Estimating Soil Nitrogen and Carbon Pools in a Northern Hardwood Forest EcosystemT.G. HUNTINGTON,* D.F. RYAN, AND S.P. HAMBURG

ABSTRACTAn intensive soil sampling design was evaluated to determine what

resolution could be obtained in N and C pool size estimates in anorthern hardwood forest soil. Pits of measured volume were ex-cavated by horizon in the forest floor and in three depth strata inthe mineral soil. Future comparisons should be able to detect dif-ferences in N and C pool sizes ranging from 8 to 25% of the observedmean values depending upon the element and depth strata. Futuresampling should detect changes of 230 and 130 kg N ha~' in theforest floor (combined O horizons) and 0- to 10-cm stratum in themineral soil respectively. Similarly, changes of 5.9 and 2.4 Mg Cha~' should be detectable for forest floor and 0 to 10 cm pools re-spectively. Soil N content for the forest floor was 1300 kg N ha~'.For the mineral soil depth strata (0-10 cm, 10-20 cm, 20 cm to thebottom of the B horizon), N contents were 1600, 1200 and 3100 kgN ha ' respectively. Total solum N content was estimated to be 7200kg N ha '. Soil C contents for the combined O horizons, 0- to10-, 10- to 20- and 20-cm strata were 30, 32, 27 and 73 Mg Cha~' respectively. The total solum C content was estimated to be160 Mg C ha '. Concentrations of soil N and C were positivelycorrelated with elevation over the 240 m range studied, but soil poolsof N and C were not correlated with elevation or soil mapping unit.

T.G. Huntington, Dep. of Geology, Univ. of Pennsylvania, 240 S.33rd St., Philadelphia, PA 19104; D.F. Ryan, Morris Arboretum,Univ. of Pennsylvania, Philadelphia, PA 19118; and S.P. Hamburg,Dep. of Systematics and Ecology, Hay worth Hall, Univ. of Kansas,Lawrence, KS 66045. Contribution of the Morris Arboretum. Re-ceived 22 May 1987. Corresponding author.Published in Soil Sci. Soc. Am. J. 52:1162-1167 (1988).

THE LONG TERM ECOLOGICAL COnSCQUCnCCS of for-est management practices upon site productivity

and soil fertility have been the subject of many recentinvestigations (Freedman et al., 1986; Johnson et al.,1982; Hornbeck et al., 1986). Most studies focusingon nutrient removal estimated total solum N but didnot characterize element pool sizes with the resolutionnecessary to detect changes within soil compartmentsresulting from disturbance. Major disturbances suchas timber harvesting are thought to reorganize nu-trients within ecosystem pools (Bormann and Likens,1979; Snyder and Harter, 1985).

Process level studies of N and C flux within the soiland between soil and other ecosystem pools are usu-ally undertaken on a plot scale and extrapolated to awatershed scale. Extrapolations of these rate studiesmay be verified if rates projected over time matchchanges measured in pools. Thus, precise estimates ofpool sizes are necessary to verify process level ratestudies. Pool size estimates also can be used to com-pare soil N and C reserves with system inputs such asprecipitation, N fixation or C assimilation, and out-puts such as harvest removal, stream water losses anddenitrification (Gessel et al., 1973; Henderson andHarris, 1975).

Estimating pool sizes on a watershed scale is com-plicated by enormous spatial variability both horizon-tally and vertically. Heterogeneity in soil thickness,

HUNTINGTON ET AL.: ESTIMATING SOIL NITROGEN AND CARBON POOLS 1163

Table 1. Descriptions of soil mapping units present on Watershed 5 in the Hubbard Brook Exp. For.t

Soilmappingunit

Tun-Lyman§

BerkshireSkerryBeckettLyme

Taxonomic class

Coarse-loamy to Loamy, mixed frigidTypic to Lithic Haplorthods

Coarse-loamy, mixed frigid Typic HaplorthodsCoarse-loamy, mixed frigid Aquic HaplorthodsCoarse-loamy, mixed frigid Typic HaplorthodsCoarse loamy, mixed frigid Aerie Haplaquepts

Numberof pits

32

20620

Rangesolumn

thickness

1-100

22-10260-8849-61

Meansolumn

thickness

48

606966

Proportionof surface

area

%54

3310

21

Mean horizonthickness}

E Bh Bsl

cm3.55 4.07 4.89

4.44 4.01 6.08

t All thickness are means of measurements taken in soil pits excavated within the corresponding units.J Horizon thicknesses are based on approximately 60 m of mapped pit faces from 21 randomly selected pits (C. Johnson, 1986, unpublished data). Rock

thicknesses are not included in the horizon thicknesses. One standard deviation about these mean values ranged from 30 to 55% of the reported mean value.§ The Tunbridge and Lyman soil series were mapped together as the Tunbridge-Lyman Rock Outcrop Complex (Tun-Lyman).

hydrplogic regime and topography may result in amosaic of different soil types. In mountainous forestedlandscapes windthrow inverts and mixes genetic ho-rizons resulting in a complex pit and mound micro-topography that further increases variability.

Our objective in this study was to determine whetheran intensive sampling design could achieve pool sizeestimates with sufficiently small confidence limits thatchanges as small as 20% of the means could be de-tected. Changes of this magnitude are possible basedon current understanding of soil response to harvestimpacts. We measured the distribution of N and Cwithin solum compartments and in relation to ele-vation and soil mapping unit.

METHODSSite Description

The study was conducted at the Hubbard Brook Exp. For.in New Hampshire. The study watershed (W-5) has an areaof 23 ha, a southeast aspect, an average slope of 20% andan elevation range of 510 to 750 m.

At the lower and mid elevations sugar maple (Acer sac-charum Marsh.), beech (Fagus grandifolia Ehrh.), and yel-low birgh (Betula alleghaniensis Britt.) were the dominantspecies. At the highest elevations (>700 m) beech, sugarmaple, balsam fir [Abies balsamea (L.) Mill.], paper birch(Betula payrifera Marsh.), yellow birch and red spruce (Picearubens Sarg.) were present. A limited selective cut of spruceprobably was carried out on the watershed in the latter partof the 1800s followed by an intensive cutting of softwoodsand hardwoods ending in 1918 (Bormann and Likens, 1979).There is no evidence of agricultural disturbance in theseforests, either by aboriginal peoples or in the historic period.At the time of our sampling in 1983, the watershed wasdominated by a 65-yr old, second-growth northern hard-wood forest.

Soils were mapped using a second order survey conductedby the Soil Conservation Service in 1983, prior to intensivesoil sampling (Fig. 1). Five soil mapping units were distin-guished on the watershed including Tunbridge-Lyman RockOutcrop Complex, Berkshire, Skerry, Beckett, and Lyme(Table 1, Fig. 1). Soil profile descriptions for the series rep-resented in these mapping units are available from the SoilConservation Service (Grafton County Soil Survey1). Thesesoils developed from glacially deposited, stony till primarilyderived from local bedrock, i.e. Littleton gneiss and Kins-man quartz monzonite.

' The Grafton County Soil Survey, Grafton County, New Hamp-shire, is scheduled for publication in 1991. Complete soil descrip-tions are currently available from the Soil Conservation Service,Woodsville, NH.

Field SamplingPrior to sampling, the watershed was surveyed and per-

manently marked into a 25- by 25-m grid which resulted in360 25- by 25-m plots (Fig. 1). Sixty plots were selected usinga stratified random design. Plots were stratified in six ele-vation bands to insure adequate representation of the ele-vation gradient, soil mapping units, topography and treespecies. The number of plots per band was weighted by theproportion of the watershed represented by the elevationband. Soil pits were located within plots by the intersectionof two coordinates randomly selected along the plot axes.Near each pit location, three additional sites which were atleast 3- but less than 6-m apart, along the contour and within

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LEGEND

Soil pitStreamElevationSoil type

——— Watershed boundary

100 200-1 I

Scale in meters

Fig. 1. Map of the study area showing elevation contours, samplelocations, the 25- by 25-m grid system, and soil mapping units:TUN = Tunbridge-Lyman Rock Outcrop Complex; BER = Berk-shire; SKE = Skerry; and BEC = Beckett.

1164 SOIL SCI. SOC. AM. J., VOL. 52, 1988

Table 2. Soil N and C contents, C/N ratio, mass and thickness by depth stratum from Watershed 5 at the Hubbard Brook Exp Forin July 1983.T

Depth stratum Content MDC§ Content MDC Thickness Mass C/N1

-kg N ha-' ——— —— Mg C ha"1 ——Horizon or depth strata

Mg ha-'

Oi + OeOa0-10 cm10-20 cm>20 cm#

Forest floorMineral soilSolum

5458595959

555955

460 (38)870 (100)

1600 (65)1200 (71)3100 (300)

1300 (120)5900 (370)7200 (370)

74200130140580

240730730

11 (1.0)20 (2.5)32 (1.2)27 (1.6)73 (6.8)

Totals30 (3.0)

130 (7.7)160 (8.2)

2.04.92.43.1

13

5.91516

9.79.3

35.0

7.1 (0.46)54 (3.0)

22 (1.7)66 (7.2)

490 (23)520 (34)

2200 (180)

88 (8.3)3200 (210)3300 (210)

23 (0.31)22 (0.40)20 (0.31)23 (0.48)23 (0.45)

232222

t Numbers in parentheses are standard errors of the means.t The sample size (n) is variable and not equal to 60 because samples from certain pits were lost or combined and were therefore not included in the analyses.§ MDC is the minimum detectable change required to measure a significant (p = 0.05) difference between the corresponding mean value and a value obtained

by resampling in the future using the same method.1 C/N is the ratio of carbon to nitrogen.# Depth strata a 20 represents the mineral soil from 20 cm to the bottom of the B horizon.

the plot were selected with similar microtopography. One ofthe four matched locations was then selected randomly forimmediate excavation. We used this procedure to allow threeunbiased but paired excavations in the future.

Pits were excavated using a modification of the proceduresdescribed by Hamburg (1984). A reference frame (71- by 71-cm inside dimension) was secured parallel to the soil surfacewith steel rods and the area within excavated. The Oi andOe horizons were excavated together and the Oa horizonwas removed separately. The separation of the Oa from theunderlying mineral soil was based on a visual estimation ofthe organic matter content, with all material >40% organicmatter being included in the Oa.

Our intention was to sample the mineral soil by genetichorizons but, because of the high frequency of buried, in-verted and mixed horizons, diffuse boundaries and highlyvariable horizon thicknesses we decided that this was im-practical. We concluded that sampling by depth strata wouldbe more repeatable. The mineral soil was excavated in threedepth strata designated nominally as 0 to 10, 10 to 20, and5*20 cm (20-cm depth to bottom of B horizon). The actualdepth of excavation for the O horizons and each mineralsoil stratum was measured relative to the plane of the ref-erence frame on a 14- by 14-cm grid centered within theframe. The pit walls were maintained perpendicular to theplane of the reference frame during excavation. Material fromeach mineral stratum was sieved through 12.5 mm screeningin the field. The material passing through the sieve wasmixed, weighed and subsampled in the field. Samples werealso collected from horizons on the faces of 30 pits.Laboratory Procedures

Mineral soil samples were air-dried, sieved to 2 mm andweighed. Samples were ground to pass 100 mesh and oven-Table 3. Soil N and C concentration by depth stratum on Water-

shed 5 at the Hubbard Brook Exp. For, in July 1983. t

Depth stratum n} N cone. C cone.

Oi + OeOa0-10 cm10-20 cm£20 cm§

5457595452

—————— g]20.6 (0.40)13.6 (0.51)3.6 (0.25)2.5 (0.18)1.6 (0.10)

465 (7.9)296 (13)

72 (4.3)55 (3.2)36 (2.6)

t Numbers in parentheses are standard errors of the means.t The sample size n is the actual number of samples analyzed. In some pits,

bedrock or C material was encounterd within 20, or even 10 cm, so nosamples of the deeper stratum were collected.

§ Depth stratum a 20 represents the mineral soil from 20 cm to the bot-tom of the B horizon.

dried to constant weight at 105 °C. Forest floor samples wereair-dried and sieved through 6.25-mm mesh and weighed.These samples were ground in a Wiley Mill to pass 60 meshand dried to constant weight at 60 C. All results for bothmineral and organic strata were reported on an oven dryweight basis (constant weight 105 °C).

Nitrogen and C concentrations were determined using anautomated C-N analyzer (NA-1500 Erba Instruments, Inc.,Peabody, MA) employing a modified Dumas combustionmethod. The N content of an individual depth stratum wascalculated by multiplying the N concentration by the soilweight for that stratum. The N contents of each depth stra-tum within a pit were summed to obtain an areal estimatefor the pit and the mean of all pits was expressed on a hec-tare basis.

In this paper we report total soil C, which in these acidicforest soils is equivalent to total organic C. Soil organic mat-ter may be estimated using an appropriate conversion factor.Total C is preferable to soil organic matter as an ecosystempool because total C can be more accurately measured (Nel-son and Sommers, 1982).

RESULTSThe minimum detectable changes (p = 0.05) (MDC)

in N and C pool sizes were estimated from the stan-dard errors (SE) of the means with the formula

MDC = 1.96 X SE. [1]This calculation is based on the assumptions of re-peated sampling by the same method, normal distri-bution and equal underlying variance. The MDC forthe forest floor and the 0- to 10-cm depth stratumTable 4. Soil N and C concentrations and C/N ratio for samples

collected from horizons exposed on the pit faces from Water-shed 5 at the Hubbard Brook Exp. For. in July 1983. t

Soil horizon N cone. C cone. C/N

gkg-EBhBslBs2C

2324242742

1.2 (0.17)bJ3.4 (0.20)a2.8 (0.14)a1.5 (0.13)b0.6 (0.07)b

25 (3.7)b68 (4.6)a63 (3.7)a36 (3.7)b14 (2.4)b

21 (0.77)20 (0.37)22 (0.46)23 (0.63)22 (0.88)

t Numbers in parentheses are standard errors of the means.I Mean values within columns which are followed by the same letter are

not significantly different (p = 0.05) using Tukey's multiple pairwise meancomparison test.

HUNTINGTON ET AL.: ESTIMATING SOIL NITROGEN AND CARBON POOLS 1165

Table 5. Pearson correlation coefficients indicating the relation-ships between N concentration, N content and soil mass withelevation by depth strata, t

Table 6. Soil pools of N and C and oven-dry mass mean valuesby elevation and soil stratum for Watershed 5 at the HubbardBrook Exp. For. sampled in July 1983.t

Depth stratum

Oi + OeOa0-10 cm10-20 cma 20 cm

N cone. vs.elevation

0.110.36**t0.30*0.37*0.47**

Soil mass vs.elevation

-0.02-0.01-0.30*-0.36**-0.27*

Soil N contentvs. elevation

-0.01-0.11

0.050.060.18

t Each sampling location was assigned an elevation corresponding to theclosest 10-m contour interval.

J Pearson correlation coefficients followed by * or ** differ significantlyfrom zero at p = 0.05 and 0.01, respectively.

should be 240 and 130 kg N ha"1 respectively (Table2). Similarly, changes of 5.9 and 2.4 Mg C ha~' shouldbe significant for the forest floor and 0- to 10-cm depthstratum respectively.

The concentration of N in the forest floor averaged20.6 and 13.6 g N kg-' in the Oi + Oe and Oa ho-rizons respectively (Table 3). The concentration of Nin the mineral soil decreased consistently with soildepth from 13.6 g N kg"1 in the 0- to 10-cm stratumto 1.6 g N kg"1 in the 3* 20-cm stratum. The analysisof mineral soil horizons from pit faces also showed adecline in N concentration with depth, with the ex-ception of the E horizon (Table 4).

The mean N content for the combined O horizons(forest floor) was 1300 kg N ha~' with a standard error(SE) of ± 120 kg N ha"1 (Table 2). The N content ofthe mineral soil was 5900 ± 370 kg N ha~'. The totalsolum N content was 7200 ± 370 kg N ha~'. The Ncontent of the 0- to 10-cm stratum was 34% higherthan the 10- to 20-cm stratum. The pool of N heldbelow 20 cm is greater than the combined contents ofthe 0- to 10- and 10- to 20-cm strata.

Pool sizes of N and C, in individual depth strataand in the total solum, were not correlated with ele-vation or soil mapping unit (Tables 5, 6, and 7). Ni-trogen concentration was positively correlated with el-evation in all strata except the combined Oi + Oehorizons while soil mass was negatively correlated withelevation (Table 5). The Tunbridge-Lyman Rock Out-crop Complex, which occupies the top third of theelevation gradient, is described as having a shallowerB horizon and being shallower to bedrock than theBerkshire or Skerry soil mapping units which domi-nate the mid and lower elevations of the watershed(Grafton County Soil Survey1). In spite of differencesin total solum thickness the two dominant soil map-ping units had mean thicknesses in the E and Bh ho-rizons which were very similar (Table 1).

The C concentration of the combined Oi + Oe ho-rizons and Oa horizon was 465 and 296 g C kg"1 re-spectively (Table 3). The concentration of C in themineral soil declined consistently with depth from 72in the 0- to 10-cm stratum to 36 g C kg~' for the > 20-cm stratum. The pit face sampling by horizon alsoindicated a decrease in C concentration with depth,with the exception of the E horizon (Table 4).

The mean C content for the combined O horizonswas 30 ± 3.0 Mg C ha"1 (Table 2). The C content ofthe mineral soil pool was 130 ± 7.7 Mg C ha"1. Thetotal solum C content was estimated to be 160 ± 8.2

Elevation

m

510-560560-610610-660660-690690-730730-750

510-560560-610610-660660-690690-730730-750

510-560560-610610-660660-690690-730730-750

n

9108

10109

9108

10109

9108

10109

SoilNcontent

kg ha-'

12001500900

100016001500

610060005200560060006200

730075006100640078007600

SoilCcontent

——— MgForest floor

273521223834

Mineral soil140130110130130140

Total solum160170130150170170

Soil mass ,

ha'1 ———

93100

7664

10090

440041002400300027002600

450042002500310029002800

t No means within any soil stratum columns differed at p = 0.05 usingTukey's multiple pairwise comparison.

Table 7. Total soil pools of N and C and oven-dry mass by soilmapping unit and soil stratum for Watershed 5 at the Hub-bard Brook Exp. For. sampled in July 1983.

Mappingunit

Tun-LymBerkshireSkerryBeckett

Tun-LymBerkshireSkerryBeckett

Tun-LymBerkshirSkerryBeckett

n

281962

281962

281962

SoilNcontent

kg ha-

1400afllOOa1300allOOa

5800a5600a7100a5800a

7200a6700a8300a7000a

SoilCcontent

——— MgForest floor

34a25a27a25a

Mineral soil130a120a150a130a

Total solum160a150a180a160a

Soil mass

ha-' ———

90a81a96a61a

2500c3800b4300ab4100abc

2700a3900b4400ab4200ab

f Mean values within columns in a soil stratum which are followed by thesame letter are not significantly different at p = 0.05 using Tukey'smultiple pairwise comparison.

Mg C ha"1. The distribution of C with soil depth wassimilar to that described for N.

The C/N ratio of the soils examined showed sur-prisingly little variation with depth (Table 2). Themean C/N ratio in the forest floor and mineral soilwas 23 and 22 respectively.

DISCUSSIONPool Size Estimates

Using this intensive sampling design we were ableto measure N and C pool sizes within the soil com-partments with confidence limits which were suffi-

1166 SOIL SCI. SOC. AM. J., VOL. 52, 1988

ciently small that changes on the order of 20% shouldbe detectable. These measurements illustrate that in-tensive soil sampling can provide important infor-mation about element storage within an ecosystem thatcan be used to make inference about changes in long-term studies.

Up to 50% of preharvest forest floor N and C isthought to be lost in the first 5 to 15 yr after timberharvest (Covington, 1981; Federer, 1984). Causes forthis change are unknown, but could result from me-chanical mixing during logging, accelerated rates ofdecomposition, and reduced rates of woody litter in-puts. Bprmann and Likens (1979) have shown thatexport in stream water and uptake in vegetation ac-count for less than 50% of the N lost from the forestfloor in the first 5 yr following a harvest.

The fate of the remaining N is important to themaintenance of soil fertility and site productivity. Re-compartmentalization may occur through mixing andeluviation of dissolved and paniculate organic N aswell as inorganic N from the forest floor and iluviationin the mineral soil (Melillo, 1981). Under this hy-pothesis, the mineral soil acts as a sink to retain Nwithin the ecosystem preventing loss to stream waterduring a short term (5-20 yr) reorganization phase.Measuring mineral soil N and C contents may makeit possible to test this hypothesis.

The estimate of 1300 kg N ha~' for the forest floorin this study is 18 to 42% higher than earlier mea-surements at Hubbard Brook employing methods dif-ferent from ours (Dominski, 1971; Gosz et al., 1976)and generally higher than values reported for othertemperate forests (Melillo, 1981; Cole and Rapp, 1981).Our estimate of N content in the mineral soil, 5900kg N ha"1, is 64% greater than a previous estimatereported for Hubbard Brook (Dominski, 1971). Theearlier estimate included only the top 38 cm of min-eral soil, compared with an average depth of 54 cmin our study. The earlier estimate was obtained witha method which involved independent determina-tions of N concentration, coarse fragment volume, bulkdensity and soil depth.

In order to compare our estimates of C contents inthe forest floor with previous estimates of forest floororganic matter, measured by loss on ignition (LOI)(Ball, 1964), we measured a conversion factor of 0.55± 0.02 (C/LOI) (kg/kg) for forest floor samples in ourstudy and applied it to organic matter estimates re-ported in other studies. The estimate of 30 Mg C ha~'in the forest floor reported in this study falls withinthe range of estimates by Gosz et al. (1976), 26 Mg Cha"1; (T.G. Siccama, 1986, unpublished data), 36 MgC ha"1; and by Covington (1981), 40 Mg C ha~', forthis and the adjacent watershed.

The difference among estimates may be the resultof different sampling methods. Our estimate was ob-tained by excavation of a large area (71 by 71 cm)from the surface downward, while the higher esti-mates were obtained with 15- by 15-cm block sampleswhere vertical faces could be examined. With the smallblocks, pedestals were removed and inverted and min-eral soil was scraped away until an O horizon wasvisible. Both Covington (1981) and Federer (1984)have reported that with this inverted pedestal tech-

nique some mineral soil was collected with the forestfloor.

A comparison of the estimated C concentration inthe Oa, or H layer, also suggests that the higher esti-mates included more mineral soil in the forest floor.In this study we measured 300 g C kg"1 in the Oa,intermediate between the estimates of Gosz et al.(1976), 390, and (T.G. Siccama, 1986, unpublisheddata), 240 g C kg'1. Federer (1984) noted that sievesize and rubbing pressure used in sieving are notstandardized for the forest floor and will introducevariability. We employed a 6-mm mesh sieve for theforest floor while Siccama used the 12-mm opening ina Wiley mill to screen forest floor. If we included coarsefragments between 6 and 12 mm, our estimate of for-est floor C would increase to approximately 34 Mg Cha~'. Our method was designed to measure forest floorand underlying mineral soil pools in a way that al-lowed these pools to be combined to estimate totalsolum pools. By measuring all soil from within a pit,no soil material was missed or counted twice.

We measured a conversion factor of 0.45 ± 0.09(C/LOI) (kg/kg) in the mineral soil and applied it toa previous estimate of mineral soil organic matter atHubbard Brook in order to compare our C estimate.We found 130 compared with 78 Mg C ha~' reportedby Bormann and Likens (1979) which was based ona sampling depth of 38 cm. Our estimate lies betweenthe mean values of 121 and 139 Mg C ha~' for cooltemperate moist and wet forests respectively based on960 locations reported by Post et al. (1982).

C/N RatioThe relative constancy of C/N between forest floor

and mineral soil was surprising, given the higher ratiosreported for leaf litter (Ryan, 1979) and dead wood(Likens and Bormann, 1970) from these species. Thefact that the C/N ratio in the O horizons is no higherthan that found in the mineral soil is probably ex-plained by rapid oxidation of C, retention of N (Bo-cock, 1963; Gosz et al., 1973; Jorgenson et al., 1980),and immobilization of N in decomposing litter. Theaverage C/N ratio for the entire solum of Watershed5 was 22 (Table 2), somewhat higher than the meanvalue of 19.2 for forest soils reported by Zinke et al.(1984) in the corresponding Holdridge life zone cli-mate, cool temperate moist forest (Holdridge, 1947).

Elevation and Soil SeriesThe spatial distribution of N and C on this wa-

tershed was surprisingly uniform with respect to ele-vation and soil mapping unit. We had anticipated thatdifferences in total elemental contents would be re-lated to elevation and soil series. The positive corre-lation between N concentration and elevation and thenegative correlation between soil mass and elevationeffectively compensate for each other, resulting inequivalent soil N contents over the range in elevationwe studied.

Many factors may influence soil N and C pool sizesover the elevation range of this study area. The tem-perature regime is lower at the higher elevations[«1.56 °C difference over the 240-m change in ele-

HUNTINGTON ET AL.: ESTIMATING SOIL NITROGEN AND CARBON POOLS 1167

vation, using the temperature lapse rate (Gordon,1972)]. Net primary productivity is greater at lowerelevations (Whittaker et al, 1974) but rates of OMdecomposition are probably faster under the highertemperature regime at lower elevations (Powers, 1980).The species composition and thus chemical charac-teristics of above- and below-ground litter also changeswith elevation. Given the changes in many factorswhich could influence soil N and C pool sizes withelevation, the constancy over the 240-m elevationrange was unexpected.

The spatial distribution of soil mapping units onthe watershed generally corresponds with elevation;therefore, it is not surprising that the similarity in Nand C pools among soils can also be explained bycompensation between soil mass and element concen-tration. Though not significant, the largest differencesbetween pool sizes in the mineral soil were found be-tween the Berkshire and Skerry units which occur atthe same elevation in the lower part of the watershed.

The high frequency of mixed or disturbed profiles,variability in depth to bedrock and heterogeneity indrainage on a scale of meters make the delineation ofmapping units very difficult. Between closely relatedsoils, like those found on W-5, intraseries variabilitymay approach interseries variability. The nearly iden-tical mean horizon (E and Bh) thicknesses in the twodominant soil mapping units further illustrates thesimilarity between these soil mapping units.

CONCLUSIONSAn intensive sampling design applied to a discrete

but highly heterogeneous soil landscape was successfulin obtaining soil N and C pool size estimates withconfidence intervals under 20%. The quantitative es-timates of total soil N and C reported here provide astatistically valid basis for comparisons with futureremeasurements. These estimates will permit the eval-uation of changes in soil compartments and total solumnutrient pool sizes over time. Large scale disturbancessuch as timber harvests may produce measurablechanges within 5 to 20 yr. Small changes such as grad-ual N depletion or accretion in an aggrading forestecosystem will require long time periods for detection.Total soil N and C were not correlated with elevationor soil series; however, N concentration increased andsoil mass decreased with increasing elevation.

ACKNOWLEDGMENTWe gratefully acknowledge assistance in the field and lab-

oratory from many people that we do not have space tothank individually. The soil mapping was done by K. Har-ward, J. Homer and S. Pilgrim of the Soil ConservationService. We would like to thank C.A. Federer, R. Pierce, P.Sollins and A. Johnson for critical reviews of earlier draftsof this manuscript.