Nitrogen availability in soil and forest floor of contrasting types of boreal mixedwood forests
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112
Nitrogen availability in soil and forest floor of
contrasting types of boreal mixedwood forests
Lucie Jerabkova, Cindy E. Prescott, and Barbara E. Kishchuk
Abstract: Boreal mixedwood forests with varying proportions of coniferous and deciduous species are found through-
out the North American continent. Maintenance of a deciduous component within boreal forests is currently favoured,
as deciduous species are believed to promote faster nutrient turnover and higher nutrient availability. Results of com-
parisons of deciduous and coniferous forests are, however, inconsistent in supporting this generalization. We compared
indices of soil nitrogen (N) availability in the forest floor and mineral soil of deciduous, mixed, and coniferous stands
of boreal mixedwood forest in northwestern Alberta. Deciduous stands had higher N availability, reflected by higher
pools of NH4-N and inorganic N in the forest floor. Forest floors of deciduous stands also tended to have higher con-
centrations of microbial N but did not have higher levels of NO3-N or higher rates of net nitrification. Mixed stands
showed the highest rates of net N mineralization. Soil N availability was more closely related to litter N content than
to litter decomposition rate. The variation among the forest types is likely attributable to vegetation, as topography is
fairly uniform, stands do not differ in soil texture, and N-availability indices correlated directly with the proportion of
deciduous trees.
Résumé : Les forêts boréales mélangées, qui contiennent des proportions variables de conifères et de feuillus, sont pré-
sentes partout sur le continent nord-américain. On encourage actuellement le maintien d’une composante feuillue dans
les forêts boréales croyant que les espèces feuillues favorisent un recyclage plus rapide et une plus grande disponibilité
des nutriments. Les résultats de comparaisons entre des forêts résineuses et feuillues sont cependant trop inconsistants
pour supporter cette généralisation. Les auteurs ont comparé les indices de disponibilité de l’azote (N) du sol dans la
couverture morte et le sol minéral de peuplements feuillus, mélangés et résineux de la forêt boréale mélangée du nord-
ouest de l’Alberta. Les peuplements feuillus avaient une plus grande disponibilité de N reflétée par des pools de N-
NH4 et de N inorganique plus importants dans la couverture morte. La couverture morte des peuplements feuillus avait
aussi tendance à avoir de plus fortes concentrations de N microbien mais n’avait pas de niveaux plus élevés de N-NO3
ou de plus hauts taux de nitrification nette. Les peuplements mélangés avaient les taux de minéralisation nette de N les
plus élevés. La disponibilité de N du sol était plus étroitement reliée au contenu en N de la litière qu’à son taux de dé-
composition. Les différences entre les types de forêts sont probablement attribuables à la végétation étant donné que la
topographie est assez uniforme, que les peuplements ont des sols dont la texture est semblable et que les indices de
disponibilité de N sont directement corrélés avec la proportion de tiges feuillues.
[Traduit par la Rédaction] Jerabkova et al. 122
Introduction but dominant species also vary along an east–west gradient.
Black spruce (Picea mariana (Mill.) BSP), white spruce (Picea
Boreal forest represents about 30% of the total forested glauca (Moench) Voss), trembling aspen (Populus tremuloides
area on the Earth (Burton et al. 2003). It has large supplies Michx.), balsam poplar (Populus balsamifera (L.) Mill.), and
of commercially utilizable timber, contains significant pools paper birch (Betula papyrifera Marsh.) have a transcontinen-
of carbon (C) (Apps et al. 1993), and provides important tal distribution, subalpine fir (Abies lasiocarpa Nutt.) and
habitats, particularly for large mammals (Thompson 1991). lodgepole pine (Pinus contorta Dougl. ex. Loud) are limited
The North American boreal forest covers 5.12 × 106 km2
to the western region, while balsam fir (Abies balsamea (L.)
(Kuusela 1992) and is a matrix of closed- and open-canopy
Mill.) and jack pine (Pinus banksiana Lamb.) are more com-
forests and bogs. Species composition changes with latitude
mon in the east (Zasada et al. 1997).
in favour of coniferous species at northern sites (Perry 1994),
Mixedwood forests, which have varying proportions of
coniferous and deciduous (broadleaf) species, depending on
Received 18 May 2005. Accepted 21 September 2005. site conditions and disturbance history, are found throughout
Published on the NRC Research Press Web site at the North American boreal forest. In the western boreal forest,
http://cjfr.nrc.ca on 20 January 2006. they usually occupy relatively nutrient-rich, fresh to moist
L. Jerabkova1 and C.E. Prescott. Department of Forest sites (Kabzems et al. 1986). Maintenance of a deciduous
Sciences, The University of British Columbia, 3041–2424 component within boreal forests is currently favoured, partly
Main Mall, Vancouver, BC V6T 1Z4, Canada. in response to perceived benefits of deciduous species for
B.E. Kishchuk. Canadian Forest Service, Northern Forestry cycling and availability of nutrients.
Centre, 5320-122 Street, Edmonton, AB T6H 3S5, Canada.
In general, deciduous forests are believed to have faster
1
Corresponding author (e-mail: luciej@interchange.ubc.ca). nutrient turnover than coniferous forests (Pastor 1987), and
Can. J. For. Res. 36: 112–122 (2006) doi:10.1139/X05-220 © 2006 NRC CanadaJerabkova et al. 113
retention in or introduction of deciduous species to conifer annual litter N input and litter decomposition rates in plots
stands has been recommended to improve nutrient availability of each forest type and assessing their degree of correlation
(Comeau 1996; Zasada et al. 1997). It has also been sug- with N-availability indicators.
gested that successional change from deciduous to conifer-
ous stands causes a decline in nutrient availability (Bormann Methods
and Sidle 1990), and that exclusion of the deciduous phase
in stand development could eventually result in site impov- Site description
erishment (Bergeron and Harvey 1997). Coniferous stands This study was a part of the Ecosystem Management Em-
generally accumulate larger quantities of organic matter and ulating Natural Disturbance (EMEND) project, a large-scale
have longer mean nutrient resident times than deciduous silvicultural project in northern Alberta (Spence et al. 1999).
stands (Vogt et al. 1986), consistent with slower nutrient The EMEND research study site is located approximately
turnover. 90 km northwest of Peace River in the Lower Foothills sub-
Nitrogen (N) dynamics are of particular interest, as the region of the Clear Hills Upland ecoregion (Beckingham et
productivity of boreal forests is largely limited by N avail- al. 1996). Elevation ranges from 677 to 880 m a.s.l. Mean
ability (Mahendrappa and Salonius 1982). Comparisons of N annual temperature at the nearest climate monitoring station
availability in boreal coniferous and deciduous stands have is –0.3 °C, with mean January and July temperatures of –18.8
not consistently demonstrated the expected greater N avail- and 14.6 °C, respectively. Mean annual precipitation is 433 mm
ability in deciduous stands. Boreal aspen stands in southern (Environment Canada 2004). Soils are primarily Gray Luvisols
Quebec (Bauhus et al. 1998) and birch stands in Finland with minor occurrences of Brunisols, Gleysols, and Solonetzic
(Priha and Smolander 1999) had higher microbial N concen- soils derived from similar glaciolacustrine and glacial-till
trations (an indicator of N availability; Myrold et al. 1989) parent materials containing few coarse fragments (Kishchuk
than spruce stands. Trembling aspen stands in Quebec had 2004). Soil texture as determined from mean particle size
faster turnover and higher accumulation of NO3-N than white content in deciduous, mixed, and coniferous stands is clay
spruce stands (Ste-Marie and Paré 1999). In contrast, higher loam (B.E. Kishchuk, unpublished data). Soils are generally
NO3-N concentrations and higher net nitrification were found well drained and exhibit little variation in pedogenetic proper-
under pine than under birch in Finnish boreal forests (Priha ties across the study area (Kishchuk 2004). Tree ages in the
and Smolander 1999). While 49-year-old aspen stands in stands ranged from 70 to 125 years (Spence and Volney 1999).
Quebec had faster potential net N mineralization and nitrifi- Dominant tree species are trembling aspen, balsam poplar,
cation than white spruce stands, no differences where found and white spruce. Black spruce, balsam fir, and lodgepole pine
between older stands of aspen and spruce (Paré and Bergeron also occur at some sites. Green alder (Alnus crispa (Ait.)
1996). There was an indication of faster N turnover under birch Pursh), high-bush cranberry (Viburnum edule (Michx.) Raf.),
than under balsam fir in the Canadian eastern boreal forest, and prickly rose (Rosa acicularis Lindl.) are the predominant
but it was inconsistent — in the first year N mineralization bushes. Tall bluebell (Mertensia paniculata (Ait.) G. Don.),
rates were higher under birch but nitrification levels were bunchberry (Cornus canadensis L.), feather moss (Pleurozium
similar; in the second year there was no difference in N min- schreberi (Brid.) Mitt.), and step moss (Hylocomium splendens
eralization but nitrification was higher in birch plots (Evans (Hedw.) BSG) are common ground-layer species.
et al. 1998).
In this study we therefore addressed the question of whether Sampling and soil analysis
deciduous stands have higher N availability and faster N Three replicates of mature deciduous-dominated, coniferous-
turnover than coniferous stands. We compared N availability dominated, and mixed stands were located within a 1000 ha
in natural mature stands with varying proportions of decidu- area. Each stand occupied approximately 10 ha. “Deciduous-
ous (mainly trembling aspen) and coniferous (mainly white dominated” stands had more than 75% deciduous trees, “co-
spruce) species in the western boreal mixedwood region. We niferous-dominated” stands had more than 75% coniferous
measured several indicators of N availability and turnover in trees, and “mixed” stands ranged between these two values.
forest floor and mineral soil of each forest type. We hypoth- In each of the stands, six samples of the FH layer of the for-
esized that a greater proportion of deciduous trees would be est floor and the upper 7 cm of mineral soil were collected
accompanied by larger pools and proportions of NO3-N, in- from randomly selected plots in June and August 2002. Sam-
organic N (NH4-N + NO3-N), and microbial N and by faster ples from each plot were analyzed separately.
N mineralization and nitrification rates. The following N-availability indicators were measured or
The positive effect of deciduous trees on nutrient cycling calculated: NH4-N, NO3-N, and soluble inorganic N (SIN)
is usually attributed to their high-quality litter causing faster concentrations and net rates of N mineralization and nitrifi-
litter decomposition and faster nutrient cycling (Pastor 1987; cation (Binkley and Hart 1989). Net rates of ammonification,
Scott and Binkley 1997). However, the decomposition rate, nitrification, and mineralization were assessed through a 6-week
especially the initial rate, has been shown to be a poor pre- in situ buried-bag incubation (Hart et al. 1994) between June
dictor of N availability (Giardina et al. 2001). Prescott (2002) and August 2002. The net ammonification rate was calcu-
suggested that amount of N returned annually in litter may lated as NH4-N accumulated after 6 weeks minus NH4-N at
be more influential than the rate of litter decay in determin- the beginning of the field incubation, the net nitrification
ing N availability at a site. Thus, we also hypothesized that rate as NO3-N at the end minus NO3-N at the beginning, and
N availability and turnover in boreal mixedwood forests were the net mineralization rate as inorganic N (NH4-N + NO3-N)
more closely related to the amount of N added in litter than at the end minus inorganic N at the beginning. We also deter-
to the litter decomposition rate. We tested this by measuring mined microbial N (Binkley and Hart 1989), as it is another
© 2006 NRC Canada114 Can. J. For. Res. Vol. 36, 2006
pool of N potentially available to plants (Hodge et al. 2000) and then settled at 4 °C for 30 min. They were then gravity-
through microbial-biomass turnover. filtered through preleached Fisher Q2 filters. The P concen-
The percentage of SIN that is NO3-N was used to describe tration in the filtrate was determined by the molybdate-blue
the relative importance of nitrate as a simpler way to express method on a Technicon Autoanalyzer (Olsen and Sommers
the NH4-N/NO3-N ratio used in other studies (Walley et al. 1982). The pH was measured with an Accumet AP62 porta-
1996). Similarly, the percentage of soluble N that is SIN was ble pH/mV meter (Fisher Scientific) in fresh samples sus-
used to describe the relative importance of inorganic N, pended in 0.01 mol/L CaCl2 in ratios (by fresh mass) of 1:2
analogous to the ratio of soluble organic N (SON) to SIN for mineral soil and 1:4 for forest floor (Kalra and Maynard
(Northup et al. 1995). This index has not been traditionally 1991). Preliminary trials showed independence of pH values
used but it shows the relative contribution of readily avail- in dry soil suspension ratios up to 1:50. Bulk density of min-
able N to the soluble-N pool and we believe that these rela- eral soil was determined by core sampling in 2000 and bulk
tive indices can be useful in comparing sites across broad density of forest floor was determined by quadrat sampling
gradients. Soluble N is defined as SIN plus SON; labile N is in 2002.
defined as soluble N plus microbial N.
Samples were kept on ice during transportation to the lab- Description of vegetation
oratory and kept at 4 °C until processing. A portion of each To characterize the vegetation structure in the vicinity of
sample was oven-dried at 105 °C for 24 h to measure gravi- the sampling plots, trees were measured and understory veg-
metric moisture content and another part of the sample was etation was described within a circle of 10 m radius around
air-dried prior to total N and C analysis. Soluble N, soluble the sampling plot center, in four out of six sampling plots in
organic carbon (SOC), and microbial biomass C and N were each stand. The diameter of trees larger than 10 cm in diameter
analyzed on field-moist samples. Samples were sieved im- was measured at 1.30 m height. The understory is described
mediately before extraction through a 4 mm mesh sieve to in terms of coverage (75%) of vegetation groups (alder, other
2001). The high clay content made it impossible to pass bushes, low shrubs, grass, herbs, mosses).
moist samples through a 2 mm mesh sieve.
For SIN (NH4-N and NO3-N) analysis, approximately 10 g Litter input and decomposition
fresh mass of soil and 6 g fresh mass of forest floor was Annual overstory litter was collected in 50 cm × 50 cm
extracted with 50 mL of 0.5 mol/L K2SO4. Samples were litter traps for 1 year at four of the six sampling plots in each
shaken for 1 h on a reciprocal shaker and then settled at 4 °C stand. Litter was air-dried and sorted into needles, leaves,
for 30 min. They were then gravity-filtered through preleached twigs, and “other”, which was mostly cones. The N concen-
Fisher Q2 filters and vacuum-filtered through 0.45 mm Duvapore tration in each of these components was measured on a Leco
PVDF membrane filters. A portion of the filtrate was set CN 2000 analyzer and total N content of annual above-
aside for NH4-N and NO3-N determination and frozen until ground litter input was calculated for each stand. To measure
analysis. Ammonium and nitrate were analyzed by a colori- rates of litter decomposition, standard samples of litter from
metric method on a Technicon Autoanalyser II (Technicon trembling aspen and white spruce were incubated on site in
Industrial Systems 1973; Technicon Industrial Systems 1977). litter bags. This litter was collected in October 2002 from
Another portion of the filtrate was used for analysis of solu- one site within the experimental area. Two grams of air-dried
ble organic C, measured as total organic C from the extract litter were placed in mesh bags with 1.5 mm diameter open-
by automated persulfate oxidation on a DOC analyzer, Astro ings, which were double-layered to minimize spillage of
2000 (Schreurs 1978). Total soluble N was measured as ni- spruce needles. Both types of litter were placed at all sites,
trate after digestion of the filtrates with persulfate oxide so the influence of site could be distinguished from that of
(Cabrera and Beare 1993). Ten millilitres of 0.5 mol/L per- litter. The bags were collected after 1 year, air-dried, weighed,
sulfate oxide was added to 5 mL of filtrate and autoclaved and analyzed for C and N concentrations. Litter masses were
for 50 min. SON was calculated as the difference between adjusted for oven-dried mass. The average decomposition
total soluble N and SIN, adjusted for water loss during auto- rate for on-site litter was calculated as the weighted average
claving. of the decomposition rates for trembling aspen and spruce
Microbial-biomass N and C were estimated by the chloro- based on their proportions in the litterfall.
form fumigation extraction method (Horwath and Paul 1994).
Samples were incubated for 48 h (Jenkinson and Powlson Statistical analysis
1976) to ensure complete lysis of microbial cells. Microbial The experimental design was a completely randomized
N was calculated as total soluble N after fumigation minus split plot with multiple measurements, where treatments were
total soluble N before fumigation, and microbial C as total fixed. The split plot was soil layer for soil characteristics
organic C after fumigation minus total organic C before fu- and litter type for litter decomposition rate. Data were ana-
migation (Martikainen and Palojärvi 1990). No conversion lyzed using SAS® statistical package version 8.02 (SAS In-
coefficient was used, as the primary goal was to compare stitute Inc., Cary, North Carolina, 2004). The GLM procedure
values among forest types. Total N and C of air-dried sam- was used to assess differences among forest types, where
ples were analyzed on a Leco CN 2000 analyzer. one stand was an experimental unit. Factor analysis and ca-
For extractable phosphorus (P) analysis, approximately 10 g nonical correlation (Tabachnick and Fidell 2001) were used
fresh mass of soil and 6 g fresh mass of forest floor was ex- to explore interactions among soil variables and vegetation
tracted with the addition of 40 mL of Bray-1 solution (Kuo characteristics, and multiple linear regression was applied to
1996). Samples were shaken for 1 h on a reciprocal shaker identify actual predictors of the soil variables. These analy-
© 2006 NRC CanadaTable 1. Concentrations and contents of total carbon (C) and several forms of nitrogen (N) in the forest floors of three forest types.
(A) Concentration.
Jerabkova et al.
Forest type Total C (g·g–1) Total N (mg·g–1) C/N ratio NO3-N (µg·g–1) NH4-N (µg·g–1) SON* (µg·g–1) Microbial N (µg·g–1)
Deciduous 0.411 (0.02)a 21.51 (0.70)a** 19.2 (0.7)a** 0.34 (0.27)a 100.40 (5.05)a** 132.29 (7.74)a 942.23 (80.21)a
Mixed 0.427 (0.02)a 17.84 (0.83)ab 24.8 (0.8)ab 1.55 (0.27)a 63.50 (4.83)b 205.31 (7.40)a 575.44 (82.84)a
Coniferous 0.389 (0.03)a 12.19 (1.17)b 32.4 (1.2)b 0.65 (0.28)a 50.86 (4.98)b 159.17 (7.40)a 422.86 (85.34)a
(B) Content.
Forest type Total C (g·m–2) Total N (g·m–2) C/N ratio NO3-N (mg·m–2) NH4-N (mg·m–2) SON* (mg·m–2) Microbial N (mg·m–2)
Deciduous 2566.18 (209.42)a 134.17 (11. 88)a — 2.15 (4.04)a 610.10 (12.61)a 626.20 (79.21)a 5876.67 (660.00)a
Mixed 2578.53 (247.79)a 107.82 (14. 06)a — 9.36 (4.04)a 383.87 (40.75)b 959.20 (727.41)a 3478.53 (681.65)a
Coniferous 2970.83 (310.62)a 109. 37 (20.58)a — 5.17 (4.17)a 405.88 (72.0)b 900.20 (75.74)a 3374.38 (702.62)a
Note: Each value is the least squares mean of three replicates of forest and layer combinations, with the standard error in parentheses. The C/N ratios were identical for part B. Values followed by
different letters are significantly different among the forest types (*, p < 0.1; **, p < 0.05). SON, soluble organic N.
Table 2. Concentrations and contents of total carbon (C) and several forms of nitrogen (N) in the upper 7 cm of mineral soil in three forest types.
(A) Concentration.
Forest type Total C (g·g–1) Total N (mg·g–1) C/N ratio NO3-N (µg·g–1) NH4-N (µg·g–1) SON (µg·g–1) Microbial N (µg·g–1)
Deciduous 0.023 (0.028)a 2.17 (1.00)a 11.3 (1.0)a 0.11 (0.27)a 3.57 (4.97)a 13.13 (7.63a) 45.70 (85.39)a
Mixed 0.026 (0.027)a 1.91 (0.96)a 13.7 (0.97)a 0.19 (0.27)a 2.17 (4.82)a 15.38 (7.40)a 62.37 (110.57)a
Coniferous 0.038 (0.021)a 2.99 (0.75)a 13.4 (0.93)a 0.44 (0.27)a 2.23 (4.67)a 18.47 (7.17)a 52.98 (108.61)a
(B) Content.
Total C (g·m–2) Total N (g·m–2) C/N ratio NO3-N (mg·m–2) NH4-N (mg·m–2) SON (mg·m–2) Microbial N (mg·m–2)
Deciduous 1864.47 (233.2)a 157.60 (16.97)a — 7.62 (4.04)a 259.56 (42.00)a 741.80 (78.07)a 3326.73 (702.62)a
Mixed 1753.94 (288.67)a 127.09 (16.38)a — 12.68 (4.04)a 144.39 (40.75)a 802.30 (75.74)a 4147.65 (909.75)a
Coniferous 2856.54 (223.60)a 218.26 (15.67)a — 29.53 (4.04)a 148.43 (39.45)a 989.20 (75.74)a 3522.85 (702.62)a
Note: Each value is the least squares mean of three replicates of forest and layer combinations, with the standard error in parentheses. The C/N ratios were identical for part B. Values followed by
different letters are significantly different among the forest types (*, p < 0.1; **, p < 0.05). SON, soluble organic N.
© 2006 NRC Canada
115116 Can. J. For. Res. Vol. 36, 2006
Table 3. The pH values, gravimetric moisture levels, and extractable phosphorus (P) and microbial carbon (C)
concentrations in the forest floor and upper 7 cm of mineral soil in three forest types.
Forest type pH(CaCl 2 ) Moisture (g·g–1) Extractable P (µg·g–1) Microbial C (µg·g–1)
Forest floor
Deciduous 5.59 (0.12)a** 235.14 (12.65)a 46.15 (4.27)a 4373.53 (357.52)a
Mixed 4.89 (0.12)b 201.03 (12.65)a 43.01 (4.27)a 4653.12 (357.52)a
Coniferous 4.44 (0.12)b 244.72 (12.65)a 42.91 (4.27)a 3999.19 (380.61)a
Mineral soil
Deciduous 4.83 (0.13)a** 36.20 (12.65)a 18.10 (4.42)a* 276.40 (397.05)a
Mixed 4.23 (0.13)b 41.91 (12.65)a 8.70 (4.28)ab 455.84 (397.05)a
Coniferous 4.71 (0.13)ab 75.46 (12.65)a 8.03 (4.55)b 348.68 (447.50)a
Note: Each value is the least squares mean of three replicates of forest and layer combinations with standard error in the
brackets. Values followed by different letters are significantly different among the forest types (*, p < 0.1; **, p < 0.05).
Table 4. Net rates of nitrogen (N) mineralization and nitrification in the forest floor and
upper 7 cm of mineral soil in three forest types.
Mineralization Nitrification
Concentration Content Concentration Content
(µg·g–1·d–1) (mg·m–2·d–1) (µg·g–1·d–1) (mg·m–2·d–1)
Forest floor
Deciduous 1.81(0.51)ab 1.13 (0.68)a 0.98 (0.33)a 0.62 (0.24)a
Mixed 3.58 (0.50)a* 2.17 (0.68)a 1.04 (0.32)a 0.64 (0.23)a
Coniferous 0.92 (0.50)b 0.73 (0.66)a 0.12 (0.32)a 1.0 (0.23)a
Mineral soil
Deciduous 0.16 (0.57)a 1.15 (0.75)a 0.03 (0.32)a 0.22 (0.27)a
Mixed 0.17 (0.50)a 1.12 (0.66)a 0.08 (0.32)a 0.50 (0.23)a
Coniferous 0.30 (0.52)a 2.01 (0.68)a 0.04 (0.32)a 0.27 (0.24)a
Note: Each value is the least squares mean of three three replicates of forest and layer combination
with standard error in the brackets. Values followed by different letters are significantly different among
the forest types (*, p < 0.1; **, p < 0.05).
ses were based on individual sampling plot data. None of the the differences in NH4-N and SIN were significant (Table 1).
data fulfilled the requirements of normality and equal vari- Nitrate concentrations were low at all sites, often below de-
ances for analysis of variance, and several transformations tection levels, and did not vary among forest types. In the
did not solve this problem, so the data were ranked. The mineral soil, there was no variation among forest types in any
ranked values were normalized (Blomm 1958) to allow sta- of the N-availability indices (Table 2). Deciduous stands
tistical tests as for original data. The significance level was also had the highest pH values and extractable-P concentra-
set at α = 0.1 for analysis of variance, to account for high tions in forest floor and mineral soil (Table 3).
within-stand variability in most soil N measurements, and Net N mineralization rates were highest in the forest floor
α = 0.05 for other analyses. Bonferroni’s adjustment was of mixed stands (Table 4). The amount of SIN accumulated
used for multiple means comparison; no adjustment was ap- per gram of forest floor during the field incubation was also
plied experimentwide. higher in mixed stands. There was no variation in net nitrifi-
NH4-N, NO3-N, and SON values are presented as the av- cation rate or NO3-N accumulation among forest types dur-
erage of the June and August samplings so that they could ing the field incubation. There was also no variation in the
be used in the common analyses with the data measured percentage of NO3-N in total inorganic N accumulated dur-
only once. Although there were some differences in these ing the field incubation.
values between sampling dates, the trend across forest types Deciduous stands had a higher proportion of labile N in
was the same. the microbial biomass (p = 0.08) in the forest floor, and in-
organic N (SIN) formed a larger proportion of the soluble-N
pool (Fig. 1). However, the proportion of SIN that was NO3-N
Results in mineral soil was higher (p = 0.025) in coniferous stands.
Forest floor in deciduous stands had the highest total N
concentration and lowest C/N ratio of the forest types Relationship of N dynamics to vegetation
(Table 1). Deciduous forest floors also had higher NH4-N, Tree density and basal area in the vicinity of the centre of
SIN, and microbial N concentrations, although the difference the sampling plots in the three forest types is shown in
in the latter was not significant (Table 1). Similar trends Fig. 2. Forest-floor N values were more strongly correlated
were seen when values were expressed as content, but only with vegetation than were mineral-soil N values. In keeping
© 2006 NRC CanadaJerabkova et al. 117
Fig. 1. Percent distribution of nitrogen (N) within the labile pool were analyzed together, but there were no significant corre-
in the forest floor and upper 7 cm of mineral soil in deciduous, lations for mineral soil.
mixed, and coniferous stands. SON, soluble organic N; micrN, We also assessed whether N-availability indices could be
microbial N. predicted from characteristics of the stand. No common predic-
Deciduous tors existed for both forest-floor and mineral-soil data, so the
layers were again analyzed separately. In the forest floor,
SON microbial N and NH4-N and the proportion of soluble N
10.9
SON NH4
made up by SIN could be reliably predicted from stand char-
NH4 21.1 acteristics. NH4-N concentration was predicted by soil pH
8.4 5.7
NO3
and moss cover (NH4 = 1.23 × pH – 0.04 × moss cover + 1.31;
NO3
0.2 R2 = 0.58, p < 0.0001) and microbial N by moisture, propor-
0.0 tion of deciduous trees, and moss cover (microbial N = 0.21 ×
micrN micrN
80.7 73.0 moisture + 0.03 × proportion of deciduous trees – 0.67 × moss
cover; R2 = 0.57, p < 0.0001). The proportion of soluble N
Mixed that was SIN was associated with a low soil soluble C/N ratio
and low moss cover, which together explained 60% of the
variation (proportion of soluble N that was SIN = –0.434 ×
SON
24.3 NH4 SON soluble C/N ratio – 0.49 × moss cover; R2 = 0.6, p < 0.0001).
7.5 19.5 NH4 In the mineral soil, 66% of the variation in NH4 concentra-
NO3 2.7 tion could be explained by total basal area (NH4 = 0.76 +
0.2 NO3 1.24 × basal area; R2 = 0.66, p < 0.0001).
micrN 0.2
micrN
68.0 77.6
Litter input and decomposition
There was no difference in total litter mass or N content
Coniferous
from the canopies in the three stand types (Fig. 3). However,
foliar litter input (leaves and needles) was significantly greater
SON
NH4 SON
in deciduous than in coniferous stands in terms of both mass
23.4
8.0 25.8 NH4 (p = 0.029) and N content (p = 0.046). Foliar N content was
3.0 positively correlated with soluble N (r = 0.64), SIN (r =
NO3
NO3 0.65) (Fig. 4a), NH4-N (r = 0.59), and microbial N (r = 0.56)
0.1
0.6 and SIN as a proportion of soluble N (r = 0.46), and nega-
micrN micrN
66.5 70.6
tively correlated with the microbial C/N ratio (r = –0.47) in
the forest floor.
Aspen leaf litter decomposed significantly faster (p = 0.064)
Forest floor Mineral soil than spruce needles during the first year, but there were no
differences in decomposition rates of either litter type among
the three forest types. Litter decomposition rates were only
with our hypothesis, the proportion, number, and basal area weakly correlated with available N (Fig. 4b) and the trends
(data not shown) of deciduous trees were all positively cor- were different for aspen and spruce litter. The decomposition
related with soluble N, SIN, NH4-N, and microbial N pools rate for aspen litter was weakly positively correlated with
and pH and negatively correlated with the forest-floor mi- NH4-N (r = 0.33) and microbial N (r = 0.27) in the forest
crobial C/N ratio (Table 5). The proportion of deciduous floor. The decomposition rate for spruce litter was negatively
trees was also positively correlated with the percentage of correlated with SIN (r = –0.33) and NH4-N (r = –0.37) in the
SIN from the soluble-N pool and inorganic N accumulated forest floor.
during the field incubation. Alder cover correlated positively
with NH4-N, microbial N, and accumulated NH4-N, and grass
Discussion
cover with nitrification and accumulated nitrate. Moss cover
was negatively correlated with ammonium and microbial N Most measures of N availability were consistent with our
and the percentage of SIN from soluble N. These trends first hypothesis of higher N availability in deciduous stands.
were confirmed by factor analysis of the forest floor. The The higher ammonium and soluble inorganic N concentra-
first vegetation factor, “deciduousness”, included the num- tions in the forest floor of deciduous stands confirmed find-
ber, basal area, and proportion of deciduous trees in the ings from a boreal forest in Quebec (Côté et al. 2000). The
stand. The second factor, “dense coniferous stands”, con- higher proportion of soluble N consisting of SIN also sup-
sisted of total tree number and basal area, number and basal ports the hypothesis of relatively higher N availability in de-
area of coniferous trees, and moss cover. The soil factor, “N ciduous stands, as a low relative abundance of SIN can be
availability”, was positively associated with SIN and NH4-N indicative of sites limited by N (Northup et al. 1995). The
and microbial N and negatively associated with the percent- tendency for a higher soil microbial N concentration in de-
age of SON from the labile N pool and the microbial C/N ciduous forests also likely indicates higher N availability, as
ratio. The N-availability factor correlated positively with de- it is suggested that soil microbial biomass is a relatively labile
ciduousness and negatively with dense coniferous stands. N pool (Myrold 1987; Wardle 1992), although Martikainen
The trend was similar but weaker when the two soil layers and Palojärvi (1990) suggested that microbial N represents
© 2006 NRC Canada118 Can. J. For. Res. Vol. 36, 2006
Fig. 2. Tree densities (a) and basal area per hectare (b) of coniferous and deciduous trees in the vicinity of sampling spots.
1000 50
Coniferous (a) Coniferous
(b)
Basal area (m ·ha )
–1
800 Deciduous 40 Deciduous
No. trees·ha–1
2
600 30
400 20
200 10
0 0
Deciduous Mixed Coniferous Deciduous Mixed Coniferous
Forest type Forest type
Table 5. Correlations between soil nitrogen (N) variables and characteristics of the vegetation in the immediate vicinity of sampling
plots in all forest types.
No. trees/ha % cover
a
Total Coniferous Deciduous % deciduous Alder Low shrub Grass Moss
[Soluble N]
Forest floor 0.00 –0.4 0.51 0.53 0.24 –0.34 0.09 –0.44
Upper mineral soil 0.12 0.21 –0.20 –0.27 0.22 –0.24 0.15 –0.01
[Inorganic N]
Forest floor –0.16 –0.56 0.49 0.60 0.32 –0.52 0.22 –0.58
Upper mineral soil –0.31 –0.35 0.11 0.12 0.15 –0.53 0.18 –0.38
[NH4-N]
Forest floor –0.25 –0.63 0.48 0.58 0.43 –0.56 0.29 –0.64
Upper mineral soil –0.29 –0.38 0.17 0.17 0.19 –0.56 0.17 –0.40
[NO3-N]
Forest floor –0.19 –0.10 –0.06 –0.06 0.19 –0.31 0.32 –0.17
Upper mineral soil –0.18 0.19 –0.37 0.44 –0.31 0.14 0.19 0.12
[Microbial N]
Forest floor –0.06 –0.53 0.56 0.47 0.39 –0.22 –0.00 –0.35
Upper mineral soil 0.16 0.05 0.1 –0.14 –0.18 0.05 0.15 0.15
Microbial C/N ratio
Forest floor 0.19 0.50 –0.35 –0.41 –0.34 0.26 –0.10 0.40
Upper mineral soil 0.15 0.28 –0.18 –0.15 –0.16 –0.03 –0.29 –0.06
Nitrification
Forest floor –0.50 –0.32 0.31 0.29 0.20 –0.30 0.49 –0.21
Upper mineral soil –0.27 –0.30 0.19 0.00 0.31 –0.17 0.30 –0.22
Accumulated NO3b
Forest floor –0.48 –0.22 0.31 0.30 0.43 –0.32 0.51 –0.26
Upper mineral soil –0.45 –0.36 0.05 0.02 0.24 –0.19 0.31 –0.27
Accumulated SINc
Forest floor –0.23 –0.39 0.33 0.46 0.23 –0.32 0.31 –0.47
Upper mineral soil –0.45 –0.36 0.05 0.02 0.24 –0.19 0.32 –0.27
SIN/soluble N
Forest floor –0.36 –0.54 0.24 0.42 0.32 –0.42 0.30 –0.52
Upper mineral soil –0.45 –0.51 0.19 0.24 0.28 –0.49 0.18 –0.40
pH
Forest floor –0.27 –0.64 0.60 0.62 0.33 –0.60 0.29 –0.50
Upper mineral soil –0.28 –0.35 0.19 0.27 0.08 –0.27 –0.07 –0.32
Note: Values in boldface type are r values that are significant at p < 0.05.
a
Proportion of deciduous trees from the total number of trees.
b
NO3-N accumulated after 6-week field incubation.
c
Soluble inorganic N accumulated after 6-week field incubation.
N that is immobilized and rather unavailable. Forest floor of tion than in coniferous stands (Hannam et al. 2004). The
deciduous stands had a higher total N concentration but sim- same trend was reported in birch–aspen and black spruce
ilar total N capital, reflecting less organic matter accumula- stands in Alaska (Troth et al. 1976).
© 2006 NRC CanadaJerabkova et al. 119
Fig. 3. Annual mass (a) and N contents (b) of canopy litter in three forest types.
250 2.0
Needles Needles
(a) (b)
Leaves Leaves
200 Twigs Twigs
1.5 Other
Mass (g·m–2)
Other
N (g·m–2)
150
1.0
100
0.5
50
0 0.0
Deciduous Mixed Coniferous Deciduous Mixed Coniferous
Forest type Forest type
Fig. 4. Relationship between soluble inorganic nitrogen (SIN) concentration in the forest floor and N input in foliar litter (a) and litter
decomposition rates (b) in three forest types.
50
2.0 (b)
(a)
1.8
40
Litter N input (g·m–2)
% decomposed
1.6
1.4
30
1.2
1.0
0.8 20
0.6 Deciduous
0.4 Mixed 10
Coniferous
0.2
0.0 0
0 20 40 60 80 100 120 140 20 40 60 80 100 120 140
SIN ( µg·g–1) SIN (µg·g ) –1
Aspen in deciduous Aspen in mixed Aspen in coniferous
Spruce in deciduous Spruce in mixed Spruce in coniferous
Deciduous stands did not have higher nitrate concentra- 1983; Ferrari 1999) and across forest types (Reich et al.
tions or net nitrification rates despite having soil characteris- 1997). Thus, we suggest that litter N content may be a better
tics generally favourable to nitrification, such as a higher indicator of N availability in forests than the litter decompo-
pH, higher ammonium concentration, and lower soil C/N ra- sition rate.
tio (Ste-Marie and Paré 1999). The lack of differences could The faster decomposition of aspen leaves during the first
be an artifact of the extraction method, as the concentrations year is consistent with the results of other studies (Alban
were very low; however, we also found no difference in 1982; Prescott et al. 2000b). This difference may, however,
amounts of NO3-N collected on ion-exchange probes incu- be short-lived, as other studies indicate that in the later stages
bated at some of these sites.2 Paré and Bergeron (1996) also the decomposition of leaf litter (including aspen) slows down
detected no differences in net nitrification rates in mature as- more than that of needle litter, and rates converge after 3 years
pen and spruce stands in which NO3-N levels were also very (Berg and Ekbohm 1991; Moore et al. 1999; Prescott et al.
low. In Finland, Priha and Smolander (1999) found higher 2000a, 2004). As there was also no correlation between soil
NO3-N levels and a higher net nitrification rate in soil in pine N availability and average decomposition rate of on-site litter,
forest compared with a birch forest, despite a lower pH. Thus, our results do not support the hypothesis that deciduous litter
at least in boreal forests, deciduous forest floor should not causes higher N availability because of its faster decomposition.
be assumed to be “nitrate environments” (sensu Kronzucker The faster decay of spruce needles at sites with low N
et al. 1997). availability may be related to their lignin concentrations be-
The positive correlation of the available-N pool in soil ing higher than in aspen leaves, as inorganic N has been
with the amount of N returned in litter supported our second shown to inhibit ligninolytic activity (Carreiro et al. 2000;
hypothesis that soil N availability is more closely related to DeForest et al. 2004). In our study, the sites with low inor-
litter N content than to decomposition rate. A correlation be- ganic N concentrations and faster spruce litter decomposi-
tween litter N content and net N mineralization rate has been tion were often spruce sites. The negative relationship may
reported within (Vitousek et al. 1982; Nadelhoffer et al. then reflect a “home-field advantage” (Hunt et al. 1988),
2
L. Jerabkova and C.E. Prescott. Manuscript in preparation.
© 2006 NRC Canada120 Can. J. For. Res. Vol. 36, 2006
where processes and organisms involved in decomposition Some N-availability indices increased with stand open-
are specific for each litter type and are more likely to occur ness, but adjustment for tree numbers or basal area through
at the site where that type of litter is dominant, resulting in analysis of covariance did not significantly affect the out-
faster decay. come of the comparisons, thus stand density is likely not the
The higher forest-floor pH in aspen stands is in keeping main influence on N dynamics.
with other comparisons of spruce and aspen (Bauhus et al.
1998) and of deciduous and coniferous forests in general Management implications
(Raulund-Rasmussen and Verje 1995; Priha and Smolander Maintaining the deciduous component in coniferous stands
1999; Hagen-Thorn et al. 2004). The higher forest-floor pH may facilitate keeping a higher proportion of N in the avail-
in aspen stands is often associated with a higher calcium able form. Mixtures might be particularly beneficial, as de-
(Ca) concentration (Brais et al. 1995). This results from Ca ciduous forests had higher ammonium concentrations, and
being redistributed in the soil profile, so the forest floor is spruce is believed to prefer ammonium as a N source. Mixed
enriched with Ca and the deeper mineral soil depleted of Ca stands may have some specific non-additive characteristics
(Alban 1982). This was evident in this study, as Ca concen- with potential for higher productivity. Hypothetically, the
trations were higher in forest floors but lower in the deepest perpetuation of pure coniferous stands may shift the soil N
mineral soil sampled (10–17 cm) in deciduous stands than in pool towards a larger proportion of relatively unavailable
coniferous stands (Kishchuk 2002). The higher pH might forms. As this study presents only short-term results, we
contribute to the observed higher availability of P in decidu- cannot speculate about the long-term consequences and how
ous stands. The higher P levels may also reflect higher P in- reversible the changes might be. As no difference in nitrate
put due to higher P concentrations in aspen litter (Flanagan availability and net nitrification was found, it does not seem
and Van Cleve 1983). that one forest type would be more prone to N losses. Litter
Although the differences in N cycling among forest types N input is a better indicator of N availability than decompo-
are consistent with the results of other studies of spruce and sition rate. Understory vegetation may serve as valuable in-
aspen, we cannot assume that the differences are directly dicator of N availability, and its active or passive role in N
and solely attributable to the species composition. Because cycling should be more closely investigated.
we studied natural forests, the possibility exists that the pat-
terns reflect other site factors such as slope position (Prescott
et al. 2000a; Chen et al. 1998) or soil texture (Reich et al. Acknowledgements
1997; Levula et al. 2003), which drive both species composi-
tion and N-cycling characteristics. However, soil moisture We thank Martin Blank, Roshini Nair, Maija Raudsepp,
did not vary among the forest types and there is no consis- and Candi Staley for assistance in the field and laboratory,
tent trend in the distribution of aspen- and spruce-dominated and Kirsten Hannam for providing forest floor bulk density
stands within the studied landscape with respect to topogra- data. This study was funded by the Sustainable Forest Man-
phy. There was also little variation in soil texture in the sam- agement Network and the fieldwork was supported by the
pled horizons among forest types (Kishchuk 2004). Neither Canadian Forest Service, Canadian Forest Products Ltd., and
slope position nor soil texture is thus likely to be the cause Daishowa-Marubeni International.
of the variation in N availability among the forest types.
Finally, there was no variation in total and available N and References
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