersen

A principle component analysis was used to various site variables into a more concise format with which to characterise the nitrogen status of the different sites (Gundersen et al., 1998), as described by Garten (1993). The first principle component ex- plained 76% of the total variance and was significantly correlated with fifteen variables such as soil N transformation rates, N concentrations in various soil and tree components and various above- and below- ground biomass pools and fluxes. The enrichment factor was observed to be highly correlated with this

B.A. Emmett et al. / Forest Ecology and Management 101 (19981 Y-18 17

first principle component (R2 = 0.90, P < 0.01) (Fig. 4~) indicating again the strong relationship between the “N enrichment factor and overall nitrogen status and rate of N cycling within a forest stand.

A positive relationship between the foliar enrichment factor and nitrification has been observed by Garten (1993) and Garten and Van Miegroet (1995). They suggest a greater use by the vegetation of 15N enriched ammonium-N resulting from the accelerated nitrification rates relative to immobilisation at the high N-deposition sites. The mechanism underly- ing the relationship observed between nitrogen in throughfall and the 6 15N enrichment factor (exclud- ing Aber) (Fig. 3) certainly appears to be the effect of nitrogen deposition on soil nitrogen transformations and the rate of N cycling rather than nitrogen deposition per se.

5. Conclusions

This study has demonstrated that the relative 15N enrichment of the vegetation to soil reflects difference in N fluxes and rates of N cycling within a group of coniferous forest stands. Measurements of natural “N abundance thus provides additional independent information regarding the N status of forest ecosystems and may be of use in identifying sites influenced by nitrogen deposition. However, care should to be taken to take account of other site characteristics and land management practices which may affect the rate of N cycling.

Acknowledgements

We would like to thank the two independent laboratories who participated in the cross laboratory comparison and for their useful comments on sample preparation and analytical techniques. The contribu- tions of Knute Nadelhoffer, Marty Downs, Brian Fry and Douwe van Dam who were involved in the planning of the “N experiments are gratefully ac- knowledged. This research was funded in part by the Commission of European Communities (EVSV- CT930264 and EV5V-CT940436).

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Natural abundance of 15N in forests across a nitrogen deposition gradient

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