Nitrogen Incorporation and Flow Through a Coniferous Forest Soil ProfileJoshua P. Schimel* and Mary K. Firestone

ABSTRACTOne of the major controls on N cycling in forest ecosystems is

the dynamics of N in the forest floor. Uptake and movement ofNHJ by the microbial component of a mixed conifer forest soil incentral California were examined by injecting 1SNHJ into either theO2 or the A horizon. Distribution of the tracer was constrained by15 X 30 cm cylinders placed in situ 1.5 yr prior to the experiment.The 15N was followed over 1- and 31-d periods to measure both theshort-term uptake and the longer-term fate of N. Recovery of 1SNwas determined in coarse roots, coarse woody detritus, fine detritus,fungal strands, and the bulk soil in each horizon; microbial biomassI5N was determined in the A horizon only. Nitrogen-dynamics inthe forest floor were characterized by a period of rapid microbialNHj-uptake after which transformations were slower. The rate ofNHJ immobilization was approximately 200 mg nr2 d~', giving aturnover time for the NHj pool of less than 1 d. In the A horizon,there was no conversion of microbial I5N into soil organic I5N, butthere was extensive lateral translocation of 15N from microbial bio-mass in the bulk soil into coarse dead roots and coarse detritus.There was little vertical translocation up the profile from either theO2 or the A horizons. This work indicates the rapidity of N turnoverin the forest floor and suggests that N sequestering in woody resi-dues may be an important fate of N in this forest soil.

NITROGEN CYCLING in forest soils has been an ac-tive research area for many years. The forest

floor has been a major focus for study, as it is criticalin controlling N cycling in forest ecosystems (Gesselet al., 1973; Cole, 1981). Litter decomposition (Meen-temeyer, 1978; Berg and Staaf, 1981; Melillo and Aber,1984; McClaugherty et al., 1985) and net mineraliza-tion and nitrification (Keeney, 1980; Vitousek et al.,1982; Binkley et al., 1986) have received particularlyintense study. This body of work has greatly increasedpur understanding of the overall patterns of N cyclingin forest ecosystems. There have been fewer reports,however, on the internal dynamics of N in forest soils.Such processes as microbial N incorporation (Schimeland Firestone, 1989) and N movement through theforest floor are important in controlling forest N cy-cling, yet they are still poorly understood (Bringmark,1980; Fahey et al., 1985).

In this study we used root-free microcosms incu-bated in situ to examine the microbially mediated Nflows in the soil of a mixed conifer forest. The role offorest floor vs. mineral soil as zones of N dynamicswere examined by injecting 15NH4 separately into theO2 and A horizons. Microbial assimilation and trans-location were determined by following the 15N for pe-riods of 1 and 31 d, to examine both the short-termuptake and the longer fate of N. By following 15N inJoshua P. Schimel, Inst. of Arctic Biology, Univ. of Alaska, Fair-banks, AK 99775-0180; and Mary K. Firestone, Dep. Plant & SoilBiology, Univ. of California, Berkeley, CA 94720. Contribution ofDep. Plant & Soil Biology, Univ. of California, Berkeley. This workwas supported by NSF grant BSR-83-06181 and Mclntire Stennisproject 4262 of the Univ. of California Agric. Exp. Stn. Received 1July 1988. 'Corresponding author.

Published in Soil Sci. Soc. Am. J. 53:779-784 (1989).

the different horizons we were able to examine bothlateral N transfers between components of a horizonand vertical transfers between horizons.

MATERIALS AND METHODSSite description

The research site was in Blodgett For. Res. Stn., George-town, CA, in the foothills of the Sierra Nevada mountains(38 °52'N, 120 °40'W). The forest is at 1400-m elevation.The climate is Mediterranean with cool wet winters and hotdry summers. Precipitation averages 1680 mm, of whichone-half to two-thirds is snow.

The site was a 60-yr-old mixed conifer stand composedof Abies concolor, Pinus lambertiana, Pseudotsuga menzie-sii, Pinus ponderosa, and Libocedrus decurrens. The soil isa well-drained, sandy loam Ultic haploxeralf of the Hollandseries, formed on granodiorite parent material. The foresthas a well-developed organic horizon (7.2 kg dry matter m~2).The experimental plot was approximately 50 m by 20 m,located in an area clear of herbaceous vegetation.

Microcosm DescriptionMicrocosms used to control the distribution of 15N were

made from 30-cm-long sections of 15.25-cm i.d. PVC pipe.Some cylinders had windows (25-cm long by 7.5-cm wide)cut in the sides which were covered with either 5.0- or 0.5-mm mesh stainless steel screen to allow root or fungal in-growth. However, there was no root ingrowth and there wereno significant differences between microcosm types, so thedata from all microcosm types were pooled for each injec-tion treatment.

Sixty microcosms were placed in the ground on 7 Nov.1984. They were placed under the canopy between 1.5 and4 m from large tree boles, avoiding fallen branches. For eachmicrocosm, a 15.25-cm diam. section of the forest floor wasremoved intact. The cylinder was then driven 15 cm intothe mineral soil. The forest floor section was separated intoOl (identifiable needle remains) and O2 (unidentifiable ma-terial) horizons which were placed in nylon mesh baskets(3.2-mm mesh). These were then replaced in the appropriatecylinder. The Ol and O2 horizons were each about 3- to 4-cm thick. The microcosms were left undisturbed until April1986 (1.5 yr) to allow fungal hyphae to regrow between thesoil horizons and to allow severed fine roots to die and de-compose. These roots would otherwise constitute a flush ofavailable C, which would not exist in undisturbed soil.

Injection ProcedureThere were two sets of injections. The microcosms for the

1-d experiment were injected on 6 Apr. 1986. A hail stormmade it impossible to do both sets of injections that day, sothe injections for the 31-d samples were done on 14 Apr.1986.

To start the experiment, (I5NH4)2SO4 (70.5 atom %) wasinjected into either the O2 horizon or the top 5 cm of theA horizon. Horizon depth was measured by inserting a 3.2-mm diam. rod into the soil to determine the depth of thenylon baskets. All injections were made using a templateand needle guide to ensure a standardized injection distri-bution at the correct depths. The templates (15-cm diam.)had 37 holes in a hexagonal grid, and injection depth wascontrolled by adjustable Al blocks that were fitted onto theneedles.

779

780 SOIL SCI. SOC. AM. J., VOL. 53, MAY-JUNE 1989

For the O2 horizon, a total of 18.5 mL of 7.03 mM(15NH4)2SO4 (0.2 g N nr2) was injected. For each injection,an 18-gauge Quincke (Babcock) spinal Needle (Popper andSons, Inc., New Hyde Park, NY) was placed so that theneedle tip was just below the bottom of the Ol basket and0.5 mL solution was injected.

In the A horizon injections a similar procedure was used.A total of 37 mL of 3.52 mM (15NH4)2SO4 (0.2 g N nr2 wasinjected. Injections [0.5 mL of (15NH4)2SO4 solution each]were done at 0- and 3-cm depth into the A horizon.

Harvest ProcedureOne set of 12 microcosms was harvested prior to the 15N

injection on 6 Apr. 1986 to determine initial soil NH4,NOi, and background 15N. One set of 24 microcosms (twelveeach injected in the O2 and A horizons) for the 1 d studywas harvested on 7 Apr. 1986 and one set of 24 microcosmswas harvested on 15 May 1986. The microcosms were dis-mantled and sampled within 4 h of being removed from theground.

The Ol and O2 baskets were removed and weighed. Thetop 5 cm of the mineral soil was collected and weighed. EachO horizon sample was separated into four subsamples. Onesubsample (25 g) was extracted with 2 M KC1 (100 mL)containing 3.5 mg L~' phenylmercuric acetate (PMA) as apreservative. These samples were stored on ice until filtra-tion and analysis of NH4 and NO^ (1 d later). A secondsubsample was analyzed for gravimetric moisture content.A third subsample was analyzed for total assimilated 15N; itwas washed in 0.5 M KC1 for 10 min to remove residualinorganic 15N and then rinsed twice with water to removeremaining salt solution. These samples were frozen untilbeing dried at 65 °C and analyzed for 15N content. The re-mainder of the sample was frozen for later analysis of roots,fungal strands, and wood.

The mineral soil was treated similarly. One sample wasused for extraction with KC1 and analysis of inorganic 15N,one for gravimetric moisture, one for root, fungus, and det-ritus analysis, and a fourth was for measurement of 15N in-corporation by the microbial biomass.

Root and Fungus Sampling ProcedureTo examine the partitioning of N into dead roots, fungal

rhizomorphs, and miscellaneous detritus, these componentswere picked from the soil samples that had been frozen atthe time of harvest.

The Ol samples were thawed, weighed, and the entiresample dissected. The Ol contained no roots and the fungalstrands present were generally long threadlike structures. Thedissected fungal material was rinsed with water, dried at65 °C, and analyzed for N and 15N content as describedbelow.

The O2 samples were thawed. A subsample (~30 g) wasthen placed in water to wash soil particles from roots andfungal material. The O2 samples contained no coarse deadroots. Fine dead roots (>2-mm long by <2-mm diam.) werepicked from the sample; smaller fragments were numerousand difficult to identify and so were considered part of thebulk soil. Fungal strands were also picked out, placed inclean water and as much adhering material as feasible wasremoved. Coarse woody material (branches, large roots etc.)was present in the O2 but was too solid to drive cylindersthrough, and so were excluded.

Mineral soil samples were sieved (4 mm), and coarse deadroots and coarse detritus were collected. A subsample of thesoil (60 g) was then slurried in water (100 mL) to float offroot pieces and organic debris. After settling, the supernatantwas sieved (0.5 mm) to collect organic fragments. The liquidwas then poured back into the residual soil and the proce-dure repeated twice more. Remaining organic fragments were

then picked from the residual soil. The organic debris wasthen composited, suspended in water, and separated intofungal material and detritus, which included fine dead rootsand unidentifiable debris. The residual soil and liquid werecentrifuged (5 000 X g for 10 min) and the soil was col-lected, dried, and analyzed for total N and 15N as describedbelow.

AnalysesAll KC1 extracts were analyzed for NH4 and NO] concen-

trations by colorimetry (Keeney and Nelson, 1982) on aLachat System IV flow injection autoanalyzer (LachatQuickChem Systems, Mequon, WI). The 15NH4 was thencollected by steam-distillation (Keeney and Nelson, 1982).Stills were cleaned between samples by steaming with aceticacid followed by ethanol. These samples were analyzed forI5N isotope enrichment by mass spectrometry (performedby Isotope Services, Los Alamos, NM).

Organic material was dried at 65 °C and ground in a Wileymill (0.4-mm mesh). This material was digested using a Kjel-dahl procedure in a block digestor. Digests were steam-dis-tilled prior to mass spectrometric analysis for 15N enrich-ment.

Mineral soil samples were dried (105 °C), ground, anddigested as above. However, the N content of the digestswas analyzed by colorimetry as above and (15NH4)2SO4 wascollected for mass spectrometric analysis by diffusion(MacKown et al., 1987).

Samples for microbial biomass analysis were washed with0.5 M KC1 for 10 min to remove inorganic 15N. Rocks andlarge root fragments were removed. After settling, the su-pernatant was sieved (0.5 mm) to remove fine organic frag-ments. The soil and supernatant were then centrifuged, de-canted, mixed with water to remove remaining KC1 andrecentrifuged. The soil was then suction filtered to dry itfurther. Two subsamples (25 g each) were analyzed for mi-crobial biomass C, N, and 15N by a modification of the pro-cedure of Voroney and Paul (1984) as described by Myrold(1987). Biomass 15N was only determined on the A horizon,as there was no appropriate technique for measuring it inthe 0 horizon. The average Kn (proportion of microbial Nmineralized) was 0.19 with a standard deviation of 0.10, therange was from 0.09 to 0.65. The initial KC1 wash was foundnot to significantly decrease biomass N (data hot shown).

CalculationsRecovery of 15N was calculated as the percentage of the

added 15N that was found in a given soil component. Theamount of 15N in a component was its 15N atom % excess(15N enrichment minus background) multiplied by the massof N in the component. Background 15N was determined forall soil components.

The rates of N incorporation during the 1-d incubationwere calculated by dividing the rate of 15N incorporation bythe average enrichment of the NH4 pool over the day. Theinitial enrichment of the NHJ pool was calculated by sum-ming the quantity and enrichment of the ambient NHJ (de-termined on the microcosms that were harvested on 6 April)with the known quantity and enrichment of added 15NH4.

StatisticsFor each combination of microcosm type, sampling time,

and injection treatment, four replicate microcosms were used.The effects of microcosm type were tested using three-wayANOVA blocking on microcosm type, injection depth, andincubation length. As there were no differences due to mi-crocosm type they were grouped together to provide 12 rep-licates for each combination of injection depth and incu-bation length. Differences between soil properties within each

SCHIMEL & FIRESTONE: NITROGEN INCORPORATION AND FLOW THROUGH A CONIFEROUS FOREST SOIL PROFILE 781

horizon across sampling dates were tested using one-wayANOVA and Duncan's Multiple Range Test. Differences be-tween incubation lengths within each injection depth treat-ment were tested using TMests. All tests were performed atthe a = 0.05 level.

RESULTSSoil Characteristics

Soil moisture and mineral N contents were rela-tively uniform across the study site with coefficientsof variation (CV) less than 50% (Table 1). The mineralN concentrations of the Ol and O2 horizons weregenerally not significantly different. The masses of theOl and O2 horizons were 3.25 and 3.95 kg dry matterm~2 with standard deviations of 0.75 and 1.85 respec-tively.

The moisture content of the Ol increased over thefirst day due to heavy precipitation but decreased overthe succeeding weeks. The O2 showed a similar pat-tern. The moisture content of the A horizon did notchange significantly over the 5 wk. By the end of theexperiment only the Ol had become dry enough tohave a possibly significant effect on microbial activity.The A horizon remained near field capacity through-out the experiment. During 6 to 7 April the temper-ature of both the O2 and A horizons was 7 °C. On 15May the temperature of the O2 had risen to 12 °Cwhile the A horizon was 10 °C.

In the Ol and O2, NH4 concentrations increasedover the course of the experiment. In the A horizonNH4 concentration peaked on 7 April but was stillhigher at the end than at the beginning of the exper-iment. The NH4 increases from April 6 to April 7 werenot due to the NH4 addition, as NH4 concentrationswere not significantly higher in horizons that had15NH4 injected than those that did not, and the totalincrease in N was roughly three times the amount ofN added to the soil. Ammonium concentrations werealways significantly greater than those of NO^. Nitrateconcentrations were significantly higher on 15 Maythan on the earlier dates.Organic Components of the Different Soil Horizons

Masses of the various soil components were ex-tremely variable over space (CV > 100%; Table 2).However, with the exception of microbial biomass,Table 1. Forest soil characteristics on each sampling date. Data are

the average of all cylinder and injection depth treatments.

Horizon

Ol

O2

A (0-5 cm)

LSD*

Samplingdate

6 April7 April15 May6 April7 April15 May6 April7 April15 May

Soil moisture

kg kg-1.51 (0.47)af2.27 (0.35)b1.15 (0.36)c1.94 (0.47)a2.28 (0.67)b1.83 (0.51)a0.51 (0.27)a0.60 (0.2 l)a0.49(0.15)a

0.29

NO;

4.8(1.4)a2.5 (0.8)3

15.0(5.4)b6.3(2.1)a2.2 (0.8)b

16.0(6.1)c1.3 (0.8)a0.5 (0.2)a3.7 (2.0)b

2.2

NH;**&41.3 (23.4)a58.7 (14.6)b76.2 (25.9)c35.9 (19.4)a76.9 (34.1)b83.0(41.9)b

4.3 ( 2.2)a13.8 ( 7.0)b8.7 ( 3.2)c

16.9

they did not change significantly over time. Microbialbiomass in the A horizon was 48.2 g C m~2 on 7 Apriland increased significantly to 76.8 g C m~2 by 15 May.Hyphal strands were a small proportion of the totalmicrobial biomass in the A horizon.15N Recovery After One Day

One day after (15NH4)2SO4 was injected into the O2horizon, approximately 17% of the added label wasstill in solution, almost completely as NHJ (Table 3).Total 15N recovery from this injection treatment av-eraged 64 ± 10% (SD) of the 15N applied. Most of the15N recovered had been immobilized in the O2 hori-zon (22%) and in the top 5 cm of the A (23%) (Table3). Labeled-N that had been immobilized in the O2was almost entirely in residual soil. Of the 15N in theA horizon, virtually all appeared to be in microbialbiomass as biomass was a component of the residualsoil. There was very little 15N in the various root, fun-gus, and detritus components. The calculated rate ofNHJ incorporation in the O2 horizon was 203 mg Nm-2 d'1.

When (15NH4)2SO4 was injected into the A horizontotal 15N recovery was only 30 ± 13%; most of theunrecoyered 15N was presumably deeper in the min-eral soil (Table 3). A smaller proportion of the labelwas found in the soil solution than when 15N was in-jected into the O2 horizon. Most of the recovered 15Nwas in microbial biomass in the A horizon. Only smallquantities of 15N were in the Ol and O2 horizons, andmuch of this was in residual O2 material. The rate ofNH4 incorporation in the A horizon was 155 mg Nnr2 d"1. The total 15N recovery was only 32 ± 13%.

I5N Recovery After 31 DaysAfter 31 d, the soil solution was depleted of 15N

(Table 4). The partitioning of 15N injected into the O2was similar to that after 1 d except that less 15N wasin solution in the O2 and of the 15N that had movedinto the mineral soil, larger quantities were in thecoarse root and detritus fractions than in microbialbiomass. Some of the increase in 15N in the coarsedetritus fraction could have been due to the largequantity of coarse detritus found in this particular setof cylinders on 15 May. The total recovery of added

Table 2. Specific organic components of the soil. Average of 7 Apriland IS May data.

Horizon

Ol MeansotO2 MeanSDA MeanSD

Coarsedead roots

0(0)0

(0)67.6

(78.8)

Finedetritust

0.1(0.4)11.6(23.0)

1477.1(927.1)

Coarsedetritus

— gm->-

o§(0)o§

(0)244.5

(921.9)

Fungalstructure

1.7(1.6)7.5

(21.8)17.0

(54.0)

Microbialbiomass-C

-#

-#

62.8ft(29.6)

t Numbers in parenthesis are standsrd deviations (n = 12 for 6 April and24 for 7 April and 15 May). Values within each horizon with the same letterare not significantly different at a = 0.05 by Duncan's Multiple Range test.

t Approximate least significant difference for any two values of that property.

f Sum of fine roots and fine detritus. In the Ol and O2 this is only fine rootsas detritus constituted the bulk of the soil material.

t Standard deviation of 24 cylinders.§ There were fallen branches in the forest floor but they were too solid for

cylinders to be driven through them.# Not measurable on organic materials using the CHC13 fumigation technique.ft 48.2 g m~2 on 7 April and 76.8 g nr2 on 15 May; these are significantly

different.

782 SOIL SCI. SOC. AM. J., VOL. 53, MAY-JUNE 1989

Table 3. ISN Recovery in specific soil components after 1-d incubation (7 April harvest).

Soilsolution

Coarse deadroots

Finedetriusf

Coarsedetritus

Fungalstructure

Residualsoil*

Microbiolbiomass-Cf

- % of added "N recovered -

Injected in O201

02

xSE§x

SE

0.3(0.1)10.7(0.9)

0(0)0

(0)

0(0)0.1

(0.0)

0.0(0.0)0.04

(0.02)

2.2(0.6)22.2(3.3)

A

Ol

O2

A

xSE

xSE§x

SEx

SELSD#

6.1(1.1)

0.1(0.1)0.4

(0.2)6.7

(1.3)1.6

0.6(0.2)

0(0)0

(0)1.9

(1.1)6.1

2.2(0.6)

0(0)

0.01(0.01)

2.4(0.6)1.0

1.2(0.9)Injected in A

0.9(0.7)13.3

0.11(0.05)

0(0)0

(0)0.07(0.03)0.2

19.0(2.5)

0.3(0.1)1.4

(0.6)16.7(2.3)

4.8

20.7(3.8)

27.1(7.3)11.8

f Sum of fine root and fine detritus I!N.j Microbial biomass is also a component of the residual soil in the A horizon. It could not be measured in the O horizon.§ Standard error of 12 replicates.# Least significant difference between any two values for that component, including both injection depths and incubation lengths.

Table 4. I5N Recovery in specific soil components after 31-d incubation (15 May harvest).

HorizonSoil

solutionCoarse dead

rootsFine

detritustCoarsedetritus

Fungalstructure Residual soil:):

Microbialbiomassj

- % of added "N recovered -

01

02

A

xSE§x

SEx

SE

0.1(0.0)0.7

(0.1)0.7

(0.2)

0(0)0

(0)8.0

(6.6)

0(0)0

(0)2.4

(0.6)

Injected in O2

24.5(16.3)Injected in A

0(0)

0.23(0.22)0.02

(0.01)

1.7(0.4)23.1(2.6)6.2

(1.2)8.9

(1.6)

01

02A

xSE§x

SEx

SELSD#

0.2(0.2)0.1

(0.1)1.0

(0.3)1.6

0(0)0

(0)7.1

(3.5)6.1

0(0)0

(0)2.7

(0.7)1.1

_———

4.7(2.0)13.3

0(0)0

(0)0

(0)0.2

0.1(0.1)0.7

(0.3)17.0(2.1)4.8

_———

18.5(3.2)1.8

f Sum of fine root and fine detritus "N.j Microbial biomass is also a component of the residual soil in the A horizon. It could not be measured in the O horizon.§ Standard error of 12 replicates.# Least significant difference between any two values for that component, including both injection depths and incubation lengths.

15N in this treatment was similar to that after 1 d,averaging 68 ± 21%.

In the A horizon injections, the 15N distribution pat-tern in the A horizon was similar to that from in theO2 injection treatment. In both injection treatments,I5N in microbial biomass and in residual soil weregenerally lower after 31 than after 1 d, though the de-crease was not significant in the A horizon injection.The 15N recovered in biomass was not significantlydifferent from the total in residual soil. After 31 d, asubstantial amount of 15N again had accumulated inthe coarse root and detritus components. The totalrecovery of 15N was 34 ± 17%, also quite similar tothat after 1 d.

DISCUSSIONN Recovery in Different Soil Components

Uptake of N in this system was rapid; 45% of the15N added to the O2 horizon was immobilized in 1 d

(73% of recovered 15N). These results are similar tothose of Weber and Van Cleve (1984), who found that15NH4C1 applied to the forest floor of an Alaskan blackspruce (Picea mariand) forest was immobilized after5d.

Microbes in the bulk soil appeared to outcompetemicrobes in the other measured components for N(Table 3). The large fungal structures were a negligiblesink for N. Uptake of N by microorganisms in thecoarse woody components of the mineral soil was slow,but large amounts of N were accumulated by 31 d,particularly in the O2-injected microcosms (Table 4).These microorganisms were thus poor competitors, butpowerful sinks for N during the course of this study.Coarse roots had been dead for at least 1.5 yr, andcoarse detritus for longer than that, yet N accumulatedin both. Nitrogen assimilated by microbes in thesematerials may be out of circulation for many years,and N transfer into this N-poor woody residue maytherefore be an important process in controlling the

SCHIMEL & FIRESTONE: NITROGEN INCORPORATION AND FLOW THROUGH A CONIFEROUS FOREST SOIL PROFILE 783

long-term fate of N in this forest soil (Sollins et al,1980; Fahey et al., 1985). Fallen branches in the 0horizon may also be a sink for N, although this pos-sibility was not tested since cylinders could not bedriven through the relatively intact branches.

The increase in 15N in coarse dead roots and coarsedetritus in the A horizon most likely comes from fun-gal translocation into these materials over the 31 d ofthe incubation. The concomitant drop in bulk soil bio-mass 15N probably resulted from this translocation.Some of the 15N in biomass after 1 d may have beenin microscopic fungal hyphae (measured as biomass,rather than as fungal strands) which extend from thebulk soil into the coarse organic material and whichtranslocated N into the woody components.

Once N had been incorporated into microbes it didnot appear to transfer rapidly into soil organic matter.After 31 d, I5N in A horizon microbial biomass wasnot significantly different from the total in the residualsoil. Thus, the residual soil 15N was still in microbialbiomass after 31 d. This suggests that microbial bio-mass-N may be protected in this soil (Van Veen et al.,1984). In the organic horizon, 15N in the residual ma-terial did not change over 31 d; after 1 d 15N waspresumably in biomass, but it is impossible to tell whatform it was in after 31 d.

There was little N flow up in the profile during the31 d of this study. Less than 2.5% of the added15Nwas recovered in horizons above where it was in-jected, and this did not increase over 31 d. Old growthponderosa pine litter in Blodgett accumulates Nequivalent to approximately 20% of its initial N con-tent between April and January (S. C. Hart, 1988, per-sonal communication). Our data suggests that the Ndoes not move up from below the Ol horizon duringthe spring. Nitrogen accumulating in the Ol wouldthus have to be translocated from within the Ol ho-rizon, come from throughfall, or be translocated dur-ing another time of the year. The Ol and O2 horizonsappear to act as distinct compartments with little in-teraction between them. The locations of the visiblefungal structures support this. White rhizomorphs oc-curred only in the Ol, while the wefty yellow bundlescommon to the O2 horizon did not occur in the Ol.

There are a number of factors which must be con-sidered in evaluating the N partitioning data. The ab-sence of live roots in the cylinders must have alteredthe natural fate of labeled-N. This distortion may bemost significant during the 1 d incubation period, thatis, during the initial period of competition. While inthe absence of functional roots we can more clearlydelineate lateral and vertical movement of N due tomicrobial processes, the N dynamics resulting fromfunctional roots and root-microbial interactions arelacking. The 1- and 31-d experiments were started ondifferent days, and 6 April was wetter than 14 April,thus the initial distribution of N may have been dif-ferent. However, the total 15N recoveries were almostidentical after both incubation periods, and the labeldistribution between O2 and A horizons were similar.It is therefore likely that the initial patterns of N dis-tribution were similar. While there were several ex-perimental perturbations that may have altered theexact amounts of 15N recovered in different soil com-

ponents, they probably did not significantly alter theoverall patterns of N distribution.Rates ofN Incorporation in the Forest Floor

We added "tag" rather than "fertilizer" levels of 15Nto avoid excessively increasing ambient N concentra-tions, and thus ambient N incorporation rates. Theamount of added N was about two thirds of the annualthroughfall in a similar site (J.G. McColl, 1979, un-published data), but was only one-third of the increasethat occurred during the first day of the experimentand would be no more than a few percent of the Ncirculating annually through the soil, thus it is not anunrealistic pulse. The rates of N incorporation werelarge, 200 and 155 mg N nr2 d^1 in the O2 and Ahorizons respectively, and the turnover times (poolsize/uptake rate) of the NHJ pools were less than 1 dfor each of these pools. Since the NH$ pools increasedover the 1 d, gross mineralization rates must be evenlarger than the uptake rates.

Although the N uptake rate in the O2 was high, itmay only be a fraction of the potential rate. We cal-culated potential NH$ incorporation rates (rates of in-corporation which would be expected to occur ifNHJ availability were not limiting) by using a Mi-chaelis-Menton equation. The NH$ solution concen-tration used was the average over the 1-d incubationand the values of the maximum incorporation rate,Fmax and the half saturation constant, Kt, were 23.3jug N g"1 hr1 and 0.35 mM (Schimel and Firestone,1988). The actual rates of NH£ incorporation by O2horizon material averaged only 9% of potential rates.The potentials were measured at 20 °C, while fieldmeasurements were done at 7 °C; if we use a Q\0 of2 (proportional rate increase for a 10° temperatureincrease, Griffin, 1981) to "normalize" or comparethese rates, field rates still appeared to be no morethan about 20% of potential N incorporation rates.Microbial N incorporation may therefore have beenN limited, which is consistent with earlier findings(Schimel and Firestone, 1989). The N limitation oc-curs despite high NHJ concentrations and turnover,suggesting that active organisms occur within NHJ-depleted microsites, thus causing diffusion to be animportant process in supplying N to microbes.

In summary, inorganic N in this forest soil was rap-idly taken up by soil microbes. Over 31 d there wasno measurable conversion of the freshly assimilatedN into soil organic matter. Lateral translocation, mov-ing N into coarse detritus, was important in control-ling the fate of N, but vertical transport up the profilewas negligible. Nitrogen transfer into C-rich woodymaterials may therefore be an important factor in theinternal N dynamics of the forest floor.

ACKNOWLEDGMENTSWe thank Yi Hsin Liu and Bob Eckert for valuable assis-

tance with the sample sorting and analyses as well as thenumerous people who helped with the field work. JimThornbury built the injection templates.

784 SOIL SCI. SOC. AM. J., VOL. 53, MAY-JUNE 1989