Soil N cycling in harvested and pristine boreal forests and peatlands

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Forest Ecology and Management 234 (2006) 227–237

Soil N cycling in harvested and pristine Boreal forests and peatlands

Cherie J. Westbrook a,*, Kevin J. Devito b, Craig J. Allan c

a Department of Geography, Centre for Hydrology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A5b Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6E 2G9

c Department of Geography and Earth Sciences, University of North Carolina at Charlotte, Charlotte, NC 28223, USA

Received 9 September 2005; received in revised form 5 July 2006; accepted 6 July 2006

Abstract

The heterogeneous Boreal Shield forest in Canada is one of the most extensive pristine forests remaining in the world and is being intensely

harvested. We studied the spatial variability of organic and inorganic N cycling processes in three Boreal Shield catchments in northwestern

Ontario for 2 years before and 1 year following clearcutting. Net N mineralization rates were similar among upland conifer, upland deciduous and

peatland stands, ranging from negligible to 150 mg kg�1 in the forest floor/peat soils and �30 to 40 mg kg�1 in mineral soils of the upland stands

over the growing season. Net nitrification rates were generally negative, <10% of net mineralization rates, and similar among the landscape units.

Reciprocal transplants of forest floor/peat and mineral soil from the uncut and cut stands indicated that changes in environmental conditions in the

clearcut influenced net N mineralization by 50-fold and nitrification rates by nine-fold in the peatlands but not the coniferous uplands. Net

inorganic N cycling rates measured the 1st year following clearcutting were within the natural range of variability, which is consistent with previous

studies in northern coniferous and aspen forests. In contrast with the literature however, no difference in soil dissolved organic N mobilization rates

(peatland stand range: 0.2 to 4.8 mg kg�1 d�1; upland coniferous stand range: �0.1 to 2.3 mg kg�1 d�1) were found between uncut and recently

clearcut stands.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Boreal forest; Canadian shield; Dissolved organic nitrogen (DON); Forest harvesting; Net N mineralization; Nitrification; Nitrogen; Peatlands

1. Introduction

The Boreal Shield ecozone of Canada is one of the most

extensive pristine forests remaining in the world, and is subject

to high logging pressure (400,000 ha annually: Environment

Canada, 2000; Kronberg and Watt, 2000). Limited N

availability, low soil temperature, and summer soil moisture

deficits have been identified as the most important growth

limiting factors in boreal forests (Post et al., 1992; Binkley and

Hogberg, 1997). Forest harvesting may increase the availability

of N through increasing soil temperature and antecedent

moisture (Chapin, 1996; Redding et al., 2002). An increase in N

may have ramifications for soil fertility, which is important in

successful forest renewal (Jurgensen et al., 1997) and the export

of waterborne nitrate (Sollins and McCorison, 1981).

A complex mosaic of peatlands and coniferous and

deciduous uplands characterizes the Boreal Shield region. Soil

* Corresponding author. Tel.: +1 306 966 1818; fax: +1 306 966 1428.

E-mail address: [email protected] (C.J. Westbrook).

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

doi:10.1016/j.foreco.2006.07.004

N availability and net N mineralization and nitrification rates

have been found to vary with tree species composition in

temperate and boreal forests (Min et al., 1999). Contrasts

among landscape units with different tree species composition

have been linked to differences in soil organic matter content,

litter N content, C/N ratios, soil moisture, temperature and

topographic position (Devito et al., 1999; Lamontagne, 1998;

Hill and Shackleton, 1989; Ohrui et al., 1999; Stottlemyer et al.,

1995; Zak and Grigal, 1991). Since trees from all landscape

units are removed during forest harvesting in this region, a

characterization of the spatial variation in N cycling rates at the

catchment scale is needed to predict the controls regulating

inorganic and potentially organic N losses.

The effects of forest harvesting on N dynamics have been

explored for coniferous and deciduous upland stands in

temperate forests. General conclusions are that net N

mineralization and nitrification commonly increase following

forest harvesting (Feller and Kimmins, 1984; Hendrickson

et al., 1989; Munson and Timmer, 1995; Paavolainen and

Smolander, 1998; Reynolds et al., 2000), and then decline as

stands age (Bormann and Likens, 1979; Piatek and Allen, 1999;

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http://dx.doi.org/10.1016/j.foreco.2006.07.004

C.J. Westbrook et al. / Forest Ecology and Management 234 (2006) 227–237228

Kranabetter and Coates, 2004). There is however a growing

body of literature showing limited effects of forest harvesting in

northern coniferous and deciduous forests (Pare and Van Cleve,

1993; Carmosini et al., 2003; Grenon et al., 2004; Lapointe

et al., 2005). The effects of forest harvesting on dissolved

organic N (DON) have been inconclusive. Studies in upland

stands have shown DON may be higher (Smolander et al.,

2001) or lower (Carignan et al., 2000; Smith et al., 2000;

Hannam and Prescott, 2003) after tree harvesting.

The effects of forest harvesting within or adjacent to

peatlands on peatland N dynamics have not been well studied.

Valley peatlands represent potentially critical interfaces

regulating N export from catchments due to their relative

abundance and position in the landscape (Devito and Dillon,

1993; Cirmo and McDonnell, 1997) and may be more sensitive

to disturbance than drier upland stands. Peatland nitrogen

dynamics are driven by their hydrological condition and thus

tend to exhibit high spatial variation in N cycling processes

(Devito and Dillon, 1993; Hill and Devito, 1997). Higher soil

moisture and degree of soil anoxia has been shown to increase

soil N availability post-harvest in a peatland (Walbridge and

Lockaby, 1994). Additionally, altered soil microclimate and

peat disturbance (rutting and compaction) may lead to higher N

cycling rates in peatlands following harvest (Grigal and Brooks,

1997; Groot, 1987).

This study was part of a larger study examining the impacts

of tree harvesting on terrestrial and aquatic ecosystems

(Steedman and Kushneriuk, 2000). Herein we report on

terrestrial N cycling processes at the catchment scale and the 1st

year effects of forest harvesting on these processes. Concurrent

measures of net N mineralization, net nitrification and DON

mobilization were conducted to: (1) compare soil N cycling

processes in coniferous upland, deciduous upland, and peatland

stands; (2) determine the 1st year effects of forest harvesting on

these three ecosystems; (3) assess whether changes in post-

harvest N cycling processes were due to changes in enviro-

nmental conditions or change in the nature of the soils.

2. General site description

This research was conducted between May 1996 and October

1998 in the Ontario Ministry of Natural Resources’ Coldwater

Lakes Experimental Watersheds (498050N and 928100W),

northwestern Ontario, Canada. The landscape is a mosaic of

coniferous (Pinus banksiana Lamb. and Picea mariana (Mill.)

BSP) and deciduous (Populus tremuloides Michx. and Betula

papyrifera) uplands, peatlands (Picea mariana (Mill.) BSP,

Ledum groenlandicum and Chamaedaphne calyculata), and

lakes. Mean annual precipitation (1971–2000) in Atikokan

(Environment Canada station 6020379) was 740 mm with 30%

falling as snow. April to October rainfall was 113%, 79%, and

98% of the long-term mean in 1996–1998. Snow water

equivalent of the late winter snowpack was 130 mm in both

1996 and 1997 and only 5 mm in 1998. The mean January and

July air temperatures are�17.6 and 19.2 8C, respectively. Mean

evapotranspiration (ET) for the study area was 350� 40 mm, as

calculated for 9 years using a simplified water budget approach

(ET = precipitation � streamflow, assuming no change in soil

moisture or groundwater storage on an annual basis). Mean

inorganic N deposition, measured on-site between 1998 and

2002 with a bulk deposition collector, was 2.3 kg N ha�1.

The bedrock geology is Archean granitic-gneissic (Zoltai,

1965). Glacial till is patchy, bouldery, and thin near the top of

hillslopes and up to 1 m thick at the bottom of hillslopes.

Mineral soils in the study area are of glacial-fluvial origin,

comprised of silty loam to coarse sand and are classified as

orthic dysteric brunisols. The forest floor 3–8 cm thick in uncut

upland stands and 0–8 cm thick in cut upland stands. Peatlands

have organic histols with a thickness of up to 1 m, and are

hydrologically connected to the hillslopes primarily via

subsurface flowpaths and occasionally by channelized surface

flow following rainfall events.

3. Study design

3.1. Catchment-scale experiment

The design was a three-catchment replication with a pre- and

post-treatment experimental approach examining the influence

of tree harvesting on net N mineralization. The three

catchments were within 5 km of one another and had similar

bedrock geology, soil depth, and pre-treatment forest cover.

The 9.8 ha reference (REF) and two experimental (9 ha EC1

and 19 ha EC2) headwater catchments were stratified by forest

cover (aspen upland, conifer upland and peatland) in May 1996

and 14, 100 m transects throughout the three forest cover types

were established in each catchment. Along each transect, five

locations (hereafter plots) spaced approximately 20 m apart

were sampled (methodology described later) during four

incubation periods in each of the pre-treatment (1996, 1997)

and post-treatment (1998) study years.

The treatment was clearcutting of the two experimental

catchments between June 1 and 20 in 1998. Approximately

90% of the trees in EC1 and 95% of the trees in EC2 were

removed with chainsaws. Trunks were dragged to the nearest

road with cable skidders before being delimbed. Peat surfaces

were highly disturbed as tree harvest occurred in June when the

soils were moist. Compression of the peat surface by skidder

tires resulted in many deep ruts that intersected the water table.

The ruts occurred in a random pattern, occupying about half of

the peatland surface. Harvesting caused much less physical

disturbance of the soils in the mineral upland stands, leaving the

soil horizons mostly intact.

3.2. Reciprocal transplant experiment

A field reciprocal transplant experiment of soils from mature

(uncut) and harvested (cut) conifer upland and peatland stands

was conducted during the time with the warmest soil conditions

to isolate the effects of altered environmental conditions versus

changes in the nature of the soil following harvesting on net N

transformations (c.f. Prescott et al., 2003). Aspen upland stands

were excluded from the experiment due to the lack of a suitable

nearby uncut stand. Ten plots were established within each of

C.J. Westbrook et al. / Forest Ecology and Management 234 (2006) 227–237 229

the four stands. Plots in the cut conifer stand and cut peatland

stand were randomly selected from the plots sampled in the

experimentally clearcut catchment, EC1. Plots within the uncut

conifer and uncut peatland stands were randomly located in the

mature forest adjacent to EC1. Intact soil cores were taken from

the cut stand and incubated in the uncut stand with similar forest

cover, and vice versa (methodology described later). Also, a

second set of cores were taken and incubated in their original

location to test for the Gadgil effect, which is the direct inhibition

of saprophytic organisms by mycorrhizal fungi (Bending, 2003).

Soil cores were incubated between 31st July and 22nd August in

1998. Temperature at 5 cm depth was continuously monitored at

one plot in each stand with a HoboTemp datalogger (Onset

Instruments, Pocasset, Massachusetts).

4. Soil chemical and physical sampling

Soil cores (�3500 in the catchment-scale experiment and 160

in the reciprocal transplant study) were taken with a 6.2 cm

diameter stainless steel corer. Forest floor and 0–10 cm mineral

(Ae, where present, and Bf horizons) soils in the upland stands,

and from 0 to 10 cm in the peatland hollows were sampled.

However, 1996 soil sampling occurred at two 10 cm depth

increments. Thus, the surface 10 cm increment included both the

forest floor and a small portion of the underlying mineral soil in

the upland stands during this year. Peatland hollows rather than

hummocks were chosen as they were more likely to be affected

by flowing water, permitting discussion of the potential for N

export from peatlands to adjacent water bodies; the topic of a

future paper. Cores for estimation of net rates and soil bulk

density were taken within 1 m of one another at each site.

Soil temperature was measured at the beginning of the soil

incubation period at each site that soil cores were collected with

a thermistor probe midway at each soil depth. Moisture was

determined by oven drying (105 8C for 24 h) a sub-sample of

soil from each sampling site and soil depth, with water content

and soil wetness (% saturation) as defined in Dingman (1994).

Specific density of mineral soil, forest floor, and well-humified

peat was assumed to be 2.65, 1.0 and 1.1 g m�3 (Brady and

Weil, 1999). Bulk density was calculated from the dry mass

over total core volume. The measured bulk densities do not

account for larger rock fragments and boulders common in the

glacial till for this area.

Soil pH was measured in a 5:1 water:soil slurry with a glass

electrode once pre- and once post-treatment in the catchment-

scale. Total C and N were determined for two replicates of a

composite sample from each transect in August 1997 (before

logging) and October 1998 (after logging). Samples were

analyzed for total C and N on a Carlo-Erba NA 1500 C/N

Analyzer at the Soil Biochemistry Laboratory, University of

Alberta, Edmonton, Alberta.

5. Soil N extractions and analyses

A time zero soil core was collected at each of the plots in

each experiment. Next to the time zero core, a second core was

collected in a 0.025 mm thick polyethylene bag and placed

back into the soil and incubated for 24 days (time 24-day core).

Time zero and time 24-day soil cores were kept on ice in the

field and returned to the field laboratory where they were kept

refrigerated until processing, within 24 h of sampling. Soil

cores taken in 1996 were shipped on ice to the University of

Toronto Biogeochemistry Lab for processing and were

analyzed within 72 h of sampling. Quality control analyses

found no difference (P = 0.664) in extractable N between

samples extracted after 24, 48, and 72 h after sampling

(Westbrook, 2000).

Inorganic N in an approximately 5 g (dry mass) sub-sample

of either the time zero or time 24 day soil core was extracted

with 50 ml 2 M KCl and shaken mechanically for 1 h to obtain

equilibrium. The KCl extracts were filtered through 1 mm

Fisher grade Q2 filters pre-rinsed with KCl, and subsequently

frozen. Extractable NH4+–N and NO3

�–N were determined

colourimetrically on a Technicon autoanalyzer. Estimates of net

nitrification rates were determined by the difference in NO3�–

N content between the time 24-day core and the time zero core

(Eno, 1960). Net ammonification rates were estimated by the

NH4+–N content of the time 24-day core minus that of the time

zero core.

Availability and mobility rates of dissolved organic N

(DON) production (from total dissolved N minus total

inorganic N) were estimated on two occasions, July and

September 1998, in the catchment-scale experiment. These

measurements were made at five randomly chosen plots in the

conifer stands and peatlands of the REF and EC2 catchments to

provide an indication of the importance of organic N to soil N

dynamics in this ecosystem. The KCl extracts (for mineraliza-

tion rates) were sub-sampled for total dissolved N (TDN)

during the July and September 1998 incubations. Total

dissolved N was determined through Zn reduction and

subsequent analysis of NH4+–N content on an autoanalyzer

as described above, at the Limnology Laboratory, University of

Alberta, Edmonton, Alberta. TDN mobilization was deter-

mined by the difference in TDN content between the time zero

and time 24-day cores.

6. Calculations

Cumulative net N mineralization and nitrification rates

(mg kg�1) for the plots in the three catchments were calculated

by summing monthly mean net N mineralization and

nitrification over the growing season. Standard errors were

computed through first order error propagation. Occasional loss

or destruction of incubating bags by animals occurred during

the three study years in the catchment-scale experiment.

Estimates of net N mineralization and nitrification for the

missing dates were conducted using linear interpolation for

sites that were missing only one incubation period of data in a

growing season, with data for the incubation periods

immediately before and after the missing period. Sites missing

data for more than one incubation period in 1 year were

excluded from the analyses. In total, 69 plots were used in the

analysis of the catchment-scale experiment. The mean bulk

density for each plot was calculated separately each year, and


was used to convert soil N concentrations from a gram to an

areal basis. There were no changes in the statistical

comparisons of mineralization and nitrification rates among

forest types and years when the data were calculated on an areal

basis, so only data on a per gram basis are reported.

7. Statistical analyses

All statistical tests were performed on the forest floor/

peatland and mineral soils separately as it was expected that

most parameters differed between the soils with very different

organic matter content. The design of the catchment-scale

experiment lent itself to direct testing of the apriori hypotheses

that net rates of mineralization and nitrification exhibited the

trend of 1998 > 1997 = 1996 for each forest stand type within

each catchment, and REF < EC1 = EC2 for each forest stand

type within each year of study. These apriori hypotheses were

tested using contrast matrices (Montgomery, 1984). Correla-

tions among soil microclimate, properties, and net rates were

assessed using a Pearson correlation matrix, correcting values

with the Bonferroni procedure. TDN mobilization differences

between the uncut and cut coniferous upland and peatland

stands were assessed using two-sample t-tests, adjusting the

probabilities for multiple tests using the Bonferroni test. Data

from the two sampling dates were combined as no difference in

TDN mobilization was found between them for forest floor/

peatland soils (P = 0.516) or mineral soils (P = 0.318).

Differences in net N mineralization and nitrification rates

between unmoved and transplanted soil cores from the mature

(uncut) and harvested (cut) stands in the reciprocal transplant

experiment were tested with one-tailed t-tests as it was

predicted that higher net rates would be measured for soils

incubated in the warmer soil conditions. All tests were

Table 1

Mean soil C/N ratios, pH and bulk density for each forest type within each catchme

catchments) with one standard error in parentheses

Forest cover Catchment C/N ratio

1997 1998

Forest floor

Conifer REF 31.3 � 0.2 31.3 � 0.1

EC1 37.5 � 1.6 37.9 � 0.5

EC2 37.5 � 0.5 40.1 � 1.6

Deciduous REF 28.3 � 0.2 28.0 � 0.0

EC1 26.7 � 0.4 36.0 � 3.5

EC2 29.8 � 0.4 30.6 � 0.1

Peatland REF 27.7 � 1.9 25.9 � 0.0

EC1 33.5 � 1.2 42.2 � 2.9

EC2 52.5 � 0.7 48.7 � 0.0

0–10 cm mineral

Conifer REF 25.1 � 0.2 22.6 � 0.2

EC1 30.9 � 1.7 26.2 � 0.7

EC2 30.3 � 0.9 26.9 � 0.2

Deciduous REF 25.0 � 0.2 25.5 � 0.5

EC1 25.6 � 2.4 21.5 � 0.1

EC2 26.3 � 0.0 20.9 � 0.4

Sample sizes are n = 2–4 for C/N ratios and n = 4–10 for pH and bulk density.

performed using SYSTAT Version 7. Significance for all

analyses was accepted at a = 0.05.

8. Results

8.1. Catchment-scale experiment

Soil C/N ratios were generally above the critical threshold of

22–25 for immobilization versus mineralization of N and values

were more variable among catchments than among years

(Table 1). The deciduous upland stands tended to have lower C/

N ratios than the upland conifer stands and peatland and there

were no differences pre- and post-harvesting. Soil pH was at the

lower limit for heterotrophic microbial activity (3.8–4.5) and

was similar before and after forest harvest (Table 1). Soil bulk

density was similar in conifer and deciduous upland stands and

higher than in peatlands (Table 1). Only the deciduous stands

showed an increase in bulk density post-harvest.

Positive net N mineralization rates were generally measured

in the forest floor and peatland soils while net immobilization

was found in the top 10 cm of mineral soils (Fig. 1). Net N

mineralization rates in the REF catchment were similar to those

measured in two experimental catchments pre-treatment except

in the REF peatland stand, which had significantly higher net

mineralization rates than in EC1 and EC2 (P = 0.004).

Generally, net mineralization rates were similar among years

(pre- and post-treatment) within a forest cover type. There were

three exceptions: (1) forest floor soils in the EC1 conifer stand

followed the trend of 1998 > 1997 = 1996 (P = 0.009); (2)

mineral soils in the EC2 upland conifer stand followed the trend

of 1997 > 1996 = 1998 (P = 0.012); (3) mineral soils in the

REF upland conifer stand followed the trend of 1998 >1997 = 1996 (P = 0.022).

nt for 1997 (before clearcutting) and 1998 (after clearcutting the two treatment

pH Bulk density (mg m�3)

1997 1998 1997 1998

4.02 � 0.18 3.94 � 0.09 0.13 � 0.02 0.17

3.83 � 0.07 3.94 � 0.08 0.20 � 0.03 0.17

3.84 � 0.09 3.99 � 0.06 0.15 � 0.01 0.22

4.02 � 0.07 4.04 � 0.04 0.16 � 0.03 0.19

4.42 � 0.17 4.51 � 0.08 0.18 � 0.02 0.28

4.27 � 0.05 4.29 � 0.06 0.18 � 0.02 0.27

4.10 � 0.06 4.21 � 0.02 0.04 � 0.01 0.04

4.16 � 0.08 4.13 � 0.04 0.07 � 0.01 0.14

3.89 � 0.09 3.83 � 0.05 0.06 � 0.01 0.07

4.22 � 0.16 4.37 � 0.11 0.53 � 0.06 0.46

4.05 � 0.08 4.04 � 0.09 0.47 � 0.02 0.58

3.97 � 0.12 3.97 � 0.13 0.51 � 0.02 0.53

4.05 � 0.05 4.10 � 0.08 0.35 � 0.05 0.50

4.48 � 0.10 4.35 � 0.08 0.48 � 0.03 0.55

4.41 � 0.14 4.16 � 0.12 0.54 � 0.04 0.54


Fig. 1. Growing season total (�S.E.) net N mineralization for the forest floor/peat (a) and mineral (b) soils within each forest type within the reference (REF) and

experimental catchments (EC1 and EC2) in 1996, 1997 (pre-harvest) and 1998 (post-harvest).

Net nitrification rates were <10% of net N mineralization

rates and usually showed net immobilization (Fig. 2). Years

with sporadically high and positive net nitrification rates were

measured for upland deciduous stands and peatland; these high

values were attributed to 1 month (August) of high rates.

Generally, few significant differences in net nitrification rates

were detected among catchments or among years (pre- and

post-treatment) for either soil horizon. Significantly higher net

nitrification rates were however measured in the peatland soils

in the REF catchment compared to those in EC1 and EC2 pre-

treatment (P = 0.045). Significantly higher net nitrification

Fig. 2. Growing season total (�S.E.) net nitrification for the forest floor/peat (a)

experimental catchments (EC1 and EC2) in 1996, 1997 (pre-harvest) and 1998 (p

rates were measured in the upland deciduous stand of the REF

catchment post-treatment (1998) than either year pre-treatment

(forest floor: P < 0.001, mineral soil: P = 0.001).

Mean TDN availability and mobilization rates were similar

in June and September 1998 (P > 0.10), thus the two dates were

averaged to provide an estimate of DON mobilization rates

during the 1st year after logging (Fig. 3). Total dissolved N was

10–25 times higher than inorganic N in soils from both uncut

and cut stands. Total dissolved N was highest in the peatlands,

intermediate in the organic soils of the coniferous upland

and lowest in the mineral soil of the coniferous upland stand.

and mineral (b) soils within each forest types within the reference (REF) and

ost-harvest).


Fig. 3. Mean (�S.E.) soil N (top) and mobilization (bottom) of total inorganic

N (TIN) and total dissolved N (TDN) for reference (REF) and cut peatland, and

conifer upland forest floor/peat and mineral soils. Data are the average of two

dates (1st July and 30th August 1998) that were not significantly different. The

difference between TDN and TIN is dissolved organic N (DON).

No difference in TDN mobilization was detected between soils

from uncut and cut peatland stands or coniferous upland stands.

Neither net mineralization nor nitrification rates were

correlated with any soil microclimate variable. Net miner-

alization rates were negatively correlated with NH4+ in the

organic (r = �0.439, P < 0.001) and mineral (r = �0.525,

P < 0.001) soils. In the organic soils, soil NH4+ was positively

correlated with percent total N (r = 0.458, P = 0.001), percent

total C (r = 0.675, P < 0.001), and C/N ratio (r = 0.674,

P < 0.001). Similarly, soil NO3� was positively related to

percent total N (r = 0.452, P = 0.001) and percent total C

(r = 0.408, P = 0.009) in the organic soils. As it was suspected

that water table depth would be a primary control on net rates, a

linear regression of net mineralization/nitrification and water

table depth was performed with data from the peatland stands.

Growing season mean water table elevations in the peatlands

(data not shown) were positively related to mineralization rates

(P = 0.021) but not to nitrification rates (P = 0.589).

8.2. Reciprocal transplant experiment

Forest floor and mineral soils in the cut conifer upland stand

were 2–3 8C on average warmer than soils in the uncut conifer

upland stand (P < 0.001) (Table 2). Moisture was significantly

higher in soils from the cut than uncut conifer upland stand in

both the forest floor (P < 0.001) and mineral soil (P < 0.001)

layers. Despite a warmer soil environment in the cut conifer

upland stand, incubating soil from the uncut stand in the cut

stand did not increase net N mineralization and nitrification

rates. Similarly, incubating soil from the cut stand in the uncut

stand did not lower net rates.

Soils in the cut peatland stand had significantly higher mean

monthly temperature (2.6 8C) and 57% higher moisture (%

saturation) than the uncut peatland stand (Table 2). Both net N

mineralization (P = 0.036) and nitrification (P = 0.005) rates

were higher when peat from the uncut stand was incubated in

the cut stand. However, in the reverse situation where soils from

the cut peatland stand were incubated in the uncut stand,

significantly lower net N mineralization (P = 0.861) or

nitrification rates (P = 0.756) were not measured.

9. Discussion

Large contrasts in net N mineralization among undisturbed

conifer, deciduous and peatland stands have been widely

reported in other forests (e.g. Hill and Shackleton, 1989; Zak

and Grigal, 1991; Stottlemyer et al., 1995; Devito et al., 1999).

However, no clear spatial patterns in growing season total N

mineralization or nitrification were observed in our study

catchments despite differences in soil organic matter content,

soil moisture, temperature and topographic position. This lack

of spatial pattern in N mineralization among various landscape

units has not been widely documented. Min et al. (1999)

proposed that N-source availability may be an important

determinate of spatial patterns in tree species in both temperate

and boreal systems as various tree species prefer different forms

of inorganic N. They also found less contrast among tree

species in the utilization of NH4+ compared to NO3

�

utilization. Fewer spatial contrasts in inorganic N cycling

rates among forest types that exist at our study site may partially

result from a NH4+-dominated inorganic N pool. However, we

suspect a more likely explanation is the similar C/N ratios

among the landscape units. Negligible to low net nitrification

rates and low nitrate losses to streams have been found for

forests that have a C/N ratio above 22–25 (Hedin et al., 1995;

Lovett et al., 2002; Ross et al., 2004). The high C/N ratios (25–

53) found in soils from our stands suggests a limited potential

for net nitrate production and loss.

Although TDN was 10–25 times higher than total inorganic

N (TIN), the rates of TDN mobilization were similar to TIN

mobilization rates, indicating DON mobilization rates were

small in these soils and similar among landscape units. DON

mobilization rates measured in the conifer stands were within

the range of those for stands within the same ecoregion

(Lamontagne, 1998), and suggest a low potential for leaching

loss from these sites under natural conditions.

Net N mineralization and nitrification rates were low when

compared to other ecosystems, although absolute comparisons

are difficult to make as methodology varies greatly among

studies. Growing season total net N mineralization (2–9 kg ha�1)

and net nitrification (�0.02 to 0.2 kg ha�1) rates for soils from

C.J.

Westb

roo

ket

al./F

orest

Eco

log

ya

nd

Ma

na

gem

ent

23

4(2

00

6)

22

7–

23

72

33

Table 2

Net mineralization, nitrification, moisture and temperature for soils from peatland and upland conifer uncut and cut stands incubated in their own location and transplanted to the other area, August 1998

Original location Uncut Cut P-values for tests of null hypotheses

Uncut (U–U) Cut (U–C) Uncut (C–U) Cut (C–C) U–U = U–C C–U = C–C U–U = C–U C–C = U–C U–U = C–C

N mineralization (mg kg�1 soil�1 d�1)

Peatland

Incubated in 0–10 cm �0.06 (0.05) 2.94 (1.41) 1.16 (0.66) 1.02 (0.43) 0.06 0.861 0.392 0.927 0.029

Conifer

Incubated in forest floor �0.02 (0.06) 0.74 (0.55) 1.45 (0.85) 1.82 (1.00) 0.217 0.792 0.481 1.000 0.100

Incubated in 0–10 cm mineral �0.01 (0.04) �0.06 (0.02) 0.06 (0.09) < 0.01 (0.06) 0.401 0.582 1.000 1.000 0.911

Nitrification (mg kg�1 soil�1 d�1)

Peatland

Incubated in 0–10 cm 0.01 (<0.00) 0.09 (0.02) <0.01 (<0.00) <0.01 (<0.00) 0.003 0.756 0.059 0.002 0.003

Conifer

Incubated in forest floor 0.04 (0.02) 0.01 (0.01) 0.03 (<0.00) 0.03 (0.01) 0.141 0.762 1.000 0.215 0.594

Incubated in 0–10 cm mineral 0.02 (0.01) 0.01 (<0.00) 0.02 (<0.00) 0.02 (<0.01) 0.154 0.565 1.000 0.332 0.707

Soil temperature (8C)

Peatland

Incubated in 0–10 cm 14.0 (0.2) NA NA 16.6 (0.2) NA NA NA NA <0.001

Conifer

Incubated in forest floor 15.4 (0.3) NA NA 17.6 (0.2) NA NA NA NA <0.001

Incubated in 0–10 cm mineral 12.8 (0.1) NA NA 15.6 (0.1) NA NA NA NA <0.001

Soil wetness (%)a

Peatland

Incubated in 0–10 cm 9.7 (1.7) NA NA 66.6 (5.0) NA NA NA NA <0.001

Conifer

Incubated in forest floor 5.8 (0.7) NA NA 17.7 (2.0) NA NA NA NA <0.001

Incubated in 0–10 cm mineral 9.0 (0.9) NA NA 22.0 (1.7) NA NA NA NA <0.001

Presented are means (n = 10) with one standard error in parentheses. Note: NA, not applicable.a Equal to % saturation, as given in Dingman (1994).


uncut coniferous stands are within the range of those measured

for similar coniferous stands in the experimental lakes area of

northwestern Ontario (Lamontagne, 1998). They also fall within

the range of growing season total net N mineralization (�4 to

8 kg ha�1) and net nitrification (�1–2 kg ha�1) rates measured

in the boreal-temperate transitional upland forests at Isle Royale

(Stottlemyer et al., 1995), and that in Alpine tundra meadows in

Colorado (0.04–0.1 kg ha�1 yr�1: Fisk and Schmidt, 1995). Net

rates in our study are 1–2 orders of magnitude less than in

deciduous and conifer-mixed uplands in the temperate Shield

forest in central Ontario (Devito et al., 1999; Foster, 1989) and in

the Adirondack Mountains of New York (Ohrui et al., 1999).

Peatland net N mineralization (0–4 kg ha�1) was lower than that

measured in deciduous swamps in Minnesota (15–

16 kg ha�1 yr�1) and in hemlock-fir dominated peatlands (15–

19 kg ha�1 yr�1) in central Ontario, although nitrification rates

were comparable to Zak and Grigal (1991) and Devito et al.

(1999).

The greatest difference in rates of net N mineralization and

nitrification for soils between uncut and cut stands should occur

in August when soil temperatures reach their maximum as

temperature has been identified as one of the most important

growth limiting factors in boreal forests (Post et al., 1992;

Binkley and Hogberg, 1997). The results of the reciprocal

transplant suggest that the microclimate conditions in the cut

peatland stand were more conducive to net N mineralization

and nitrification than in the uncut peatland stand. The enhanced

net rates measured in the peat soils that originated in the uncut

plot and incubated in the cut plot may result from high

decomposition of Sphagnum spp. in the warmer soil environ-

ment (Trettin et al., 1997). In contrast, peat soils taken from the

cut stand did not show lower net N cycling rates when they were

incubated in the cooler uncut stand. We think this result reflects

the considerably higher moisture content of the peat in the cut

stand (67% of saturation) than in the uncut stand (10% of

saturation), and its control on redox status (Devito and Dillon,

1993), rather than a change in the nature of the peat material

following forest harvesting. Support for this inference is the

significantly lower aerobic process of net nitrification

(P = 0.003) measured in the cut stand than the uncut stand.

Our reciprocal transplant study indicated that higher

temperature in the upland coniferous cut stand did not enhance

net N mineralization nor nitrification rates, and that net rates

were similar in soils from the uncut and cut stands. In contrast,

higher soil temperatures in Alaska, achieved through a soil

warming experiment, have been shown to enhance net N

mineralization but not nitrification rates (Van Cleve et al.,

1990). That soil net rates were similar in the uncut and cut

upland coniferous stands, and that incubation of soil in different

microclimate environments did not affect net rates suggests

something other than a change in the nature of the soil from the

upland coniferous stands was responsible for regulating

inorganic N production. The most likely explanations are that:

(1) there was allelochemical inhibition of N cycling processes.

The high phenolic concentrations typically measured in upland

coniferous stands have been shown to inhibit N cycling

(Persson and Wiren, 1995; Janssen, 1996; Paavolainen and

Smolander, 1998) or (2) the organic matter quality was poor

and thus limited heterotrophic microbial activity. Our findings

are consistent with the in situ experimental results of Carmosini

et al. (2003). In contrast, Prescott et al. (2003) reported that a

change in the nature of the soil material was responsible for

higher net nitrification in undisturbed soils from high-elevation

sites in British Columbia incubated in gaps.

So why did the altered microclimate conditions after

harvesting not result in an increase in net N mineralization and

nitrification rates in the catchment-scale experiment, even in

the peatland stands? A 2–3 8C temperature increase should

translate into approximately a 30–60% increase in net N rates

(Stark, 1996; Burns and Murdoch, 2005) but an increase of

100–1000% would have been necessary to detect a significant

post-harvesting increase for the stands given the natural spatial

and temporal variability in net rates. The result is that there was

no detectable increase in net N mineralization and nitrification

for the year following forest harvest. Our findings are consistent

with those of other researchers working in northern upland

forests (Gordon and Van Cleve, 1983; Fisk and Fahey, 1990;

Carmosini et al., 2003; Grenon et al., 2004).

Several studies in coniferous forests have followed net N

mineralization and nitrification rates for the decade following

harvest and have shown a peak at 3–7 years (e.g. Frazer et al.,

1990; Prescott et al., 2003) supporting Vitousek et al. (1982)

suggestion of a lag in the nitrification response following

disturbance. Aspen forests may not however experience a

delayed peak in net N mineralization or nitrification following

clearcutting (White et al., 2004; Lapointe et al., 2005). Long-

term effects of clearcutting on soil N cycling in peatlands are

not available. Net nitrification rates in our forested ecosystems

may increase within a similar timeframe as harvesting did

immediately increase NH4+ availability in most stands

(Westbrook, 2000) and it may take more than 1 year post-

harvest for the nitrifier population to significantly respond to

greater substrate availability (Hendrickson et al., 1985).

However, we think it is unlikely that a similar peak will be

observed at our study site. Pools with a high turnover rate, such

as those in N-limited forests, tend to be able to recover quickly

from disturbance (Aber and Melillo, 1982). Measurements of

NH4+ and NO3

� turnover rates at our study site show extremely

fast (in the range of hours) recycling of inorganic N in both

mature uncut and harvested stands (Westbrook and Devito,

2004). Further, it was found that there was a greater capacity to

consume nitrate than was produced as NO3� assimilation was

limited by NO3� availability (Westbrook and Devito, 2004).

Support for expected effective longer-term N retention is the

stable concentration of inorganic and organic N in surface

runoff from the two treatment catchments during the first 8

years following harvest (Allan, unpublished data).

The finding of similar DON in the uncut and cut coniferous

upland and peatland stands constrasts with the findings of

Nieminen (1998) and Smolander et al. (2001) who measured

higher soil DON in recently clearcut than mature stands. This

was likely because little slash, which is rich in DON (Qualls

et al., 2000), remained in the conifer and peatland stands in our

study after harvesting. Tree trunks were delimbed and slash


piles were burned at loading docks located outside the

catchments. Soil DON and DON mobilization were also not

lower in clearcuts than mature stands, as was found by Hannam

and Prescott (2003). We attribute this to the maintenance of

relatively intact LFH and mineral soil layers following

clearcutting. While significant soil disturbance and redistribu-

tion did occur in localized areas within the two treatment

catchments, especially where soils were thin, these areas did

not occur within our study plots. The lack of a difference in the

soil DON pool and mobilization rates between uncut and cut

conifer and peatland stands suggests that plants are likely not

directly relying on DON as a source of N nutrition or that

increases or decreases resulting from clearcutting are within the

natural variability.

10. Conclusions

This study shows remarkably consistent net N cycling rates

among peatland and upland coniferous and deciduous ecosys-

tems in these headwater catchments despite differences in soil

organic matter content, soil moisture, temperature, and topo-

graphic position. Of the total N production in the soil, it appears

that the dissolved organic fraction may not be as important as

suggested in other studies. DON mobilization rates were similar

to total inorganic N mobilization despite that the DON fraction

dominated the soil N pool. Clearcutting minimally influenced

soil net organic and inorganic N cycling as rates measured the 1st

year following logging of the two experiment catchments were

within the natural range of variation even though soil

temperatures were 2–3 8C warmer. We expect that the

characteristic 3–7 yr delayed peak in nitrification measured in

other northern upland forests is unlikely to occur at our site due

to: (1) relatively high soil C/N ratios that favour microbial

immobilization; (2) the removal of C-rich slash from the

catchments. However, further long-term study of N cycling and

catchment hydrologic outputs will be required to test this

hypothesis. In particular, attention should be paid to the recovery

of the clearcut peatlands as higher net N mineralization and

nitrification rates were measured in peat soils taken for the uncut

stand and incubated in the warmer cut stand. Even small

increases in peatland net N cycling rates may lead to an enhanced

N loss from the catchments as peatlands have a direct hydrologic

connection to streams in these catchments.

Acknowledgements

We are grateful to J. Latour, G. Taylor, D. Hiscox, L. Dolce,

J. Truhn and S. Young for their help during field samplings. G.

Taylor provided laboratory assistance and P. Furlong and M.

Friday provided logistical support. R. Kushneriuk provided

meteorological data for the study site. We thank D. Binkley and

W.B. McGill for helpful comments on an earlier version of this

manuscript. We thank two anonymous reviewers who

significantly improved this manuscript. Total C and N analyses

were conducted at the Soil Biochemistry Lab, Department of

Renewable Resources, University of Alberta and TDN analysis

at the Limnology Laboratory, Department of Biological

Sciences, University of Alberta. This project was funded by

a Natural Sciences and Engineering Research Council of

Canada award and a New Faculty Start-up grant to K. Devito, a

Circumpolar Boreal Alberta Research Grant from the Canadian

Circumpolar Institute of the University of Alberta to C.

Westbrook, a Student Temporary Employment Program grant

for field assistance, a University of North Carolina at Charlotte

Faculty Research Grant to C. Allan, and in-kind funding

provided by Dr. R. Steedman and the Centre for Northern Forest

Ecosystem Research, Ontario Ministry of Natural Resources.

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Soil N cycling in harvested and pristine boreal forests and peatlands

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