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Microsite variations in soil chemistry and nitrogen mineralization in a beech-maple forest
S&i Bid. Biochcm. Vol. 21, No. 6. pp. 795-801, 1989 Printed in Great Britain. All rights reserved
MICROSITE NITROGEN
0038-0717~89S3.00 + 0.00 CopyrightSC1989 Pergamon Pms pk
VARIATIONS IN SOIL CHEMISTRY AND MINERALIZATION IN A BEECH-MAPLE FOREST RALPH
E. J. B~ERNER and SHARI D. KOSLOWSKY’
Department of Botany, The Ohio State University, 1735 Nell Avenue, Columbus, OH 43210, U.S.A. (Accepted I5 January 1989) Summary-Though whole forest stand and ecosystem estimates of soil nitrogen dynamics are common, few studies have included measurements of within-stand variations in N mineralization and nitrification among microsites. We evaluated soil chemical status, net mineralization and nitrification potential during Autumn 1986 in A- and B-horizon soils taken from near the bases of Fagup grandi/olia (American beech). Acer sacchorum (sugar maple), and Fravincu unrericaae (white ash) trees and from sites > 2 m from trees using buried bag exposures. Fugus sites had significantly lower pH in both horizons than did other sites. Soils near Acer trees had higher organic C and NH: concentrations and Fraxinw soils had significantly higher PO:- availability. Moisture+ and NO; concentrations were similar among microsites. Using stepwise discriminant analysis, we constructed functions which differentiated among site types at P < 0.01 on the basis of these chemical measures. A-horizon N mineralization rates were generally 50% greater near Acer trees than away from trees, with Fugus and Fruxinur sites intermediate. B-horizon mineralization rates were highest under Fruxinus, but only 2-36% of A-horizon rates. Relative nitriftcation was approached 100% in all site types despite soil pH c 4. Calculation of N mineraIi~tion on an area basis with and without accounting for the differences among microsite types indicates that whole-ecosystem estimates of N minerali~tion generated by sampling schemes which do not explicitly include unique microsites around tree bases may be biased low by 820%.
INTRODUCTION
Our current understanding of how functional processes unite and coordinate the many components of terrestrial ecosystems is mainly based on ecosystemand watershed-level studies in which the focus has been on determining the rates of, and controls on, these processes at a whole-system level (e.g. Vitousek ef al., 1982). Such studies have produced estimates of N minerali~tion from organic matter and relative nitri~cation for a number of ecosystems and have subsequently been used to produce large-scale predictive models. In contrast, studies of within-ecosystem patterns often focus on smaller scale spatial and temporal variability for a particular process. For example, smaller scale patterns of soil nutrient status may occur within deciduous forest ecosystems as a result of variations in parent material distribution (Vankat er al., 1977). small scale disturbances (Vitousek, 1985). microclimate (Boerner, 1984) and differences in the volume and nutrient content of throughfall and stemflow under different tree species (Parker, 1983; Croder and Boemer, 1986). Quantifying microsite differences in soil properties and processes is important for two reasons. First, the most frequent disturbances in mesic forests (e.g. single treefall gaps, small mammal activity, etc.) and the spatial gradients along which forest plants are distributed are typically small in scale (Bratton, 1976; *Current address: Ohio Environmental Protection Agency, 1800 Watermark Drive, Columbus, OH 43215. U.S.A.
Crazier and Boerner, 1984; Pickett and White, 1985). Thus the smaller the scale of study, the closer one can estimate those factors which are important to germinating seeds, plant roots and soil fauna (Coleman ef al., 1983). Second, in a mature forest stand, microsites of various types may make up an appreciable portion of the ground area, thus making differences in the chemical properties of these microsites and the more uniform, more often sampled forest floor important for calculation and validation of whole-ecosystem means. Among the most studied microscale patterns of variation in deciduous forests are those associated with tree bases. The chemistry of surface soils around the bases of individuals of a given tree species may differ significantly from soils around other tree species or away from tree bases because of differences in the volume and chemistry of stemflow (Zinke, 1962; Gersper and Holowaychuk, 1970; Crozier and Boemer, 1986). differential rates of root exudation, or the decay of leaf masses with different nutrient concentrations or amounts of defensive compounds (e.g. tannins, lignin). Extractable amounts of PO:-, SOiand Ca*+, as well as both pH and base saturation have all been reported to differ significantly among soils near the bases of different tree species (Zinke, 1962; Gersper and Holowaychuk, 1970; Crozier and Boemer, 1986). As that suite of soil factors has been correlated with N mineralization rates and relative nitritication (e.g. Robertson, 1982; Nadelhoffer et al., 1983; Plymale er al.. 1987), we hypothesized that these soil processes would differ significantly among samples taken from near the bases of a variety of tree
795
796
RALPH
E. J. B~ERW and
species. If the variance among such samples was large, the utility of a whole-system mean for predicting within-system fluxes in this forest would be weakened. Our specific objectives, therefore, were to: (1) determine the nature and magnitude of differences in A- and B-horizon chemistry in plots around the bases of different tree species and away from trees, (2) determine the rates of N mineralization and relative nitrilication in these microsites, and (3) develop an empirical multiple regression model for rates of N turnover at the whole-stand level which takes these microscale variations into account.
SHARI
D.
KO.SLOWSKY
content was determined by Walkley-Black oxidation (Allison, 1965). Net nitrification was calculated as the differences between NO; content in subsamples extracted immediately and those extracted after exposure in situ. Net N mineralization was calculated as the sum of the differences between initial and final NO; and NH: content of paired bags. Relative nit~fication was calculated as the ratio of net nitrification to net N mineralization. To characterize the mass and species composition of litter at each of the sampling plots. we took two 0.25 rn’ litter samples next to each sampling point during April 1987. We chose this time for sampling as it was after the major period during which leaf MATERIALS AND METHOLX3 litter redistribution occurs (Boemer and Kooser, The Bohannan Preserve is a 40 ha mature hard1989) but before most mass and nutrient Ioss begins wood forest in eastern Delaware County, Ohio in this area (Boemer, 1984). (40’21’N, 82”X’W). Importance percentages (mean Differences in initial conditions and N mineralizof relative density and relative basal area in five ation were analyzed by analysis of variance and 0.04 ha plots) of the common species are Rcer SUC- covariance, fonowed by Ryan-Einot-Gabriel&rum : 40.3, Fraxims ume?ica~a : 24.9, Fag= gran - Welsch Multipie F tests. This post-test was chosen to difofia: 14.5 and Quercus borealis: 11.2; the site has minimize the probability of Type I errors (Statistical 35.7 m’ ha-’ basal area > lOcm diameter at breast Analysis System, 1985). Pairwise stepwise discrimiheight and has not been disturbed since approx. 1900. nant analysis was also used to resolve differences in ~nnington series soils (aeric ~hraqua~fs) have deinitial conditions among sites. MultipIe regression veloped on the carbonate-bearing glacial till substrawas used to develop empirical models for N mineraltum of the site (Matanzo PI al., 1969). ization among sites. Ail pH values were transformed All sampling was carried out within a 1 ha area of to hydrogen ion concentrations before analysis. level. relatively uniform topography. Sampling plots were established around the bases (annuius O-30 cm RESULTS from the rootcrown) of 5 randomly chosen canopy Initial conditions: A -lrori:on individuals of each of the three dominant tree species, and at 5 randomly chosen points > 2 m from any tree Soils taken from around the bases of Acer trees base. This set of sites was chosen to represent a had significantly higher concentrations of organic C range of potential stemflow influence with increasing and NH: than did soils from No Tree sites during the stemflow in the order: No Tree < Fruxinus < first exposure period; organic C and NH.,+ concenAcer < Fagus (Gersper and Holowaychuk, 1970; trations in Fugus and Fruxinus soils were intermediate Crozier and Boerner, 1986). At the beginning of each (Table I). In contrast, extractable PO:- concenexposure period, A- and B-horizon samples were trations were significantly higher in Fraxinus soils taken from each plot with a 2cm corer and sieved than in any other site type (Table I). Though we (~6 mm) to remove large roots, leaves and cobbles. observed the same patterns of organic C, NH:, and Each sample was divided into aliquots of approx. PO:- concentrations among site types during the 100-150 g; one aliquot was returned to the lab for second exposure period, the differences were not immediate extraction white one ~B-horizon) or two significant at P c 0.05. (A-horizon) aliquots were placed in polyethylene A-horizon pH was significantly lower in the Acer bags and buried for exposure at the depth from which and Fagus sites than in the Fraxinus and No Tree they were taken @no, 1960). Exposure periods began types during both exposure periods. There were no on 30 September and 22 October 1986 and lasted 22 significant differences in soit moisture or NO; pools and 27days. respectively. We chose to measure N among sites during either of the sampling periods dynamics in autumn because studies in this region (Table 1). Despite relatively low pH, 72-92% of the have shown that the magnitude of intersite differences extractable inorganic N was NO;. decreases from spring to autumn (Plymale ef al., We used stepwise discriminant analysis to deter1987). Thus, if significant differences existed in mine if combinations of these patterns of variation in autumn, we could he confident that such differences single soil chemical factors among site types produced existed throughout the growing season. an overall pattern of distinct soil chemical microsites. NO; and NHf were extracted from 4Og subDespite relatively high variation in some individual samples with 2 M KC1 (Bremner. 1965); PO:- was chemical factors within site types, the pairwise stepextracted with 1 M NH, OAc (Chapman, 1965). NO;. wise discriminant analysis constructed discriminant NH: and PO:- concentrations were determined functions which distinguished ail four site types from by the ultraviolet, phenate and stannous chloride each other at P c 0.01 or better (Table 2). Acer sites methods, respectively (American Public Health were distinguished from the others on the basis of Association, 1976). Soil pH was determined in a I:5 high organic C content, low pH and low PO:-. The slurry of oven-dried soil in lOmM CaCl, (Peech. discriminant functions which distinguished Fraxinus 1965). Soil moisture was determined by drying 5Og sites from the others were based on the high PO:- and subsamples at IIO”C to constant mass. Organic C pH of those sites, whereas the Fagus sites were
Microscale
soil chemistry
19-l
patterns
Table I. Means of initial soil conditions in sites established next to tree bases or away from any trees. Standard errors of the means are given in parenthcscs. Organic C is expressed in mg g-’ dry soil, PO:-, NH: and NO; in pmol IOOg-’ soil and moisture in percent by mass. Means followed by the same lower case letter were not significantly different Exposure period 1 No tree Organic C PO;
34.2b (2.1)
Fro.rinW
Act-r
Exposure period 2 No tree
Fr2gu.r
43.9ab
50.&a
(3.2)
(6.8)
A-horizon 44.&b 30.la (3.9) (1.4)
0.79a
Fr0.rillU.S
Acer
Fagus
32.8a (0.4)
42.4a
(0.6)
45.3a (5.8)
0.26b (0.02)
(0.08)
0.16b (0.01)
0.19b (0.01)
0.7Oa (0.16)
I .03a (0.08)
0.74a (0.09)
0.86a (0.02)
NH;
I .89b (0.14)
2.45b (0.19)
4.OSa (OSI)
2.53b (0.24)
1.82a (0.16,
l.78a (0.66)
2.27a (0.33)
2.45a (0.38)
NO,
4.8a (0.7)
I2Sa (2.9)
11.9a (0.7)
7.8a (1.0)
II.4a (1.5)
19.la (5.0)
19.0 (4.0)
l9.6a (3.1)
Moisture
36.7a (0.9)
38Sa (2.1)
38. la (1.4)
35.7a (1.5)
35.6a (1.1)
32.6a (0.8)
31.8a (2.1)
37.2a (0.7)
4.41a
PH
4.I2a
3.23b
3.46b
4.29a
7.4b (0.9)
12.5ab (1.6)
2l.k (3.9)
B-horizon 6.2b 18.9a (3.4) (0.7)
0.14a (0.03)
0.45a (0.12)
0.33a (0.09)
(0.W
NH;
0.73b (0.12)
I .09ab (0.07)
I .42a (0.23)
NO;
3.5a (0.8)
3.3a (0.9)
Moisture
22.2a (0.5)
25.3a (0.7)
Organic C PO:
3.50a
DH
3.5Oa
4.02a
I I .4ab
3.67b
3.73b
(1.3)
15.6a (1.5)
lS.lab (1.9)
I .OOa (0.13)
0.97a (0.11)
l.l2a (0.13)
0.96a (0.05)
I .30ab (0.13)
0.8la (0.13)
0.88a (0.09)
1.13a (0.19)
I .24a (0.14)
8.Oa (1.5)
S.Oa (1.4)
2.4a (1.1)
6.0a (0.6)
9.2a (2.0)
7.9a (0.8)
27.6a (1.4)
26.la (0.8)
23.6a (1.0)
24.6a (0.8)
27.7a (2.2)
25.9a (1.3)
3.5Sa
distinguished by their low pH (Table 2). The discriminant functions for No Tree sites were based on low organic C, high pH and low inorganic N concentrations (Table 2). Thus, when the soil nutrient status of these four sites analyzed in a multivariate fashion, each was easily resolved as a unique microsite. Initial conditions: B-horizon
Many of the patterns of variation in B-horizon soils among site types paralleled those of the A-
0.27a
3.35a
3SSb
3.72a
3.6lb
horizon, but at lower absolute values (Table I). Acer sites again had the highest organic C and NH: concentrations, and No Tree sites the lowest. PO:and NO; concentrations were lower than in the A-horizon, and no significant difierences among site types were observed. An average of 75-98% of the extractable inorganic N was NO; despite pH < 3.75 in all sites. Once again, stepwise discriminant analysis revealed significant differences in the overall chemical status of
Table 2. Results of stepwix discriminant analyses of initial conditions among all combinations of sites. The F-statistic for Wilk’s 1. and the soil factors used in calculating the discriminant function. are given for each pair. l = discriminant function significant at P < 0.01, l * = P < 0.001. l ** = P < 0.0001. Soil factors are C = organic C content. PO4 = extractable PO:-, H = hydrogen ion activity, NH4 = initial NH:. NO3 = initial NO, Site types No Tree Fra.rinus
F = 13.91”’ C. P04. H, NO3
A-horizon
-
Act-r
F = 17.16*** NH4. C. H
F = 10.24** P04. H
-
Fugus
F = 50.08*** H No Tree
F = 18.93*** P04.H Fmxinw
Fz7.44’ NH4, H Acer
Fagus
B-horizon
Fagus
F = 19.93” C F = 26.81”’ C F = 13.35*** NH4. C. H No Tree
3.4Sb
F = 5.37’ C. NO3 F = 10.22*” NH4, H, NO3 Fmxinuc
F=6.21* H Act-r
Fagus
798
RALPH
E. J. B~ERNEII and SHARI D. Kosrows~v
these four site types (Table 2). Fugus sites were distinguished from the others by high NH,+ concentrations and low pH. and Fruxinus sites were distinguished on the basis of low NO;, low organic C and high pH (Table 2). The discriminant function which separated Acer sites from the others was based on high organic C content. As in the A-horizon, No Tree sites were discriminated from the others by their lower concentrations of NH.,+, organic C and NO;. Thus, not only were the A- and B-horizon soils near the bases of these three tree species easily discriminated from soils away from trees on the basis of a suite of soil chemical factors, the No Tree sites always had the lowest absolute concentrations of nutrients, regardless of horizon.
Tabk 4. Rates of N minenliitioa (mg N 100 g-’ soil day-‘) in four site types. Standard errors of the means are given in parentheses. Means followed by the same lower case letter were not significantly diliercnt at P < 0.05 Site types Exposure period
Horizon
I
A
2
A
Pooled
A
1
B
2
B
Pooled
B
No Tree
There were no significant differences in the total dry mass of leaf litter or the proportion of Acer or Fraxinus litter among the various site types (Table 3). There were significantly lower proportions of Fugus and Q. boreaLis litter near the bases of Acer trees than any other species (Table 3); when these proportions were converted to absolute mass, these differences were no longer significant. Rates of N mineralization
and relative nitrifcation
Three-way Analysis of Variance indicated that rates of N mineralization were significantly higher during the first exposure period (mean air temperature 14. I “C) than during the second (mean air temperature 8.7’C) (P < O.Ol), and were significantly higher overall in A- than B-horizons (P ~0.01) (Table 4). There were, however, no significant differences among site types in either horizon or exposure period. When time periods were pooled and air temperature used as a covariate in l-way Analysis of Variance by horizon, the differences among site types did become significant at P c 0.05, with Acer sites having significantly higher rates than No Tree sites in both horizons and Fagus sites in the B-horizon (Table 4). The means and variances of the site types were strongly correlated: Acer sites had the highest mean N mineralization rate and the highest variance (Table 4). When N mineralization rates were converted to a g-l organic C basis, A-horizon Acer sites Table 3. Total mass (g dry mass 0.25 m-*) and proportions of leaf liwr of various species in plots near the bases of four site types. Standard errors of the means are given in parentheses. For factors where analysis of variance on log transformed proportions indicated significant differences at P < 0.05. means with ditTercnt lower case letters were significantly different. N = IO for each species No Tree
/+l7Xi~uS
Acer
Fagw
Dry mass
91.6 (8.9)
85.3 (8.0)
107.6 (8.2)
110.2 (11.0)
% Arer litter
30.8 (6.1)
22.3 (2.4)
35.0 (4.4)
24.2 (2.5)
l4.la (3.8)
I7.2a (I.81
7.4b (2.1)
18.7a (I.21
% Froxinus litter
3.8 (1.1)
10.8 (2.4)
IS.0 (6.2)
8.6 (2.7)
% Qurrcus borealis litter
l9.0a (4.6)
9.0a (1.4)
4.3b (4.3)
8.6a (0.8)
% Fagus litter
Act-r
Fagw 1.79 (0.24) I .34 (0.14)
2.05 (0.50) 1.08 (0.13)
2.59 (0.8 I) 1.69 (0.26)
1.37 (0.10) -0.1s (0.02) 0.14 (0.07)
1.62 (0.22) 0.62 (0.12) 0.54 (0.07)
1.85 (0.49) 0.76 (0.21) 0.57 (0.12)
0.43 (0. IO) 0.21 (0.17)
0.59ab (0.08)
0.64a (0.09)
0.29bc (0.04)
O.Ok (0.04)
initial conditions: leaf litter mass and composition
Fra.zinw
I .a9 (0.12) 1.02 (0.08)
(A::,
had significantly higher N mineralization rates than Fagus sites; in contrast, in the B-horizon, soils under Fruxinus trees mineralized a larger proportion of the available organic matter than did soils from the other site types (Table 5). Means of relative nitrification among site types and horizons varied between 95-105%; no mean differed significantly from 100% at P < 0.05. Thus any NH: not immediately immobilized by microbes was nitrified despite the low pH. To convert our mass-based N mineralization rates lo an area basis, we multiplied them by bulk density and horizon depths (data from Matanzo et al., 1969). From this we estimate A + B-horizon N mineralization in a site with no trees to be 5005 mg m-* of forest floor yr (A-horizon: 4829, B-horizon: 176). This estimate should reflect what would result from a sampling program which consciously or unconsciously avoided sampling at tree bases. To determine how incorporating these tree base microsites might effect this estimate, we calculated the forest floor area occupied by tree trunks plus annuli with 60 or 100 cm radius. These radii were chosen to represent common distances for stemflow effects from medium-sized trees (Zinke, 1962; Gersper and Holowaychuk, 1971, Crozier and Boerner, 1986). Tree bases plus 60 cm annuli covered approx. 683 m* ha-’ (6.8%) of the forest floor in our study site; extending the annular radius to 100 cm increased this to approx. 1600 m* ha-’ (16.0%). Multiplying the N mineralization rate of each microsite type by the area covered by that type, bulk density and horizon depth yielded estimates of N mineralization per unit area which then reflected microscale differences. With tree bases and 6Ocm annuli accounted for, the total N Table 5. Rates of N mineralization (mg N g-’ organic C day-‘) in four site types. Means followed by the same lower case letter were not significantly dimerent at P < 0.05 Site types Exposure period I 2 Pooled
I
2 Pooled
Horizon A A A B B B
No Tree Fra.thu 0.SS.d 0.34b 0.43ab -0.2Oc 0.23~ 0.04d
0.47b 0.33b 0.42ab osoa 0.74a 0.49a
Acer 0.5lab 0.40a 0.49a 0.36b 0.37b 0.35b
Fagur 0.40.~ 0.30b 0.37b 0.23b 0.14c 0.17c
Microscale soil chemistry patterns mineralization rate was 5437 mg m2 yr-’ , an increase of 8.7% over the No Tree estimate. Extending the annular radius to IOOcm increased the overall site estimate to 6017 mg m* of forest floor yr-‘, 20.2% greater than that for a site without tree base effects. ~e~~tio~~ips with en~iro~entu~ factors We used multiple regression to assemble empirical models of N mineralization rates among all site types. All four horizon-by-exposure period models were significant at P < 0.05 and 69-97% of the variance in N mineralization could be explained by regression on initial chemical conditions (Table 6). As was the case for initial conditions, the residual variation not explained by the model was lower for exposure period 1 than exposure period 2. The two soil factors most closely and consistently related to N mineralization rates were organic C (positive) and NO;- (negative). The relationship of pH to mineralization rate was more variable: a positive ~lationship during the first exposure period, and a negative or no relationship during the second. DISCUSSON
Sampling schemes for soil chemical properties and nutrient fluxes for whole forest stands may assume these properies are normally distributed in space, with residual variances unrelated to any nonrandomly distributed factors. Zinke (1962), Bratton (1976), Crozier and Boerner (1984, 1986) and ourselves have demonstrated that this assumption is not always warranted. Distinct differences in soil chemical properties often exist between sites near tree bases vs those away from trees, among sites under different tree species, as well as differences related to disturbances. As a result, many microsite types present in these forests have soil chemical properties which differ considerably from the mean values for those stands, especially if the sampling schemes used to generate those means do not explicitly address microscale patterns. A number of observations suggest that a major factor responsible for inducing the microsite differentiation we observed was stemflow, including the lower pH around the base of F. grand@iia (cf. Gersper and Holowaychuk, 1971; Crazier and Tabk 6. Multiple regression models of N mineralization for each horizon-time combination. The sign of the slope component for each significant model factor is air0 given EXpOSUtY period Horizon I
A
Mod.4 Coctlicicnt of significance determination P < o.ooo1
0.930
Significant model components Organic C (+
PO:-(-) ;;I (;) 2
A
P co.oo1
0.808
Organic C f +) PH (+)
1
B
P < o.ooo3
0.970
Moisture (+) NOj(-1 FH(-1
2
B
P < 0.035
0.687
Organic C(+) NO,- i-)
)
799
Boemer. 1986). higher PO:- values near the bases of trees producing little stemflow, i.e. Frarinus (cf. Crazier and Boemer, 1984), higher organic C near Acer bases (cf. McClaugherty et al., 1985), and higher inorganic N pool sizes near tree bases than away from trees (cf. Gersper and Holowaychuk, 1971). ~ffe~nti~ litter deposition and decay could also contribute to some of these differences. For example, Acer litter decays more quickly and contains higher concentrations of soluble sugars and N compounds than does the litter of Fagus or Q. bore&s (McClaugherty et al., 1985). As a result, if differential litter deposition or decay did occur around bases of trees of different species, this mechanism could potentially be as important as stemflow or throughfall in generating microsite differences. Though the proportion of total litter made up by Fugus and Q. borealis litter did vary among site types, we did not find significant differences in either total litter mass or the mass of litter of any major tree species in our microsites; thus on the basis of our data set, we cannot implicate differentiat litter deposition or decay as an important mechanism in this stand. As static pool sizes are only instantaneous representations of the surplus in the balance of inorganic N production vs uptake and loss, it is important to focus on among-site type differences in dynamic properties such as rates of mineralization and nitrification in order to establish the importance of these microscale patterns. A-horizon mineralization rates differed by as much as 50% among site types, though high within-type variability sometimes rendered these differences statistically non-signi~cant. They may well have been biologically signifi~nt nonetheless since as autumn measures they represent minimum levels of difference (cf. Plymale et al., 1987). The overall mineralization rates we observed were well within the ranges reported for other deciduous forests (Ellenberg, 1977; Pastor et al., 1984). Our multiple regression models accounted for 69-93% of the variance in mineralization rates on the basis of the same chemical factors as have other studies (e.g. Plymale et ul., 1987). As we observed no significant deviations from randomness in the residual variances from these multiple regressions, we have no evidence that chemical or physical factors unique to a single tree species are significantly influencing mineralization rates. For example, if higher concentrations of phenohc defensive compounds were accumulating near a tree base because of stemflow, differential litter deposition, decay or root exudation, we would expect samples from around the bases of trees of that species to have lower rates of mineralization than predicted by organic matter, N pool sizes or moisture alone. In no case did we observe such a bias in the residual variances, so we conclude that the differences in mineralization among site types are, to the limits of our data, related to differences in the soil chemical properties on which the regression models were based. We fee1 these small-scale patterns of variation in soil chemistry may have important implications at both within- and among-forest ecosystem scales.
RALPH
800
E. J. BOER.XER and SHARID. K~XLOWSKY
Broad-scale modeling of important ecosystem processes requires estimates of rates from a large number of sites (e.g. Meentemeyer, 1978). If the wholeecosystem estimates used to assemble these amongsystem models have inherent biases in them so will the overall model. One of the original motivations for carrying out this research was to determine how important biases in sampling might be in estimating overall ecosystem N turnover. Most studies we have reviewed either say little about the spatial pattern of sampling or refer simply to a random sampling scheme. As it is considerably easier to sample soils and perform in situ incubations away from trees than in a tangle of roots, such sampling schemes may consciously or unconsciously avoid sampling at the bases of trees. Our calculations indicate that ignoring microsites near tree bases could reduce the overall estimate of N mineralization on an area basis by as much as 20%, even in autumn. Thus for broad-scale studies, it is important to utilize a stratified-random design for sampling forest soils which explicitly accounts for within-stand patterns of microscale variation. Crozier and Boerner (1984, 1986) have demonstrated correlations between perennial herb distribution and soil chemical patterns associated with tree bases in a mixed mesophytic forest, and have attributed these soil chemical differences to stemflow effects. Persistence of these differences into the B-horizon may have implications for tree seedlings as well, as these plants quickly extend their root systems deeper into the soil than do perennial, rhizomatous herbs such as those discussed by Crozier and Boemer (1984). As the competitive success of various Quercus species seedlings is related as much to the nutrient status ofa microsite as its light regime (Racine, 1971). the resource mixes produced by both differences in light penetration through different species’ canopies and differences in static and dynamic soil nutrient properties produced by stemflow and other processes may produce a mix of resource site types which helps maintain diversity in these forests. Our results demonstrate that microsites exist at the bases of Acer. Fraxinus and Fagus trees, and in sites away from trees, and that these patterns of differences may establish a gradient of N availability within this forest stand from relatively high availability near Acer trees to relatively low N availability away from trees. The existence of this microscale gradient has not diminished our ability to predict whole-forest level rates of N mineralization on the basis of a relatively small number of chemical factors but it has established the requirement that we be conscious of these microscale gradients in establishing sampling programs or interpreting patterns of community structure. Acknowledgements-We thank Dr Jon Sanger and Ohio Wesleyan University for permission to work in the Bohannan Preserve. We also thank Do-soon Cho and Rick Yang for field assistance and Bob Madej for lab assistance.
REFERENCES Allison L. E. (1965) Organic C. In Merhodc of Soil Analwis (C. A. Black, ed.). pp. 1367-1378. American Society of Agronomy, Madison.
American
Public Health Association
(A.P.H.A.)
(1976)
Sfanakrd Methoak for the Examinarion of Wafer and Wasrewater, 14th Edn. American Public Health Associ-
ation, New York. Boemer R. E. J. (1984) Nutrient fluxes in litterfall and decomposition along a gradient of soil fertility in southern Ohio. Canadian Journal of Foresr Research 14, 794-802.
Boemer R. E. J. and Kooser J. G. (1989) Leaf titter redistribution among forest patches within an Allegheny Plateau watershed. Lrrndrcnpe Ecology 2(2), 81-92. Bratton S. P. (1976) Resource division in an understory herb community: responses to temporal and microtopographic gradients. The American Naruralisr 110, 679-693. Bremner J. M. (1965) Inorganic forms of N. In Method of Soil Analysis (C. A. Black, Ed.), pp. 1179-1206. American Society of Agronomy, Madison. Chapman H. D. (1965) Cation exchange capacity. In Metho& of Soil Analysis (C. A. Black, Ed.), pp. 891-895. American Society of Agronomy. Madison. Coleman D. C.. Reid C. P. and Cole C. V. (1983) Biological strategies of nutrient cycling in soil. Aduances in Ecological Research 13, l-55. Crazier C. R. and Boemer R. E. J. (1984) Correlations of understory herb distribution with microhabitats under different tree species in a mixed mesophytic forest. Oecologia 62, 337-343.
Crazier C. R. and Boemer R. E. J. (1986) Stemtlow induced soil nutrient heterogeneity in a mixed mesphytic forest. Barronia 52. l-8.
Eno C. F. (1960) Nitrate production in the field by incubating the soil in polyethylene bags. Soil Science Society of America
Proceedings
24, 277-279.
Gersper P. L. and Holowaychuk N. (1970) Some effects of stemflow from forest canopy trees on chemical properties of soils. Ecology 52, 691-702. McClaugherty C. A.. Pastor J.. Aber J. D. and Melillo J. M. (1985) Forest litter decomposition in relation to soil N dynamics and litter quality. Ecology 66, 266-275. Matanzo F.. Stone K. L. Jr, Thompson R. D. and Wenner K. H. (1969) Soil Survey of Delaware County, Ohio. U.S.D.A. Soil Conservation Service, Washington. Meentemeyer V. (1978) Macroclimate and lignin control of litter decomposition rates. Ecology 59, 465472. Nadelhoffer K. J.. Aber J. D. and Melillo J. M. (1983) Leaf-litter production and soil organic matter dynamics along a N availability gradient in southern Wisconsin (U.S.A.). Canadian Journal of Foresr Research 13, 12-21.
Parker G. G. (1983) Throughfall and stemllow in the forest nutrient cycle. Adoances in Ecological Research 13, 57-133. Pastor J., Aber J. D. and McClaugherty C. A. (1984) Aboveground production and N and P cycling along a N mineralization gradient on Blackhawk Island, Wisconsin. Ecology 65, 256-268. Peech M. (1965) Hydrogen-ion activity. In Merhoak of Soil Analysis (C. A. Black, Ed.), pp. 914-926. American Society of Agronomy, Madison. Pickett S. T. A. and White P. S. (1985) The Ecology of Narural Disturbance and Patch Dynamics. Academic Press, New York. Plymale A. E., Boerner R. E. J. and Logan T. A. (1987) Relative N mineralization and nitritication in soils of two contrasting hardwood forests: effects of site microclimate and initial soil chemistry. Forest Ecology and Monagemenr 21, 21-36.
Racine C. H. (1971) Reproduction of three species of oaks in relation to the vegetational and environmental gradients in the southern Blue Ridge. Bullerin of the Torrey Boranical Club 98, 297-3 IO.
Microscale soil chemistry patterns Robertson G. P. (I 982) Nitriftcation in forested ecosystems. Bulletin of Transactions 445-457.
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London 296,
Statistical Analysis Systems (1985) S.A.S. User’s Guide: Starirrics, Version 5. Statistical Analysis System, Cary NC. Vankat J. L., Anderson D. S. and Howell J. A. (1977) Plant communities and distribution factors in Abner’s Hollow, a south central Ohio watershed. Cusrmrea 42, 216-226.
801
Vitousek P. M. (1985) Community turnover and ecosystem nutrient dynamics. In The Ecology of Natural Disturbance and Parch Dynamics (S. T. A. Pickett and P. S. White, Eds), pp. 325-334. Academic Press, New York. Vitousek P. M., Goss, J. R.. Grier C. C., Melillo J. M. and Reiners W. A. (1982) A comparative analysis of potential nitrification and nitrate mobility in forest ecosystems. Ecological Monographs 52, I55-177.
Zinke P. J. (1962) The pattern of influence of individual forest trees on soil properties. Ecology 43, 130-133.
MICROSITE NITROGEN
0038-0717~89S3.00 + 0.00 CopyrightSC1989 Pergamon Pms pk
VARIATIONS IN SOIL CHEMISTRY AND MINERALIZATION IN A BEECH-MAPLE FOREST RALPH
E. J. B~ERNER and SHARI D. KOSLOWSKY’
Department of Botany, The Ohio State University, 1735 Nell Avenue, Columbus, OH 43210, U.S.A. (Accepted I5 January 1989) Summary-Though whole forest stand and ecosystem estimates of soil nitrogen dynamics are common, few studies have included measurements of within-stand variations in N mineralization and nitrification among microsites. We evaluated soil chemical status, net mineralization and nitrification potential during Autumn 1986 in A- and B-horizon soils taken from near the bases of Fagup grandi/olia (American beech). Acer sacchorum (sugar maple), and Fravincu unrericaae (white ash) trees and from sites > 2 m from trees using buried bag exposures. Fugus sites had significantly lower pH in both horizons than did other sites. Soils near Acer trees had higher organic C and NH: concentrations and Fraxinw soils had significantly higher PO:- availability. Moisture+ and NO; concentrations were similar among microsites. Using stepwise discriminant analysis, we constructed functions which differentiated among site types at P < 0.01 on the basis of these chemical measures. A-horizon N mineralization rates were generally 50% greater near Acer trees than away from trees, with Fugus and Fruxinur sites intermediate. B-horizon mineralization rates were highest under Fruxinus, but only 2-36% of A-horizon rates. Relative nitriftcation was approached 100% in all site types despite soil pH c 4. Calculation of N mineraIi~tion on an area basis with and without accounting for the differences among microsite types indicates that whole-ecosystem estimates of N minerali~tion generated by sampling schemes which do not explicitly include unique microsites around tree bases may be biased low by 820%.
INTRODUCTION
Our current understanding of how functional processes unite and coordinate the many components of terrestrial ecosystems is mainly based on ecosystemand watershed-level studies in which the focus has been on determining the rates of, and controls on, these processes at a whole-system level (e.g. Vitousek ef al., 1982). Such studies have produced estimates of N minerali~tion from organic matter and relative nitri~cation for a number of ecosystems and have subsequently been used to produce large-scale predictive models. In contrast, studies of within-ecosystem patterns often focus on smaller scale spatial and temporal variability for a particular process. For example, smaller scale patterns of soil nutrient status may occur within deciduous forest ecosystems as a result of variations in parent material distribution (Vankat er al., 1977). small scale disturbances (Vitousek, 1985). microclimate (Boerner, 1984) and differences in the volume and nutrient content of throughfall and stemflow under different tree species (Parker, 1983; Croder and Boemer, 1986). Quantifying microsite differences in soil properties and processes is important for two reasons. First, the most frequent disturbances in mesic forests (e.g. single treefall gaps, small mammal activity, etc.) and the spatial gradients along which forest plants are distributed are typically small in scale (Bratton, 1976; *Current address: Ohio Environmental Protection Agency, 1800 Watermark Drive, Columbus, OH 43215. U.S.A.
Crazier and Boerner, 1984; Pickett and White, 1985). Thus the smaller the scale of study, the closer one can estimate those factors which are important to germinating seeds, plant roots and soil fauna (Coleman ef al., 1983). Second, in a mature forest stand, microsites of various types may make up an appreciable portion of the ground area, thus making differences in the chemical properties of these microsites and the more uniform, more often sampled forest floor important for calculation and validation of whole-ecosystem means. Among the most studied microscale patterns of variation in deciduous forests are those associated with tree bases. The chemistry of surface soils around the bases of individuals of a given tree species may differ significantly from soils around other tree species or away from tree bases because of differences in the volume and chemistry of stemflow (Zinke, 1962; Gersper and Holowaychuk, 1970; Crozier and Boemer, 1986). differential rates of root exudation, or the decay of leaf masses with different nutrient concentrations or amounts of defensive compounds (e.g. tannins, lignin). Extractable amounts of PO:-, SOiand Ca*+, as well as both pH and base saturation have all been reported to differ significantly among soils near the bases of different tree species (Zinke, 1962; Gersper and Holowaychuk, 1970; Crozier and Boemer, 1986). As that suite of soil factors has been correlated with N mineralization rates and relative nitritication (e.g. Robertson, 1982; Nadelhoffer et al., 1983; Plymale er al.. 1987), we hypothesized that these soil processes would differ significantly among samples taken from near the bases of a variety of tree
795
796
RALPH
E. J. B~ERW and
species. If the variance among such samples was large, the utility of a whole-system mean for predicting within-system fluxes in this forest would be weakened. Our specific objectives, therefore, were to: (1) determine the nature and magnitude of differences in A- and B-horizon chemistry in plots around the bases of different tree species and away from trees, (2) determine the rates of N mineralization and relative nitrilication in these microsites, and (3) develop an empirical multiple regression model for rates of N turnover at the whole-stand level which takes these microscale variations into account.
SHARI
D.
KO.SLOWSKY
content was determined by Walkley-Black oxidation (Allison, 1965). Net nitrification was calculated as the differences between NO; content in subsamples extracted immediately and those extracted after exposure in situ. Net N mineralization was calculated as the sum of the differences between initial and final NO; and NH: content of paired bags. Relative nit~fication was calculated as the ratio of net nitrification to net N mineralization. To characterize the mass and species composition of litter at each of the sampling plots. we took two 0.25 rn’ litter samples next to each sampling point during April 1987. We chose this time for sampling as it was after the major period during which leaf MATERIALS AND METHOLX3 litter redistribution occurs (Boemer and Kooser, The Bohannan Preserve is a 40 ha mature hard1989) but before most mass and nutrient Ioss begins wood forest in eastern Delaware County, Ohio in this area (Boemer, 1984). (40’21’N, 82”X’W). Importance percentages (mean Differences in initial conditions and N mineralizof relative density and relative basal area in five ation were analyzed by analysis of variance and 0.04 ha plots) of the common species are Rcer SUC- covariance, fonowed by Ryan-Einot-Gabriel&rum : 40.3, Fraxims ume?ica~a : 24.9, Fag= gran - Welsch Multipie F tests. This post-test was chosen to difofia: 14.5 and Quercus borealis: 11.2; the site has minimize the probability of Type I errors (Statistical 35.7 m’ ha-’ basal area > lOcm diameter at breast Analysis System, 1985). Pairwise stepwise discrimiheight and has not been disturbed since approx. 1900. nant analysis was also used to resolve differences in ~nnington series soils (aeric ~hraqua~fs) have deinitial conditions among sites. MultipIe regression veloped on the carbonate-bearing glacial till substrawas used to develop empirical models for N mineraltum of the site (Matanzo PI al., 1969). ization among sites. Ail pH values were transformed All sampling was carried out within a 1 ha area of to hydrogen ion concentrations before analysis. level. relatively uniform topography. Sampling plots were established around the bases (annuius O-30 cm RESULTS from the rootcrown) of 5 randomly chosen canopy Initial conditions: A -lrori:on individuals of each of the three dominant tree species, and at 5 randomly chosen points > 2 m from any tree Soils taken from around the bases of Acer trees base. This set of sites was chosen to represent a had significantly higher concentrations of organic C range of potential stemflow influence with increasing and NH: than did soils from No Tree sites during the stemflow in the order: No Tree < Fruxinus < first exposure period; organic C and NH.,+ concenAcer < Fagus (Gersper and Holowaychuk, 1970; trations in Fugus and Fruxinus soils were intermediate Crozier and Boerner, 1986). At the beginning of each (Table I). In contrast, extractable PO:- concenexposure period, A- and B-horizon samples were trations were significantly higher in Fraxinus soils taken from each plot with a 2cm corer and sieved than in any other site type (Table I). Though we (~6 mm) to remove large roots, leaves and cobbles. observed the same patterns of organic C, NH:, and Each sample was divided into aliquots of approx. PO:- concentrations among site types during the 100-150 g; one aliquot was returned to the lab for second exposure period, the differences were not immediate extraction white one ~B-horizon) or two significant at P c 0.05. (A-horizon) aliquots were placed in polyethylene A-horizon pH was significantly lower in the Acer bags and buried for exposure at the depth from which and Fagus sites than in the Fraxinus and No Tree they were taken @no, 1960). Exposure periods began types during both exposure periods. There were no on 30 September and 22 October 1986 and lasted 22 significant differences in soit moisture or NO; pools and 27days. respectively. We chose to measure N among sites during either of the sampling periods dynamics in autumn because studies in this region (Table 1). Despite relatively low pH, 72-92% of the have shown that the magnitude of intersite differences extractable inorganic N was NO;. decreases from spring to autumn (Plymale ef al., We used stepwise discriminant analysis to deter1987). Thus, if significant differences existed in mine if combinations of these patterns of variation in autumn, we could he confident that such differences single soil chemical factors among site types produced existed throughout the growing season. an overall pattern of distinct soil chemical microsites. NO; and NHf were extracted from 4Og subDespite relatively high variation in some individual samples with 2 M KC1 (Bremner. 1965); PO:- was chemical factors within site types, the pairwise stepextracted with 1 M NH, OAc (Chapman, 1965). NO;. wise discriminant analysis constructed discriminant NH: and PO:- concentrations were determined functions which distinguished ail four site types from by the ultraviolet, phenate and stannous chloride each other at P c 0.01 or better (Table 2). Acer sites methods, respectively (American Public Health were distinguished from the others on the basis of Association, 1976). Soil pH was determined in a I:5 high organic C content, low pH and low PO:-. The slurry of oven-dried soil in lOmM CaCl, (Peech. discriminant functions which distinguished Fraxinus 1965). Soil moisture was determined by drying 5Og sites from the others were based on the high PO:- and subsamples at IIO”C to constant mass. Organic C pH of those sites, whereas the Fagus sites were
Microscale
soil chemistry
19-l
patterns
Table I. Means of initial soil conditions in sites established next to tree bases or away from any trees. Standard errors of the means are given in parenthcscs. Organic C is expressed in mg g-’ dry soil, PO:-, NH: and NO; in pmol IOOg-’ soil and moisture in percent by mass. Means followed by the same lower case letter were not significantly different Exposure period 1 No tree Organic C PO;
34.2b (2.1)
Fro.rinW
Act-r
Exposure period 2 No tree
Fr2gu.r
43.9ab
50.&a
(3.2)
(6.8)
A-horizon 44.&b 30.la (3.9) (1.4)
0.79a
Fr0.rillU.S
Acer
Fagus
32.8a (0.4)
42.4a
(0.6)
45.3a (5.8)
0.26b (0.02)
(0.08)
0.16b (0.01)
0.19b (0.01)
0.7Oa (0.16)
I .03a (0.08)
0.74a (0.09)
0.86a (0.02)
NH;
I .89b (0.14)
2.45b (0.19)
4.OSa (OSI)
2.53b (0.24)
1.82a (0.16,
l.78a (0.66)
2.27a (0.33)
2.45a (0.38)
NO,
4.8a (0.7)
I2Sa (2.9)
11.9a (0.7)
7.8a (1.0)
II.4a (1.5)
19.la (5.0)
19.0 (4.0)
l9.6a (3.1)
Moisture
36.7a (0.9)
38Sa (2.1)
38. la (1.4)
35.7a (1.5)
35.6a (1.1)
32.6a (0.8)
31.8a (2.1)
37.2a (0.7)
4.41a
PH
4.I2a
3.23b
3.46b
4.29a
7.4b (0.9)
12.5ab (1.6)
2l.k (3.9)
B-horizon 6.2b 18.9a (3.4) (0.7)
0.14a (0.03)
0.45a (0.12)
0.33a (0.09)
(0.W
NH;
0.73b (0.12)
I .09ab (0.07)
I .42a (0.23)
NO;
3.5a (0.8)
3.3a (0.9)
Moisture
22.2a (0.5)
25.3a (0.7)
Organic C PO:
3.50a
DH
3.5Oa
4.02a
I I .4ab
3.67b
3.73b
(1.3)
15.6a (1.5)
lS.lab (1.9)
I .OOa (0.13)
0.97a (0.11)
l.l2a (0.13)
0.96a (0.05)
I .30ab (0.13)
0.8la (0.13)
0.88a (0.09)
1.13a (0.19)
I .24a (0.14)
8.Oa (1.5)
S.Oa (1.4)
2.4a (1.1)
6.0a (0.6)
9.2a (2.0)
7.9a (0.8)
27.6a (1.4)
26.la (0.8)
23.6a (1.0)
24.6a (0.8)
27.7a (2.2)
25.9a (1.3)
3.5Sa
distinguished by their low pH (Table 2). The discriminant functions for No Tree sites were based on low organic C, high pH and low inorganic N concentrations (Table 2). Thus, when the soil nutrient status of these four sites analyzed in a multivariate fashion, each was easily resolved as a unique microsite. Initial conditions: B-horizon
Many of the patterns of variation in B-horizon soils among site types paralleled those of the A-
0.27a
3.35a
3SSb
3.72a
3.6lb
horizon, but at lower absolute values (Table I). Acer sites again had the highest organic C and NH: concentrations, and No Tree sites the lowest. PO:and NO; concentrations were lower than in the A-horizon, and no significant difierences among site types were observed. An average of 75-98% of the extractable inorganic N was NO; despite pH < 3.75 in all sites. Once again, stepwise discriminant analysis revealed significant differences in the overall chemical status of
Table 2. Results of stepwix discriminant analyses of initial conditions among all combinations of sites. The F-statistic for Wilk’s 1. and the soil factors used in calculating the discriminant function. are given for each pair. l = discriminant function significant at P < 0.01, l * = P < 0.001. l ** = P < 0.0001. Soil factors are C = organic C content. PO4 = extractable PO:-, H = hydrogen ion activity, NH4 = initial NH:. NO3 = initial NO, Site types No Tree Fra.rinus
F = 13.91”’ C. P04. H, NO3
A-horizon
-
Act-r
F = 17.16*** NH4. C. H
F = 10.24** P04. H
-
Fugus
F = 50.08*** H No Tree
F = 18.93*** P04.H Fmxinw
Fz7.44’ NH4, H Acer
Fagus
B-horizon
Fagus
F = 19.93” C F = 26.81”’ C F = 13.35*** NH4. C. H No Tree
3.4Sb
F = 5.37’ C. NO3 F = 10.22*” NH4, H, NO3 Fmxinuc
F=6.21* H Act-r
Fagus
798
RALPH
E. J. B~ERNEII and SHARI D. Kosrows~v
these four site types (Table 2). Fugus sites were distinguished from the others by high NH,+ concentrations and low pH. and Fruxinus sites were distinguished on the basis of low NO;, low organic C and high pH (Table 2). The discriminant function which separated Acer sites from the others was based on high organic C content. As in the A-horizon, No Tree sites were discriminated from the others by their lower concentrations of NH.,+, organic C and NO;. Thus, not only were the A- and B-horizon soils near the bases of these three tree species easily discriminated from soils away from trees on the basis of a suite of soil chemical factors, the No Tree sites always had the lowest absolute concentrations of nutrients, regardless of horizon.
Tabk 4. Rates of N minenliitioa (mg N 100 g-’ soil day-‘) in four site types. Standard errors of the means are given in parentheses. Means followed by the same lower case letter were not significantly diliercnt at P < 0.05 Site types Exposure period
Horizon
I
A
2
A
Pooled
A
1
B
2
B
Pooled
B
No Tree
There were no significant differences in the total dry mass of leaf litter or the proportion of Acer or Fraxinus litter among the various site types (Table 3). There were significantly lower proportions of Fugus and Q. boreaLis litter near the bases of Acer trees than any other species (Table 3); when these proportions were converted to absolute mass, these differences were no longer significant. Rates of N mineralization
and relative nitrifcation
Three-way Analysis of Variance indicated that rates of N mineralization were significantly higher during the first exposure period (mean air temperature 14. I “C) than during the second (mean air temperature 8.7’C) (P < O.Ol), and were significantly higher overall in A- than B-horizons (P ~0.01) (Table 4). There were, however, no significant differences among site types in either horizon or exposure period. When time periods were pooled and air temperature used as a covariate in l-way Analysis of Variance by horizon, the differences among site types did become significant at P c 0.05, with Acer sites having significantly higher rates than No Tree sites in both horizons and Fagus sites in the B-horizon (Table 4). The means and variances of the site types were strongly correlated: Acer sites had the highest mean N mineralization rate and the highest variance (Table 4). When N mineralization rates were converted to a g-l organic C basis, A-horizon Acer sites Table 3. Total mass (g dry mass 0.25 m-*) and proportions of leaf liwr of various species in plots near the bases of four site types. Standard errors of the means are given in parentheses. For factors where analysis of variance on log transformed proportions indicated significant differences at P < 0.05. means with ditTercnt lower case letters were significantly different. N = IO for each species No Tree
/+l7Xi~uS
Acer
Fagw
Dry mass
91.6 (8.9)
85.3 (8.0)
107.6 (8.2)
110.2 (11.0)
% Arer litter
30.8 (6.1)
22.3 (2.4)
35.0 (4.4)
24.2 (2.5)
l4.la (3.8)
I7.2a (I.81
7.4b (2.1)
18.7a (I.21
% Froxinus litter
3.8 (1.1)
10.8 (2.4)
IS.0 (6.2)
8.6 (2.7)
% Qurrcus borealis litter
l9.0a (4.6)
9.0a (1.4)
4.3b (4.3)
8.6a (0.8)
% Fagus litter
Act-r
Fagw 1.79 (0.24) I .34 (0.14)
2.05 (0.50) 1.08 (0.13)
2.59 (0.8 I) 1.69 (0.26)
1.37 (0.10) -0.1s (0.02) 0.14 (0.07)
1.62 (0.22) 0.62 (0.12) 0.54 (0.07)
1.85 (0.49) 0.76 (0.21) 0.57 (0.12)
0.43 (0. IO) 0.21 (0.17)
0.59ab (0.08)
0.64a (0.09)
0.29bc (0.04)
O.Ok (0.04)
initial conditions: leaf litter mass and composition
Fra.zinw
I .a9 (0.12) 1.02 (0.08)
(A::,
had significantly higher N mineralization rates than Fagus sites; in contrast, in the B-horizon, soils under Fruxinus trees mineralized a larger proportion of the available organic matter than did soils from the other site types (Table 5). Means of relative nitrification among site types and horizons varied between 95-105%; no mean differed significantly from 100% at P < 0.05. Thus any NH: not immediately immobilized by microbes was nitrified despite the low pH. To convert our mass-based N mineralization rates lo an area basis, we multiplied them by bulk density and horizon depths (data from Matanzo et al., 1969). From this we estimate A + B-horizon N mineralization in a site with no trees to be 5005 mg m-* of forest floor yr (A-horizon: 4829, B-horizon: 176). This estimate should reflect what would result from a sampling program which consciously or unconsciously avoided sampling at tree bases. To determine how incorporating these tree base microsites might effect this estimate, we calculated the forest floor area occupied by tree trunks plus annuli with 60 or 100 cm radius. These radii were chosen to represent common distances for stemflow effects from medium-sized trees (Zinke, 1962; Gersper and Holowaychuk, 1971, Crozier and Boerner, 1986). Tree bases plus 60 cm annuli covered approx. 683 m* ha-’ (6.8%) of the forest floor in our study site; extending the annular radius to 100 cm increased this to approx. 1600 m* ha-’ (16.0%). Multiplying the N mineralization rate of each microsite type by the area covered by that type, bulk density and horizon depth yielded estimates of N mineralization per unit area which then reflected microscale differences. With tree bases and 6Ocm annuli accounted for, the total N Table 5. Rates of N mineralization (mg N g-’ organic C day-‘) in four site types. Means followed by the same lower case letter were not significantly dimerent at P < 0.05 Site types Exposure period I 2 Pooled
I
2 Pooled
Horizon A A A B B B
No Tree Fra.thu 0.SS.d 0.34b 0.43ab -0.2Oc 0.23~ 0.04d
0.47b 0.33b 0.42ab osoa 0.74a 0.49a
Acer 0.5lab 0.40a 0.49a 0.36b 0.37b 0.35b
Fagur 0.40.~ 0.30b 0.37b 0.23b 0.14c 0.17c
Microscale soil chemistry patterns mineralization rate was 5437 mg m2 yr-’ , an increase of 8.7% over the No Tree estimate. Extending the annular radius to IOOcm increased the overall site estimate to 6017 mg m* of forest floor yr-‘, 20.2% greater than that for a site without tree base effects. ~e~~tio~~ips with en~iro~entu~ factors We used multiple regression to assemble empirical models of N mineralization rates among all site types. All four horizon-by-exposure period models were significant at P < 0.05 and 69-97% of the variance in N mineralization could be explained by regression on initial chemical conditions (Table 6). As was the case for initial conditions, the residual variation not explained by the model was lower for exposure period 1 than exposure period 2. The two soil factors most closely and consistently related to N mineralization rates were organic C (positive) and NO;- (negative). The relationship of pH to mineralization rate was more variable: a positive ~lationship during the first exposure period, and a negative or no relationship during the second. DISCUSSON
Sampling schemes for soil chemical properties and nutrient fluxes for whole forest stands may assume these properies are normally distributed in space, with residual variances unrelated to any nonrandomly distributed factors. Zinke (1962), Bratton (1976), Crozier and Boerner (1984, 1986) and ourselves have demonstrated that this assumption is not always warranted. Distinct differences in soil chemical properties often exist between sites near tree bases vs those away from trees, among sites under different tree species, as well as differences related to disturbances. As a result, many microsite types present in these forests have soil chemical properties which differ considerably from the mean values for those stands, especially if the sampling schemes used to generate those means do not explicitly address microscale patterns. A number of observations suggest that a major factor responsible for inducing the microsite differentiation we observed was stemflow, including the lower pH around the base of F. grand@iia (cf. Gersper and Holowaychuk, 1971; Crazier and Tabk 6. Multiple regression models of N mineralization for each horizon-time combination. The sign of the slope component for each significant model factor is air0 given EXpOSUtY period Horizon I
A
Mod.4 Coctlicicnt of significance determination P < o.ooo1
0.930
Significant model components Organic C (+
PO:-(-) ;;I (;) 2
A
P co.oo1
0.808
Organic C f +) PH (+)
1
B
P < o.ooo3
0.970
Moisture (+) NOj(-1 FH(-1
2
B
P < 0.035
0.687
Organic C(+) NO,- i-)
)
799
Boemer. 1986). higher PO:- values near the bases of trees producing little stemflow, i.e. Frarinus (cf. Crazier and Boemer, 1984), higher organic C near Acer bases (cf. McClaugherty et al., 1985), and higher inorganic N pool sizes near tree bases than away from trees (cf. Gersper and Holowaychuk, 1971). ~ffe~nti~ litter deposition and decay could also contribute to some of these differences. For example, Acer litter decays more quickly and contains higher concentrations of soluble sugars and N compounds than does the litter of Fagus or Q. bore&s (McClaugherty et al., 1985). As a result, if differential litter deposition or decay did occur around bases of trees of different species, this mechanism could potentially be as important as stemflow or throughfall in generating microsite differences. Though the proportion of total litter made up by Fugus and Q. borealis litter did vary among site types, we did not find significant differences in either total litter mass or the mass of litter of any major tree species in our microsites; thus on the basis of our data set, we cannot implicate differentiat litter deposition or decay as an important mechanism in this stand. As static pool sizes are only instantaneous representations of the surplus in the balance of inorganic N production vs uptake and loss, it is important to focus on among-site type differences in dynamic properties such as rates of mineralization and nitrification in order to establish the importance of these microscale patterns. A-horizon mineralization rates differed by as much as 50% among site types, though high within-type variability sometimes rendered these differences statistically non-signi~cant. They may well have been biologically signifi~nt nonetheless since as autumn measures they represent minimum levels of difference (cf. Plymale et al., 1987). The overall mineralization rates we observed were well within the ranges reported for other deciduous forests (Ellenberg, 1977; Pastor et al., 1984). Our multiple regression models accounted for 69-93% of the variance in mineralization rates on the basis of the same chemical factors as have other studies (e.g. Plymale et ul., 1987). As we observed no significant deviations from randomness in the residual variances from these multiple regressions, we have no evidence that chemical or physical factors unique to a single tree species are significantly influencing mineralization rates. For example, if higher concentrations of phenohc defensive compounds were accumulating near a tree base because of stemflow, differential litter deposition, decay or root exudation, we would expect samples from around the bases of trees of that species to have lower rates of mineralization than predicted by organic matter, N pool sizes or moisture alone. In no case did we observe such a bias in the residual variances, so we conclude that the differences in mineralization among site types are, to the limits of our data, related to differences in the soil chemical properties on which the regression models were based. We fee1 these small-scale patterns of variation in soil chemistry may have important implications at both within- and among-forest ecosystem scales.
RALPH
800
E. J. BOER.XER and SHARID. K~XLOWSKY
Broad-scale modeling of important ecosystem processes requires estimates of rates from a large number of sites (e.g. Meentemeyer, 1978). If the wholeecosystem estimates used to assemble these amongsystem models have inherent biases in them so will the overall model. One of the original motivations for carrying out this research was to determine how important biases in sampling might be in estimating overall ecosystem N turnover. Most studies we have reviewed either say little about the spatial pattern of sampling or refer simply to a random sampling scheme. As it is considerably easier to sample soils and perform in situ incubations away from trees than in a tangle of roots, such sampling schemes may consciously or unconsciously avoid sampling at the bases of trees. Our calculations indicate that ignoring microsites near tree bases could reduce the overall estimate of N mineralization on an area basis by as much as 20%, even in autumn. Thus for broad-scale studies, it is important to utilize a stratified-random design for sampling forest soils which explicitly accounts for within-stand patterns of microscale variation. Crozier and Boerner (1984, 1986) have demonstrated correlations between perennial herb distribution and soil chemical patterns associated with tree bases in a mixed mesophytic forest, and have attributed these soil chemical differences to stemflow effects. Persistence of these differences into the B-horizon may have implications for tree seedlings as well, as these plants quickly extend their root systems deeper into the soil than do perennial, rhizomatous herbs such as those discussed by Crozier and Boemer (1984). As the competitive success of various Quercus species seedlings is related as much to the nutrient status ofa microsite as its light regime (Racine, 1971). the resource mixes produced by both differences in light penetration through different species’ canopies and differences in static and dynamic soil nutrient properties produced by stemflow and other processes may produce a mix of resource site types which helps maintain diversity in these forests. Our results demonstrate that microsites exist at the bases of Acer. Fraxinus and Fagus trees, and in sites away from trees, and that these patterns of differences may establish a gradient of N availability within this forest stand from relatively high availability near Acer trees to relatively low N availability away from trees. The existence of this microscale gradient has not diminished our ability to predict whole-forest level rates of N mineralization on the basis of a relatively small number of chemical factors but it has established the requirement that we be conscious of these microscale gradients in establishing sampling programs or interpreting patterns of community structure. Acknowledgements-We thank Dr Jon Sanger and Ohio Wesleyan University for permission to work in the Bohannan Preserve. We also thank Do-soon Cho and Rick Yang for field assistance and Bob Madej for lab assistance.
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