transport of carbon and nitrogen between litter and soil organic matter in a northern hardwood...

Transport of Carbon and NitrogenBetween Litter and Soil OrganicMatter in a Northern Hardwood

Forest

Timothy J. Fahey,1 Joseph B. Yavitt,1* Ruth E. Sherman,1

Peter M. Groffman,2 Melany C. Fisk,3 and John C. Maerz4

1Department of Natural Resources, Cornell University, Ithaca, New York 14583, USA; 2Cary Institute of Ecosystem Studies, Millbrook,

New York 12545, USA; 3Department of Zoology, Miami University, Oxford, Ohio 45056, USA; 4Warnell School of Forestry and NaturalResources, University of Georgia, Athens, Georgia 30602, USA

ABSTRACT

We used sugar maple litter double-labeled with 13C

and 15N to quantify fluxes of carbon (C) and nitro-

gen (N) between litter and soil in a northern hard-

wood forest and the retention of litter C and N in

soil. Two cohorts of litter were compared, one in

which the label was preferentially incorporated into

non-structural tissue and the other structural tissue.

Loss of 13C from this litter generally followed dry

mass and total C loss whereas loss of 15N (20–30% in

1 year) was accompanied by large increases of total

N content of this decaying litter (26–32%). Enrich-

ment of 13C and 15N was detected in soil down to 10–

15 cm depth. After 6 months of decay (November–

May) 36–43% of the 13C released from the litter was

recovered in the soil, with no differences between

the structural and non-structural labeled litter. By

October the percentage recovery of litter 13C in soil

was much lower (16%). The C released from litter

and remaining in soil organic matter (SOM) after

1 year represented over 30 g C m-2 y-1 of SOM

accumulation. Recovery of litter 15N in soil was

much higher than for C (over 90%) and in May 15N

was mostly in organic horizons whereas by October

it was mostly in 0–10 cm mineral soil. A small pro-

portion of this N was recovered as inorganic N (2–

6%). Recovery of 15N in microbial biomass was

higher in May (13–15%) than in October (about

5%). The C:N ratio of the SOM and microbial bio-

mass derived from the labeled litter was much

higher for the structural than the non-structural

litter and for the forest floor than mineral SOM,

illustrating the interactive role of substrates and

microbial activity in regulating the C:N stoichiom-

etry of forest SOM formation. These results for a

forest ecosystem long exposed to chronically high

atmospheric N deposition (ca. 10 kg N ha-1 y-1)

suggest possible mechanisms of N retention in soil:

increased organic N leaching from fresh litter and

reduced fungal transport of N from soil to decaying

litter may promote N stabilization in mineral SOM

even at a relatively low C:N ratio.

Key words: carbon; forest soil; litter decay;

nitrogen; sugar maple; isotope tracer.

Received 15 July 2010; accepted 10 December 2010;

published online 3 February 2011

Author Contributions: Fahey coordinated project, primary responsi-

bility for writing. Yavitt assisted in project coordination, data analysis and

interpretation and writing. Sherman coordinated field data collection and

data analysis. Groffman—project conceptualization and coordination and

writing primarily for microbial aspects. Fisk—project conceptualization,

field and laboratory analysis and writing. Maerz—project conceptualiza-

tion, field and laboratory analysis.

*Corresponding author; e-mail: [email protected]

Ecosystems (2011) 14: 326–340DOI: 10.1007/s10021-011-9414-1

� 2011 Springer Science+Business Media, LLC

326

INTRODUCTION

A principal source of carbon (C) and nitrogen (N)

supply to forest soil is from transformation of

aboveground litter, including dissolution and

leaching of organic C and N, fragmentation and

particle transport (either biotic or abiotic) and

biological mineralization of N to inorganic soluble

forms. Filamentous soil fungi promote N transport

via extensive hyphal networks that connect litter

and soil (Lindahl and others 2001; Frey and others

2003). In many acidic forest soils litter accumulates

on the soil surface because of limited invertebrate

activities capable of fragmentation, particle trans-

port and soil mixing. These processes are crucial to

forest soil formation and fertility and may be al-

tered by changes in soil biota (for example, invasive

species such as earthworms; Alban and Berry 1994)

or environment (for example, N deposition, tem-

perature, and so on). Although the process of leaf

litter decay has been characterized in great detail

over the years, the transport of C and N between

litter and soil and its sensitivity to environmental

change are not fully understood.

Atmospheric pollution associated with fossil fuel

combustion and agriculture results in high levels of

nitrogen deposition in downwind areas (Holland

and others 1997) that could lead to an overabun-

dance of N in soils and ecosystems, a condition

known as N saturation (Aber and others 1989). In

this condition availability of mineral N exceeds

plant N demand, resulting in high rates of nitrifi-

cation and consequent nitrate leaching and soil

acidification. However, despite prolonged high N

deposition, many forests in the northeastern US do

not exhibit high nitrate leaching (Martin and oth-

ers 2000; Goodale and others 2003), presumably in

part because of a high capacity to retain N in soil

organic matter (SOM). For example, it is well

known that decaying leaf litter acts as a strong sink

for N (Bocock 1964) including N from atmospheric

deposition (Micks and others 2004). Moreover,

isotopic tracer experiments using 15N illustrate

short-term immobilization of added N in microbial

biomass and SOM (Zogg and others 2000). How-

ever, the forms and mechanisms of N retention in

forest SOM are not well understood. There is par-

ticular uncertainty about how N retention interacts

with the stabilization of soil C derived from plant

detritus. According to the reasoning of Waksman

and Starkey (1931) and elaborated in the Redfield

ratio hypothesis (Redfield 1958), the C:N ratio of

the soil medium should approach that of the soil

microbial community as C is lost as CO2 but N is

retained. Although general patterns of soil and

microbial C:N support this hypothesis, the mecha-

nistic underpinnings are not fully explored

(Cleveland and Liptzin 2007). For example, how

does variation in the biochemistry of detrital sub-

strates affect the C:N ratio of stabilized SOM?

The objective of the present study was to quan-

tify the co-transport of C and N between leaf litter

and an acid forest soil. Using dual labeled (13C and15N) leaf litter, we traced C and N flux during the

first-year of litter decay. Two types of litter were

compared, one in which the 13C was incorporated

preferentially into non-structural leaf tissue and

the other structural components. We also quanti-

fied N transport from older litter and soil to fresh

leaf litter. We hypothesized that (1) the C:N ratio of

organic matter derived from plant litter and accu-

mulating in soil is lower than the C:N of the litter

substrate because during stabilization in SOM C is

lost as CO2 whereas N is conserved; (2) the 13C

from the non-structural label is more readily uti-

lized by soil microbes than the structural label; and

(3) fungal transport of N to fresh litter is supplied

from older litter as well as mineral soil.

METHODS

Study Site

The research was conducted at Cornell University’s

Arnot Forest located in Tompkins County, central

New York State (42�15¢N, 76�40¢W) and situated on

the northern Allegheny Plateau. For a detailed site

description see Fain and others (1994). Briefly, the

study plots were located at 600–620 m elevation in

forests dominated by Acer saccharum Marsh. The

stands are mature, 2nd-growth forests originating

following clear-cut harvest in the 1870s. Basal area

ranges from about 30 to 35 m2/ha and canopy

height 23–25 m. Soils are acidic Dystrochrepts (pH

4.5–5.0) derived from glacial till overlying Upper

Devonian shales. Clay content of the less-than-2-

mm fraction ranges from 24–28% in the 0–10 cm

soil and sand content from 13–17%. Soils are stony

averaging 22% by volume coarse fraction (>2 mm)

in the 0–10 cm soil. Although invasive earthworms

are common in the Arnot Forest (Bohlen and others

2004), the study plots were chosen in earthworm-

free sites with a well-developed organic horizon (ca.

4 cm thick) overlying mineral soil. The climate is

temperate continental with mean temperature of

-4�C in January and 22�C in July, and mean an-

nual precipitation of 90 cm, evenly distributed

through the year. The region receives moderately

high annual N deposition (ca. 10 kg N ha-1 y-1;

Fahey and others 1999).

Transport of Carbon 327

Isotopic Labeling of Leaf Litter

Sugar maple leaf litter was labeled with 13C and 15N

and applied to 0.5 m2 quadrats in the field study

plots. The process for labeling leaf litter is described

in detail in Horowitz and others (2009). Briefly,

during summer 2006, seven 2.5 m tall 9 3 m

diameter aluminum chambers were positioned

over dense sapling stands of sugar maple that had

been released by heavy overstory thinning in 2000.

The litter layer (Oi) was removed from the soil

surface, and the root systems of the saplings inside

the chambers were isolated by trenching to about

0.5 m. Saplings were labeled with 15N by applying

0.25 g 15N m-2 as 99 atom % enriched 15NH4Cl to

each chamber soon after leaf expansion on 22 May

2006. To ensure even distribution the 15N was ap-

plied in solution (8 l per chamber) immediately

after a large rain event.

The saplings were labeled with 13C after enclosing

the chambers with polyethylene sheeting. Labeled13CO2 (40 atom % enriched) was added to each

chamber on 13 sunny days between 1 September

and 20 September 2006. To ensure maximum uti-

lization of the label, the procedure involved scrub-

bing ambient CO2 (to ca. 50–60 ppm) after sealing

the chambers, injecting 13CO2 until chamber con-

centration reached about 500 ppm, and re-opening

the chambers when CO2 concentrations were re-

duced to nearly constant levels (typically after

0.5–1 h). The procedure was successful in enriching

foliage to d13C over 300 per mil (Horowitz and

others 2009).

All fresh leaf litter was collected from each

chamber in September and October 2006, returned

to the laboratory, and air-dried to constant mois-

ture content. Litter was thoroughly mixed and

subsamples taken for chemical analysis. In addi-

tion, litter was again collected from the chambers

in September–October 2007. We expected the

biochemical nature of the isotopic label to differ

markedly between the two cohorts of litter: the 13C

should be primarily in non-structural components

(polysaccharides) of the 2006 litter because the

label was added at the end of the growing season,

whereas in the 2007 leaves the 13C primarily would

be in structural components. The 2007 litter was

processed in the same way as for 2006; however,

the drying and storage period were brief as litter

was added to the field plots about 1 month after the

end of the litterfall period in the chambers.

To characterize the differences in the biochemi-

cal distribution of the isotope labels between the

structural and non-structural litter, a sequential

extraction procedure was conducted on several

subsamples of the initial litter following McLeod

and others (2007). Briefly, litter tissue was ex-

tracted with 10% formic acid to remove loosely

bound polysaccharides; with phosphate buffer to

dissolve and remove loosely bound pectin; with

cyclohexanediamine tetra acetic acid (CDTA) to

remove calcium, which releases bound pectin; with

urea to break hydrogen bonds and separate cellu-

lose from hemicelluloses; with sodium carbonate to

remove remaining pectin; with sodium hydroxide

to dissolve hemicelluloses; and finally with formic

acid to remove all remaining non-structural sugars.

A subsample of the residual solid phase was re-

tained after each step for isotope analysis, as de-

scribed below. Also, lignin concentration of

structural and non-structural litter was analyzed by

the standard detergent extraction method (Van

Soest 1963) by Dairy One, Inc. (Ithaca, NY).

Field Plot Establishment and Sampling

In each of three sugar maple-dominated stands 16

0.5 m2 quadrats were established in fall 2007. Half

of the quadrats were assigned randomly to the

structural-litter treatment and half to the non-

structural treatment. Fresh litter from 2007 was

removed from each quadrat and a coarse-mesh

nylon screen (hole size = 6 cm2) was positioned on

the underlying forest floor and anchored at the

corners. About 200 g (weighed to ±0.01 g) of iso-

tope-labeled litter was added to each quadrat to

roughly match leaf litterfall in the study area. A

second coarse-mesh screen was positioned on the

added litter and anchored to confine the litter and

prevent subsequent addition of litterfall.

Fungal ingrowth bags were installed at approxi-

mately 5 cm depth in the mineral soil in January

2008, to quantify incorporation of label into fungal

hyphae. Bags were placed adjacent to 2 quadrats

and inside 2 quadrats per litter type in each plot.

Ingrowth bags were constructed of nylon mesh

(50 lm mesh size, ca. 5 cm diameter). Each bag

was filled with 27 g of acid-washed, autoclaved

sand, and sealed with a solder iron. Ingrowth bags

were collected in September 2008, and refrigerated

prior to hyphal extraction.

The field quadrats were destructively sampled on

21 May and 10 October 2008. Two quadrats from

each litter type treatment and each stand were

chosen randomly for harvest on each date. First,

the corner anchors on the screens were removed

and all the litter remaining between the two

screens was collected. Next, the underlying forest

floor horizons (Oe + Oa) were collected by exca-

vating with hand spades to the top of the mineral

328 T. J. Fahey and others

soil. Finally, mineral soil was cored to 20 cm depth

using 5 cm diameter, sharpened split-PVC corers.

Soil samples from several cores (generally 6–8)

were composited for each quadrat by 5 cm depth

increment. All samples were returned to the labo-

ratory for processing the same day as collected.

To quantify N transport from 1-year-old litter to

fresh litter the remaining (unharvested) quadrats of

each litter treatment type (structural and non-

structural label) in each stand were used. In fall

2008, fresh sugar maple litter was collected in the

stands and about 200 g m-2 of this unlabeled litter

was added to each quadrat and secured in place

with a third coarse-mesh screen, as above. In mid-

July and mid-November 2009, subsamples of this

leaf litter were collected from each plot taking great

care to avoid any of the labeled older litter. These

litter samples were processed for isotope analysis, as

described below.

Lab Processing of Samples

Litter and forest floor samples were weighed moist

and a subsample was taken for moisture determi-

nation by re-weighing after oven-drying to con-

stant mass at 70�C. The subsample was stored for

chemical analysis as described below. A second

subsample of forest floor was taken for microbial

biomass and related measurements and stored at

about 2�C. For mineral soil cores, coarse fragments

(>1 cm) were removed and the rest of the bulk

sample was weighed moist. Subsamples were taken

for moisture determinations, isotope analysis, and

microbial biomass (and related) measurements.

The subsamples were either stored at 2�C (for

microbial biomass) or dried to constant mass at

70�C (for moisture determination) and sieved to

remove the less-than-2 mm fraction.

Samples of litter, forest floor, and soil were finely

ground and homogenized for isotope analysis. The

elemental and isotopic (13C, 15N) composition of

these samples was measured on a Finnigan isotope

ratio mass spectrometer at the Cornell Stable Iso-

tope Laboratory with appropriate standards for

normalization correction, instrument linearity, and

precision purposes. Samples were run in batches

with expected similarity of isotope enrichment to

avoid sample carryover errors.

For inorganic N and microbial biomass mea-

surements some pooling of samples was conducted:

mineral soil samples were pooled into 0–10 and 10–

20 cm depth increments and samples were pooled

by litter type within plots. Inorganic N (NH4+ and

NO3- ) was extracted from soil with 2 M KCl fol-

lowed by colorimetric analysis on a flow injection

analyzer. Microbial biomass C and N content were

measured using the chloroform fumigation-incu-

bation method (Jenkinson and Powlson 1976). The

flush of carbon dioxide was measured by thermal

conductivity gas chromatography, and a propor-

tionality constant (0.45) was used to calculate

biomass C from the CO2 flush. Inorganic N flush

data were not corrected with a proportionality

constant. KCl-extracted samples were prepared for15N analysis by diffusing inorganic N onto acidified

disks (Stark and Hart 1996) which were subse-

quently analyzed at the University of California

Davis Stable Isotope Lab on a Europa Integra iso-

tope ratio mass spectrometer with an integral

combustion unit. CO2 flush samples were analyzed

for 13C at the same facility.

Fungal hyphae were extracted from ingrowth

bags by suspending sand from each bag in 100 ml

deionized water and shaking for 5 min on a rotary

shaker at 120 rpm. Hyphae floated out of the sand

and were collected by filtration (25 mm IsoporeTM

membranes, 0.22 lm pore size). Successive ali-

quots of water were added until no more hyphae

could be retrieved. Hyphae were rinsed from filter

membranes, frozen and lyophilized. Organic C and

N content, 13C, and 15N of fungal hyphae were

quantified by mass spectroscopy at the University

of California, Davis.

Reference Soils and Isotopes

Calculation of isotope pools and fluxes requires

accurate and precise estimates of reference (pre-

treatment) soil mass and bulk density, element

contents, and isotope natural abundance (Nadelh-

offer and Fry 1994). Soil mass, bulk density, and

coarse fragment content were determined in each

stand by the soil pit excavation method (Rowell

1994). Four soil pits (0.2 9 0.2 m) were excavated

to 20 cm depth in 5 cm depth increments at ran-

dom locations in each stand. The fine (<2 mm)

fractions were stored and processed for elemental

and isotope analysis, as described earlier.

Pools of 13C and 15N in litter on each plot were

calculated at time zero and at the time of plot col-

lection as the product of dry weight, carbon con-

centration, and isotopic atom % (13C and 15N). The

release of the isotope from each plot during decay

was estimated as the difference between initial and

final isotope pools in litter; these values were used

to estimate percentage of isotope recovered in

underlying soil.

Isotopic enrichment of forest floor, mineral soil,

inorganic N pools, and microbial biomass was esti-

mated in reference to the mean natural abundance


in samples from the four soil pits in each stand.

(Note: The use of separate reference samples for the

three stands proved to be necessary for detecting

small quantities of the litter-derived isotopes in the

mineral soil because of significant—though minis-

cule—differences in isotope natural abundance

among the three stands.) We calculated the initial

total pool of 13C and 15N in each soil layer in each

stand from the mean isotope natural abundance,

mean element (C or N) concentration and dry

weight (based on bulk density) of the fine soil

fraction (<2 mm) for each depth. We calculated

the final isotope pool for each plot at the time of

collection from isotope enrichment and element

concentration and assuming bulk density and fine

fraction content was equivalent to the stand-level

values. Similarly, isotopic enrichment of microbial

biomass and fungal hyphae was calculated in ref-

erence to control samples collected within each

plot. The differences between initial and final iso-

topic pool estimates for each depth, component and

quadrat were used to calculate % recovery of excess

isotopes released from litter.

% recovery

¼ ðfinal soil isotope pool� initial soil isotope poolÞinitial litter isotope pool - final litter isotope poolð Þ

� 100

where

Soil isotope pool ¼ soil isotope atom%

� element½ � � soil mass

This approach assumes no changes over time in

the natural abundance of 13C and 15N in the soil

pools considered.

Because this percent recovery parameter accounts

for between-plot variation in litter decay (that is,13C, 15N changes) and litter treatment differences in

isotope signatures, we used % recovery for statistical

comparisons between plots, litter types, and depths.

These data were not normally distributed, nor could

they be normalized by transformation; hence, non-

parametric Mann–Whitney and Kruskal Wallis tests

were used to test for differences in percent recovery

between litter types. For microbial biomass 13C and15N where n = 3 (due to sample pooling) a relaxed

threshold of P = 0.10 was used to evaluate signifi-

cant treatment effects. Finally, differences in dry

weight, 13C and 15N loss from decaying litter were

analyzed using Student’s t statistic.

Transport of 15N from the isotopically labeled 1-

year-old litter to overlying fresh litter in 2009–2010

was calculated on the assumption that overall dry

weight loss from the fresh litter followed the same

time course as observed in 2008–2009 (Figure 1).

The increase in atom % 15N and total N concen-

tration were used to estimate the transport from

labeled litter, assuming that 15N from unlabeled

sources (older litter, soil, atmospheric deposition)

carried the same reference atom % 15N as unla-

beled litter; this assumption would likely result in

minor error as local atmospheric deposition (del15N = 0.86&, Goodale and others 2009) and older

litter are only very slightly higher in 15N than fresh

litter (del 15N = -1.79&).

RESULTS

Reference Soils

Soils in the study area were very stony with coarse

fraction volume of about 30% in the upper 20 cm.

An organic horizon comprising about 1 kg dry

weight/m2 covered the mineral soil surface

(Table 1). Bulk density of the less-than-2 mm

fraction increased with depth in mineral soil from

0.51 to 0.77 g cm-3. SOM content (and % C and %

N) decreased sharply from the 0–5 to 5–10 cm

layer. The C:N ratio declined with depth from 17 in

forest floor to 13.5 at 0–20 cm depth. The natural

Figure 1. Decomposition of sugar maple litter, (upper)

dry weight and 13C loss and (lower) total N content and15N loss, for structural (S) and non-structural (NS) la-

beled litter. Error bars indicate standard errors.


abundance of 13C and 15N increased steadily with

depth in soil and background variation was rela-

tively low (Table 1).

Litter Chemistry

The isotopically labeled sugar maple leaf litter was

strongly enriched in both 13C and 15N (Table 2).

Enrichment of 13C was much greater in the structural

(d 13C = 417.8& or ‘‘per mil’’) than the non-struc-

tural litter (d 13C = 142.1&) whereas the reverse was

true for 15N (d 15N = 188.0 vs. 337.1&), so that the13C:15N ratio was higher in structural litter (62.5) than

the non-structural litter (33.6), compared with total

C:N of 40–42.

The distribution of 13C isotopes among the

extractive biochemical fractions differed markedly

between the ‘‘structural’’ and ‘‘non-structural’’

litter for several fractions. The isotope distributions

are expressed as a percentage difference from the

bulk label in Table 2. Most notably, the 13C label

was particularly high in the lignocellulose fraction

in the structural litter and low in this fraction in the

non-structural litter. Conversely, the hemicellulose

fraction was much more highly labeled in the non-

structural litter. Surprisingly, the lignocellulose

fraction was highly labeled in the non-structural

litter even though the labeling was conducted only

a few weeks before leaf abscission. The differences

in 15N label distribution between litter types were

relatively minor, and the label was relatively uni-

formly distributed among the extractive fractions.

The most notable 15N enrichment was in the urea-

soluble fraction (hemicellulose complexes) and the

most notable 15N depletion was in the lignocellu-

lose residue, which also had low total N concen-

tration (0.35%N).

Weight Loss and Nitrogen ContentChanges in Decaying Litter

Litter decay followed a typical weak exponential

pattern through the first-year of decay (Figure 1).

Over the first 6 months weight loss was signifi-

Table 1. Selected Physical and Chemical Properties of Soil (<2 mm fraction) from the Study Area in ArnotForest, Central New York

Soil layer Bulk density

(g/cm3)

Mass/area

(kg/m2)

% C %N C:N Per mil

d 13C d 15N

Forest floor – 0.945 37.91 (1.21) 2.23 (0.06) 17.0 -27.330 (0.174) 0.385 (0.133)

0–5 cm 0.51 (0.04) 20.8 9.308 (1.144) 0.584 (0.059) 15.9 -25.941 (0.128) 7.988 (0.567)

5–10 cm 0.66 (0.05) 24.6 3.246 (0.450) 0.231 (0.023) 14.1 -25.583 (0.107) 12.206 (0.815)

10–15 cm 0.77 (0.08) 27.8 2.684 (0.355) 0.186 (0.010) 14.4 -25.391 (0.091) 12.587 (0.558)

15–20 cm 0.76 (0.07) 28.5 2.199 (0.261) 0.163 (0.014) 13.5 -25.205 (0.168) 13.988 (0.677)

Standard errors in parentheses (n = 12).

Table 2. Chemistry of Isotope-labelled Sugar Maple Leaf Litter and Percent Enrichment (+) or Depletion (-)of 13C in Six Biochemical Extractive Fractions of Structural and Non-structural Litter Relative to the BulkLitter

Fraction %

Structural Non-structural

% Carbon 44.8 (0.18) 45.1 (0.13)

% Nitrogen 1.12 (0.03) 1.07 (0.01)

Atom % 13C 1.6330 (0.0209) 1.2608 (0.0041)

Atom % 15N 1.0423 (0.0204) 1.5823 (0.0409)

Lignin 24.6 23.2

Free sugars -9.6 (1.6) +5.9 (1.4)

Weakly-bound polysac/pectin -2.2 (0.2) -9.9 (3.1)

Strongly-bound polysac/pectin +1.9 (0.0) -2.5 (3.7)

Hemicelluloses -13.5 (1.3) +20.8 (4.2)

Inaccessible sugars +3.7 (0.2) 0.0 (6.3)

Lignocellulose +19.9 (0.4) -16.1 (6.2)

Standard errors in parentheses.


cantly higher for the non-structural than the

structural-labeled litter, but this difference was not

significant after 1 year. Loss of C and 13C generally

tracked dry weight loss. In contrast, N concentration

in leaf litter increased dramatically as decomposi-

tion proceeded, nearly doubling over the 1-year

period. As a result, the total N content of the litter

increased to 26–32% of the initial over the first-year

of decay (Figure 1). No significant differences were

observed between the structural and non-structural

litter. Despite this apparent addition of N, significant

release of 15N occurred during the first-year of de-

cay (Figure 1). The loss of 15N was significantly

(P < 0.01) higher for the non-structural litter than

the structural litter, and about 20–30% of the initial15N was lost over the first year. Release of 15N was

observed both during winter/spring (especially for

non-structural litter) and during the growing sea-

son (both litter types; Figure 1).

Fresh litter that was added to the unharvested

quadrats in Fall 2008 increased significantly in both

total N and d 15N by July 2009 and further in-

creased by November 2009 (Table 3). No significant

differences were observed between the structural

and non-structural plots which were pooled for

subsequent calculations. Because the only source

of 15N enrichment of this unlabeled litter was the

labeled litter from the previous year, we can esti-

mate the proportion of N transport to first-year

litter derived from second-year litter. This value

averaged 11.6% by July and 17.7% (cumulative)

by November. Presumably, the rest of the N accu-

mulating in first-year litter (Figure 1) is derived

from atmospheric deposition, older litter, and SOM.

Isotope Recovery in Soil and MicrobialBiomass

In May 2008, 6 months after addition of labeled

litter, we were able to detect 13C enrichment of soil

down to the 10–15 cm depth layer; no significant13C enrichment was observed at 15–20 cm depth

for either the structural or non-structural litter

quadrats (Table 4). Similarly, microbial biomass

was significantly enriched in 13C in forest floor and

0–10 cm mineral soil (Table 5) but not in 10–20 cm

soil. For purposes of comparison between litter

types and depths, we analyzed the recovery of 13C

as a percentage of the measured 13C released from

the litter on each individual plot; this calculation

utilizes stand-level measurements of reference 13C

and bulk density (<2 mm fraction mass per depth)

together with experimental quadrat-level mea-

surements of litter mass and 13C and soil % C. In

May, we recovered about 43% of the 13C released

from the structural litter in soil, with the highest

proportion of recovery in 0–5 cm in mineral soil

(Figure 2). The corresponding value for non-

structural litter appeared to be lower (36%), but

this difference was not significant (P = 0.25). A less

distinct depth pattern was observed for the non-

structural litter with no measurable recovery below

10 cm depth.

Percent recovery of 13C in microbial biomass

ranged from 1.8% to 2.8% with roughly equal

amounts recovered in forest floor and 0–10 cm

mineral soil (Table 5). Fungal hyphae collected on 1

September 2008 from bags in the upper mineral soil

were enriched in 13C and enrichment was signifi-

cantly higher in structural (9 ng 13C/g soil ± 3.3)

than non-structural (3 ng 13C/g soil ± 0.6) quad-

rats. The 13C in the respirable carbon pool in soil

(10-day incubation) averaged about twice as

large in the structural versus non-structural litter

quadrats (Table 6), but this difference was not

statistically significant.

By October 2008 both the 13C enrichment in soil

(Table 4) and the % recovery of 13C released from

litter (Figure 2) were significantly lower than in

May, and little enrichment was detected below

10 cm depth for either structural or non-structural

litter. Percent recovery of 13C was similar for both

litter types. Microbial biomass 13C was similar in

October as in May but again % 13C recovery in

microbial biomass was lower in October (Table 5).

The 13C in the respirable 13C pool also appeared to

Table 3. Estimated Translocation of Nitrogen from Decaying 1-Year-Old Sugar Maple Litter into OverlyingFresh Litter at Arnot Forest Plots, Based on Changes in N Concentration and d 15N of Fresh Litter andAssuming Decay Rates Observed in Figure 1

Date % N d 15N % N from old litter

November (initial) 1.157 -1.989 –

July 2.14 (0.13) +28.68 (5.03) 11.6%

November 2.27 (0.10) +44.48 (5.45) 17.7%

Standard errors in parentheses.


decline by October, especially for the non-struc-

tural litter quadrats (Table 6).

Not surprisingly, recovery of 15N was much

higher than 13C. On both collection dates over 90%

of the 15N released from litter was recovered in the

upper 10 cm of soil. Significantly higher soil pool

enrichment was observed in the non-structural

than the structural litter plots; however, no differ-

ences in % recovery were observed between

structural and non-structural litter. No 15N

enrichment was detected below 10 cm depth (Ta-

ble 4). The depth pattern of 15N recovery changed

between May and October as enrichment was ini-

tially higher for the forest floor horizon and later

for 0–5 cm mineral soil (Figure 3). Most of the

excess 15N recovered was in organic form. In May,

the proportion of inorganic 15N recovered ranged

from about 2% to 6% of total N with significantly

higher values in mineral soil than in forest floor

horizons.

Microbial biomass also was enriched in 15N in the

forest floor and upper mineral soil (Table 5).

Average enrichment was higher (but not signifi-

cantly) in the non-structural than the structural

Table 4. Mass Excess of 13C and 15N (Enrichment above Background) in Soils Collected from QuadratsAmended with13C and 15N Labeled Leaf Litter in Arnot Forest, NY

Depth Excess 13C mg/m2 Excess 15N mg/m2

May 2008 Oct 2008 May 2008 Oct 2008

A. Structural

Oa 23.85 (5.46) 25.26 (9.39) 2.14 (0.30) 2.06 (0.62)

0–5 45.97 (17.89) 21.91 (5.73) 2.00 (1.18) 3.17 (0.68)

5–10 24.55 (11.19) 3.48 (1.41) 0.23 (0.09) 1.33 (0.38)

10–15 15.17 (4.94) 2.36 (0.71) 0 0

B. Non-structural

Oa 9.66 (2.11) 4.92 (0.72) 6.08 (0.99) 2.07 (0.22)

0–5 14.56 (4.57) 12.22 (2.80) 1.62 (0.39) 7.13 (1.05)

5–10 17.19 (8.80) 1.39 (0.83) 0.36 (0.14) 3.00 (0.21)

10–15 0 0 0 0

A. Structural label; B. non-structural label (standard errors in parentheses).

Table 5. Carbon and Nitrogen Content of Microbial Biomass, Excess 13C and 15N in Microbial Biomass inForest Floor and 0-10 cm Mineral Soil, and % Recovery of Isotopes Released from Litter

Soil

depth (cm)

Litter

type

Microbial biomass

(mg per g soil)

Isotopic enrichment of

microbial biomass

(lg per g soil)

% Recovery in

microbial biomass

C N Microbial

C:N

13C 15N 13C 15N

A. May

Forest floor S 11.31 (1.58) 0.899 (0.96) 12.6 2.87 (0.64) 0.177 (0.03) 0.99 (0.13) 9.9 (3.6)

NS 11.14 (0.55) 0.953 (0.150) 11.7 2.16 (0.37) 0.498 (0.109) 1.57 (0.25) 5.7 (1.5)

0–10 S 0.66 (0.07) 0.108 (0.012) 6.1 0.046 (0.04) 0.002 (0.001) 0.81 (0.08) 08.9 (5.8)

NS 0.63 (0.10) 0.121 (0.018) 5.2 0.031 (0.02) 0.002 (0.001) 1.23 (0.91) 4.1 (1.4)

B. October

Forest floor S 12.63 (0.24) 1.018 (0.068) 12.4 3.59 (0.94) 0.158 (0.015) 0.56 (0.19) 1.39 (0.60)

NS 11.94 (1.18) 0.844 (0.035) 14.1 1.45 (0.30) 0.207 (0.029) 0.51 (0.22) 0.71 (0.38)

0–10 S 0.86 (0.23) 0.096 (0.016) 9.0 0.034 (0.020) 0.005 (0.001) 0.53 (0.23) 3.47 (0.48)

NS 1.25 (0.23) 0.131 (0.008) 9.5 0.028 (0.022) 0.014 (0.003) 1.11 (0.87) 4.55 (0.81)

Sugar maple litter labeled with 13C and 15N in structural (S) and non-structural (NS) tissue was added to quadrats in November 2007 with soil collections in A. May 2008 andB. October 2008. Standard errors in parentheses.


litter plots in May but values were more similar by

October. Percent recovery of 15N in microbial bio-

mass (10–20 % in May) was much higher than for13C and generally higher in May than October

(about 5%), especially for the forest floor horizons

(Table 5). Fungal hyphae in the upper mineral soil

also were significantly enriched in 15N (2 ng 15N/g

soil) with no difference between structural and

non-structural quadrats.

The ratio of 13C:15N in the SOM formed from the

decaying litter was generally much lower than for

Figure 2. Recovery of 13C from decaying sugar maple

litter in three soil depth layers, expressed as a percentage

of 13C lost from structural and non-structural labeled

litter in May 2008 and October 2008. Error bars indicate

standard errors.

Table 6. Respirable Carbon in Soil (10 Day Aerobic Incubation), Excess Respirable 13C (Relative to Refer-ence 13C) and the Ratio of Microbial C to Soil C for Total C and Excess 13C in Structural (S) and Non-structural(NS) Litter Plots at Arnot Forest

Depth Litter type Respirable13C (lg/g day)

Excess respirable13C (ng/g day)

Microbial C:

Soil C

Microbial 13C:

Soil 13C

A. May

Forest floor S 879 (70) 213 (50) 2.98% 10.2%

NS 791 (68) 121 (27) 2.94% 17.7%

0–10 S 25.3 (5.8) 3.59 (1.27) 1.05% 2.9%

NS 39.8 (3.7) 1.80 (0.55) 1.00% 4.4%

B. October

Forest floor S 816 (52) 173 (61) 3.33% 6.6%

NS 472 (125) 36 (13) 3.15% 8.3%

0–10 S 21.8 (3.6) 2.56 (0.66) 1.37% 5.7%

NS 44.0 (1.9) 1.16 (0.69) 1.99% 8.6%

A. May 2008 and B. October 2008.

Figure 3. Recovery of 15N from decaying sugar maple

litter in three soil depth layers, expressed as a percentage

of 15N lost from structural and non-structural labeled

litter in May 2008 and October 2008. Error bars indicate

standard errors.


the litter itself (2–12 vs. 34–63), and significant

(P < 0.01) differences between structural and

non-structural litter plots reflected the higher13C:15N in the former litter type (Figure 4). Patterns

of differences among litter types and soil depths in

the 13C:15N ratio of new SOM consistently reflected

those of the soil microbial biomass (Figure 4). The13C:15N ratio of microbial biomass was consistently

much lower in the non-structural than the struc-

tural litter and this difference was statistically sig-

nificant (P < 0.10) in the forest floor on both

collection dates (Figure 4). Similarly, the 13C:15N

ratio in new fungal hyphae was significantly lower

in the non-structural (1.4 ± 0.3) than the struc-

tural litter quadrats (4.4 ± 0.5).

DISCUSSION

Leaf litter is a principal source of organic matter in

forest soils. In many cold and acidic forest soils

where physical mixing of litter into soil is minimal

because of limited soil macroinvertebrate commu-

nities, a thick organic horizon develops on the soil

surface, and transport of organic matter to mineral

soil occurs primarily by leaching (Park and Matzner

2003). We double labeled sugar maple litter with13C and 15N and traced these isotopes to soil. We

compared isotope movement between litter cohorts

in which either structural or non-structural tissue

components were preferentially labeled. Our

overall goal was to better understand this key

process of soil formation and nutrient recycling and

to provide insights into the role and mechanisms of

anthropogenic N deposition in altering this pro-

cessing.

Structural versus Non-Structural 13CLabeling

We observed large differences in the preferential13C enrichment of litter tissue fractions between

the structural and non-structural litter, especially

for lignocellulose, hemicellulose-pectin complexes,

and free sugars (Table 2). The labeling of the non-

structural litter was conducted immediately before

leaf abscission and senescence and considerable

retranslocation of 13C was observed within 1 week

of the end of the labeling period (Horowitz and

others 2009). The structural litter acquired its label

almost entirely from the 13C stored in perennial

tissues the previous fall and remobilized during leaf

expansion and shoot elongation (Horowitz and

others 2009); hence, growing cell walls were

strongly labeled. The non-structural litter was

especially strongly labeled in the hemicellulose

fraction (urea extractable; McLeod and others

2007), 21% stronger labeling than for the bulk

litter tissue (Table 2). This hemi-cellulose fraction

is associated with pectins and not with lignin

(Popper and Fry 2005). Therefore, in late summer

the labeled sugars apparently coalesced quickly into

hemi-cellulose (Hoch 2007) and pectins that bind

to hemicelluloses. Presumably this fraction is more

labile than the lignocellulose in the cell walls

(Cosgrove 2005) that dominated the labeling in the

structural litter (20% stronger label than bulk litter,

Table 2). Not surprisingly, the free sugar fraction

was also much more strongly labeled in the non-

structural than the structural litter.

Litter and Dissolved Organic Carbon

It has long been known especially from studies of

stream ecosystems that a considerable proportion of

initial leaf litter weight loss is associated with

leaching by water (Meyers and Tate 1983). The

same is true for terrestrial litter; for example, King

and others (2001) noted that over 111 days of

laboratory incubation, 78% of weight loss from

Populus litter was by DOC versus only 22% from

microbial respiration. Not only soluble and labile

sugars but also more recalcitrant tannins are lea-

Figure 4. C:N ratio of soil organic matter and microbial

biomass derived from decaying sugar maple litter in or-

ganic and surface mineral soil in May 2008 and October

2008. Error bars indicate standard errors.


ched from decomposing leaf litter (Tiarks and oth-

ers 1992). Hence, much of the weight loss observed

from November to May in the present study (Fig-

ure 1) probably resulted from leaching of soluble

organic matter. We observed differences in over-

winter weight loss between ‘‘structural’’ and ‘‘non-

structural’’ litter (Figure 1), likely due to the fact

that the non-structural litter was stored for a year

before use. However, after 1 year, weight loss and13C release from litter were not significantly dif-

ferent between the litter types, and the rate of

weight loss was comparable to previous observa-

tions for sugar maple in northern hardwood forests

(Gosz and others 1973).

The proportion of 13C released from litter be-

tween November and May that was recovered in

soil was surprisingly high (36–43%, Figure 2),

indicating that much of the organic matter was not

rapidly utilized by soil microorganisms. Using a

similar approach, Rubino and others (2010) ob-

served that about two-thirds of C released from

litter was recovered in soil, the rest being released

as CO2. Qualls and Haines (1992) demonstrated

that only a small proportion of DOC in forest soils

was likely utilized by microbes during the leaching

process with most DOC removal by adsorption to

the soil solid phase. Hence, we suspect that much of

the overall 13C loss occurred after DOC was

deposited in the underlying soil.

Recovery of litter 13C in soil in May was slightly

higher for structural than non-structural litter

(Figure 2), perhaps reflecting the higher lability of

the label in the non-structural litter. Don and

Kalbitz (2005) argued that the most labile organic

matter (for example, soluble sugars) is released first

followed by an increasing proportion of more re-

calcitrant DOM as microbial activity generates

additional soluble compounds in the decaying lit-

ter. Hence, a sequence in which an increasing

proportion of the DOC released from litter is sta-

bilized in SOM could be hypothesized. However,

we observed decreasing recovery of 13C in soil from

May to November even though some additional

leaching of DOC from litter undoubtedly occurred.

Apparently, the DOC that was added to soil from

litter between November and May was subse-

quently utilized by microbes at a much higher rate

during the summer (probably in part because of

higher soil temperature) than any additional litter

organic matter that was added to soil by summer

leaching. Hence, the seasonal dynamics of C stor-

age in temperate forest soils are influenced by

seasonality in the supply from litter, its lability, and

microbial activity. A moderately large proportion

(<16%, Figure 2) of the C lost from litter during

the first-year of decay remained in SOM, and this

process could contribute significantly to the accu-

mulation of C in forest soils. For example, given leaf

litterfall C flux at Arnot Forest (200 g C m-2 y-1;

Bohlen and others 2004), this process could add

about 32 g C m-2 y-1 to the SOM pool, including

15–20 g C m-2 y-1 in mineral soil. The process of

stabilization of soil C is reflected by the fact that

the ratio of microbial biomass 13C:soil 13C was sev-

eral-fold higher than microbial biomass C:soil C

(Table 5). Additional processing of 13C in SOM by

microbes would be expected to cause a reduction in

the former ratio, but that was not clearly evident

over the first-year of litter decay.

Interactions of Carbon and Nitrogen

Carbon and nitrogen are intimately related in both

the leaching of organic matter from plant litter and

the decomposition and mineralization of litter

(McGill and Cole 1981). In general, the C:N ratio of

forest SOM decreases with depth (Table 1), but the

evolution of soil C:N is not completely understood

and undoubtedly involves both differential supply

and removal processes. For example, Schoenau and

Bettany (1983) attributed declining C:N with depth

to preferential leaching of N-rich DOM, whereas

Qualls and Haines (1992) noted the role of hydro-

lysis of DON linked to mineralization of DOC, ra-

ther than selective adsorption of DON, in evolution

of forest soil C:N. Microbial processes clearly play a

key role in soil organic N retention; for example,

Perakis and Hedin (2001) observed rapid transfor-

mation of inorganic N into SOM in unpolluted, old-

growth forest. Our dual labeling of litter with 13C

and 15N provided further insights into the interac-

tions of these elements in SOM formation.

It has long been known that decaying litter of

high initial C:N can be a strong sink for N (Bocock

1964) apparently transported from soil to litter by

fungal hyphae (Wessen and Berg 1986; Hart and

Firestone 1991). In our study, this process is indi-

cated by the large increase in N content of decaying

sugar maple litter over the first-year of decay

(Figure 1), matching previous observations of Gosz

and others (1973). However, these measurements

indicate only the net flux of N into litter; others

have noted that considerable loss of native litter N

can accompany the process of incorporation of

exogenous N into litter (Blair and others 1992;

Zeller and others 2000). Our observation of 15N loss

from decaying litter illustrates that the gross N flux

into first-year litter is nearly twice as great as the

net flux. Hence, gross N transport to decaying leaf

litter is among the largest N flux pathways in these


forests. Several potential sources of this exogenous

N transport to decaying litter have been identified,

including atmospheric deposition (Micks and oth-

ers 2004), asymbiotic N fixation in litter (Russell

and Vitousek 1997), and fungal transport from

older litter and soil (Fahey and others 1985; Hart

and Firestone 1991). We were able to estimate that

the previous cohort of litter apparently supplies

about 18% of this N, with the remainder from

these other potential sources.

Presumably, most of the 15N was leached from

decaying litter in the form DON because the

immobilization potential for inorganic N in litter

apparently is very high (Micks and others 2004).

This N was strongly retained in the forest floor and

upper 5 cm of soil over the winter/spring. Our

double labeling of the litter allowed us to estimate

the C:N ratio of the litter-derived organic matter

that was retained in soil (Figure 4). This ratio was

much lower for non-structural than structural litter

plots and in spring it was much lower in mineral

soil than forest floor. The lower value in the non-

structural litter plots could result in part from the

lower 13C/15N ratio in the litter itself (33.6 vs.

62.5); however, the difference in the 13C:15N ratio

of SOM derived from the non-structural versus

structural litter was much greater (four to eight-

fold) than the less than two-fold difference in the

litter itself (Figure 4). Hence, the more labile C

from the non-structural litter was probably utilized

to a greater extent and perhaps accompanied by

more effective microbial N immobilization.

The idea that non-structural material is more

labile than structural material is further supported

by the observation that soil 13C:15N and microbial13C:15N showed similar patterns, with much lower

ratios in the non-structural than structural litter

plots (Figure 4). A similar pattern also was ob-

served in fungal hyphae collected in soil bags

(C:N = 1.4 vs. 4.4). These low 13C:15N ratios should

not be mistaken as reflecting the C:N of the new

microbial tissue but rather the C:N of what was

assimilated from the labeled litter; most of the C

and N in the new microbial tissues would be de-

rived from unlabeled sources. The patterns in Fig-

ure 4 support a strict Redfield ratio interpretation

of the C:N stoichiometry of SOM: the DOM derived

from litter and adsorbed to soil presumably had a

much higher C:N ratio than what was eventually

assimilated by the microbial community, and the

SOM that accumulated in soil after 1 year carried

the C:N signature of that assimilated organic mat-

ter.

One possible explanation for the relatively high

lability of the non-structural material is that the

depolymerization of pectins and hemicelluloses may

produce high amounts of simple carbohydrates. If N

availability is limiting to microbial growth as these

substrates are being utilized, they may be respired to

CO2 with little growth and conservation of biomass,

in so-called overflow metabolism (Schimel and

Weintraub 2003). An alternative but less likely

explanation is that breakdown of products of non-

structural C are preferentially allocated to respira-

tory pathways whereas those of structural C are

allocated to biosynthesis. In any case, the similarity

in soil and microbial 13C:15N supports the idea that

stabilized SOM reflects (is derived from) microbial

organic matter (Six and others 2004; Jastrow and

others 2007; Simpson and others 2007) and

emphasizes the importance of microbial processing

to soil N storage.

The increasing C:N values of litter-derived, re-

tained SOM with increasing soil depth for both

litter types could represent either/both processes of

dissolved organic matter adsorption or its sub-

sequent mineralization. For example, lower C:N

compounds could be preferentially adsorbed in

surface horizons leaving DOM of higher C:N to be

transported more deeply; however, previous

observations suggest the reverse should be true, at

least for mineral soil horizons (Qualls and Haines

1992; Kaizer and others 1996), because of a higher

affinity of hydrophobic DOC with lower N con-

centration for adsorption to mineral surfaces.

Notably, the behavior of surface organic horizons is

poorly understood because they are both a sink and

source of DOC (Kalbitz and others 2000).

The fate of the C and N adsorbed to forest soil has

received limited study. We observed large declines

in C:N of the retained SOM in the mineral soil from

spring to fall 2008 (Figure 4), suggesting that

mineralization of C was accompanied by stabiliza-

tion of N-rich compounds, at least within the

timeframe of our study. Apparently, the mineral

soil OM derived from litter DOM was not a major

source of N for plant root uptake as 15N in this pool

actually increased during the growing season (Ta-

ble 4). In contrast, the C:N of retained organic

matter in the forest floor increased slightly over the

growing season (Figure 4), and the 15N recovery

declined substantially, presumably reflecting min-

eralization and transport from this pool. It is also

notable that the proportion (relative to total N) of

inorganic 15N in the forest floor was much higher

in fall (about 6%) than spring (2%) whereas the

reverse was true in surface mineral soil (1% vs.

5%). Clearly, surface organic horizons behave very

differently from surface mineral horizons regarding

the processing of litter-derived C and N.


The C:N of retained SOM derived from 1 year of

the structural label litter decay (7 to 13) was

somewhat lower than the overall average SOM (14

to 17) but it exhibited the same pattern of decrease

from forest floor to mineral soil (Table 1; Figure 4).

This litter-derived organic matter is probably a

significant source of stabilized forest SOM and

eventually provides a small supply of N for sub-

sequent biological cycling in the ecosystem. Other

sources of forest SOM include particulate material

from the aboveground detrital–microbial complex,

and root-derived organic matter including rhizo-

deposition and root turnover. The relative impor-

tance of these sources is not well constrained

although root C often appears to have a longer

residence time in soil than shoot C (Rasse and

others 2005).

Our observations provide the basis for elaborat-

ing upon the mechanisms contributing to contin-

uing forest soil N retention despite high

atmospheric N deposition (Aber and others 1998,

2003; Martin and others 2000; Goodale and others

2003). Experimental studies suggest that most of

this N retention is associated with stabilized SOM

(Magill and others 2004). Our results indicate that

a substantial amount of organic N leached from

decaying litter is stabilized (at least temporarily) in

mineral SOM (Figure 3) with significant move-

ment of 15N into the mineral soil after 1 year.

Differences in d 15N of different pools provide

insight into how N retention remains high in the

face of persistent N deposition. We observed much

lower d 15N of surface organic horizons than

underlying mineral soil (over 7&, Table 1).

Together with the low d15N of fresh leaf litter

(-1.79&) and the apparent accumulation of litter-

derived DON in mineral soil during the growing

season, this suggests relatively tight recycling of N

within and between forest floor and vegetation.

Nitrogen accumulation in fresh decaying litter is

supplied from three principal sources: 1-year-old

litter (apparently about 18% in present study,

Table 3), older SOM and atmospheric deposition. A

substantial proportion of this N supply to decaying

litter is now derived from the latter source because

inorganic N in rainfall appears to be efficiently

immobilized in Oi litter (Zogg and others 2000;

Micks and others 2004; Raciti and others 2008).

The d15N of bulk precipitation in the study region

averages about -0.86& (Goodale and others 2009)

which would reinforce the low d15N values ob-

served in forest floor organic matter. Under pristine

(low N deposition) conditions a higher proportion

of N supply to decaying litter must have been de-

rived from older SOM because little would come

from atmospheric deposition. Moreover, presumed

higher C:N of fresh litter under pristine conditions

would favor higher microbial demand for N and

higher flux into first-year litter (Frey and others

2000). Hence, it seems likely that decreased de-

mand for fungal translocation to first-year litter

could contribute to continuing N retention in soil

under high N deposition. Forest floor and mineral

soil horizons exhibit highly contrasting behavior

and presumably mechanisms of DOC and DON

dynamics (Figure 4). Mineral soil appears to pro-

vide a less transient sink for organic N derived from

leaf litter, and perhaps this N retention process has

been affected by reduced fungal N demand. Clearly,

additional detailed study of the dynamics of DON

adsorption and retention in forest floor and mineral

soil will be needed to better understand the future

capacity of cold temperate forest soils to retain

anthropogenic N.

ACKNOWLEDGEMENTS

For their assistance in various aspects of this re-

search the authors wish to thank Patrick Bohlen,

Fusheng Chen, Mark Dempsey, Ted Feldpausch,

Lisa Martel, Joe Milanovich, Leo Stoscheck, Robin

Schmidt and David Lewis. This research was sup-

ported by a grant from the National Science

Foundation (DEB-0542065).

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transport of carbon and nitrogen between litter and soil organic matter in a northern hardwood...

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