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Published online 12 March 2007
Published in Soil Sci Soc Am J 71:347-354 (2007)
DOI: 10.2136/sssaj2006.0188
© 2007 Soil Science Society of America
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SOIL BIOLOGY & BIOCHEMISTRY

Low Dissolved Organic Carbon Input from Fresh Litter to Deep Mineral Soils

M. Fröberga,*, P. M. Jardinea, P. J. Hansona, C. W. Swanstonb, D. E. Toddc, J. R. Tarverc and C. T. Garten, Jr.c

a Environmental Sciences Division, Oak Ridge National Laboratory Oak Ridge, TN 37831-6422
b Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550
c Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422

* Corresponding author (froebergmj{at}ornl.gov).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Dissolved organic carbon (DOC) leached from recent litter in the forest floor has been suggested to be an important source of C to the mineral soil of forest ecosystems. To determine the rate at which this flux of C occurs, we have taken advantage of a local release of 14C at Oak Ridge National Laboratory Reservation, Oak Ridge, TN (35°58'N, 84°16'W). Eight replicate 7- by 7-m plots were established at four field sites on the reservation in an upland oak forest setting. Half of the plots were provided with 14C-enriched litter ({Delta}14C {approx} 1000{per thousand}), and the other half with near-background litter ({Delta}14C {approx} 220{per thousand}) for multiple years. Differences in the labeled leaf litter were used to quantify the movement of litter-derived DOC through the soil profile. Soil solutions were collected for several years with tension lysimeters at 15- and 70-cm depths and measured for DOC concentration and 14C abundance. The net amount of DOC retained between 15 and 70 cm was between 2 and 10 g m–2 yr–1. There were significant effects of the litter additions on the 14C abundance in the DOC, but the net transport of 14C from the added litter was small. The difference in {Delta}14C between the treatments with enriched and near-background litter was only about 130{per thousand} at both depths, which is small compared with the difference in {Delta}14C in the added litter. The primary source of DOC within the mineral soil must therefore have been either the Oe or Oa horizon or the organic matter in the mineral soil. During a 2-yr time frame, leaching of DOC from recent litter did not have a major impact on the C stock in the mineral soil below 15 cm in this ecosystem.

Abbreviations: DOC, dissolved organic carbon • EBIS, Enriched Background Isotope Study • ORR, Oak Ridge Reservation • TVA, Tennessee Valley Authority


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Dissolved organic carbon (DOC) is an important factor in both aquatic and terrestrial ecosystems and is significant for transport of nutrients (Qualls and Haines 1991), metals (Tipping and Hurley 1992), and organic pollutants (Chiou et al., 1986). It also plays a key role in the biogeochemistry of C by redistributing it with depth in the soil profile. The DOC is leached from the forest floor by percolating soil water and becomes immobilized in the mineral soil, where it contributes to the soil C stock. Fluxes of DOC directly underneath the O horizon are usually in the range of 10 to 40 g m–2 yr–1 and under the B horizon usually well below 10 g m–2 (Michalzik et al., 2001). The difference between O and B horizons is widely thought to be mainly due to physical and chemical retention rather than rapid mineralization (Kalbitz et al., 2000). The retention of DOC occurs mainly in the upper decimeters of the mineral soil (Michalzik et al., 2001), where the majority of soil C resides (Jobbágy and Jackson, 2000). Numerous studies of DOC concentrations and fluxes in forest ecosystems can be found in the literature. In the majority of those studies, however, no attempt has been made to separate the sources of DOC leaching. Thus, the relative contribution, overall and by depth, of recently photosynthesized C and humified soil organic matter is still poorly known (McDowell, 2003).

Radiocarbon is an excellent tool for studying C turnover in soils and can be used as a tracer of different C sources in the ecosystem. The bomb–14C peak in the mid-20th century is commonly used in studies of C cycling. It has also been used to get information about the movement of DOC in the soil profile (Trumbore et al., 1992; Michalzik et al., 2003; Fröberg et al., 2005). It is not, however, specific to any one form of C (e.g., root vs. leaf litter). In this study, a unique local release of 14C on the Oak Ridge Reservation (Trumbore et al., 2002) was used to track DOC movement from canopy litter sources. The main objective of this study was to use 14C-enriched litter to quantify the proportion of DOC in the mineral soil originating from decomposing fresh litter.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description and Experimental Design
The Enriched Background Isotope Study (EBIS) project was established in the autumn of 2000 on the Oak Ridge Reservation (ORR), in the U.S. Department of Energy's National Environmental Research Park near Oak Ridge, TN (35°58'N; 84°16'W). The aim of the EBIS is to take advantage of a local release of 14C from a nearby incinerator to study soil C cycling in an intact forest ecosystem. The approach is similar to the widespread use of the global atmospheric radiocarbon peak ("bomb peak") in the mid-20th century in C cycling research, but has a different time resolution as well as a control.

Mean annual precipitation at the EBIS sites is 1358 mm and mean annual temperature 14°C (Johnson and Van Hook, 1989). All EBIS research plots are located on upslope, ridge-top positions in the upland oak forest type (Quercus spp., Acer spp.) with scattered pine (Pinus echinata Mill. and P. virginiana Mill.), mesophytic hardwoods (Liriodendron tulipifera L., Fagus grandifolia J.F. Ehrh.), and some hickory (Carya spp.). The ages of the overstory trees cover a broad range from about 40 to 150 yr, and the maximum canopy height is approximately 26 m. Maximum leaf area index is approximately 6 m2 m–2. The EBIS sites on the ORR encompass two soil types and two levels of 14C exposure in 1999. At all sites, replicated permanent plots were established for the manipulation of forest litter through reciprocal transplants of enriched vs. near-background litter among sites.

Before leaf senescence in the fall of 2000, leaf litter was collected on Pine Ridge in the vicinity of the 1999 14C release and on Walker Branch Watershed at the east end of the ORR, which had experienced near-background 14C conditions. Litter was collected from tarps weekly from September through mid-December 2000. Collected litter was air dried and stored in large (1- by 1.5-m) vacuum bags until needed for the experimental manipulations. Sufficient enriched (~1000{per thousand}) and near-background (220{per thousand}) litter was collected in the fall of 2000 to conduct a multiyear, plot-level litter manipulation study (3 yr of litter additions). The {Delta}14C of the near-background litter was slightly elevated relative to the expected northern hemisphere values (Levin and Hesshaimer, 2000) because the 1999 14C-release also exposed eastern portions of the Oak Ridge Reservation to a limited degree (Trumbore et al., 2002).

Four research sites were established on the ORR (Fig. 1 ). On the west end of the ORR, two "enriched" sites were established: one site on Ultisol soils of Pine Ridge, and one site on Inceptisol soils on Tennessee Valley Authority (TVA) land on Chestnut Ridge. Approximately 10 km further east, away from the source of the 1999 14C pulse, two "near-background" sites were established: one site on Ultisols within Walker Branch Watershed on Chestnut Ridge, and one site on Inceptisols on Haw Ridge. The Ultisols are deep, highly weathered soils derived from dolomitic parent material. The Inceptisols are shallow, less weathered soils derived from limey shale and sandstone formations. The humus type for both of these soils is classified as a Leptomoder (a transitional state between mor and mull forest humus) according to the classification scheme of Green et al. (1993). Characteristics of the sites are summarized in Table 1 and a detailed description of the hydrologic, physical, geochemical, and mineralogical properties of these soils can be found in previous studies (Wilson and Luxmoore, 1988; Jardine et al., 1988; Wilson et al., 1989, 1990).


Figure 1
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  		Fig. 1. Experimental treatments in the Enriched Background Isotope Study.

 

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  		Table 1. Soil properties (mean ± SE) and litterfall amounts for the four experimental sites.

 
At each of the four research sites on the ORR, eight square 7- by 7-m plots were delineated with metal fence posts and plastic fencing (~61 cm tall). The forest floor within each plot was covered with landscape cloth from late September through early December of 2000 and ambient litterfall was periodically blown off the landscape cloth. The landscape cloth was removed in mid-December. In May 2001, and January/February 2002, and 2003, 500 g dry mass m–2 of 14C-enriched or near-background litter (approximating the mean annual litterfall, Table 1) was added back to the respective treatment plots. Litter treatments were randomly applied to the eight plots at each of the four research sites. The following combination of replicated research plots was created by the experimental design:
  1. Plots with 14C-enriched root litter, 14C-enriched soil C, and applied 14C-enriched leaf litter (Pine Ridge and TVA western ORR)
  2. Plots with 14C-enriched root litter, 14C-enriched soil C, and near-background 2000 litter, (Pine Ridge and TVA, western ORR)
  3. Plots with background roots and soil C, and 14C-enriched 2000 litter (Walker Branch and Haw Ridge, eastern ORR)
  4. Plots with background roots and soil C and near-background 2000 litter (Walker Branch and Haw Ridge, eastern ORR)

Sampling and Analysis of Dissolved Organic Carbon and Dissolved Organic Carbon-14
Two near-background and two enriched plots from each of the four EBIS sites (16 total plots) were each instrumented with four tension lysimeters (ceramic cups, Soilmoisture Equipment Corp., Santa Barbara, CA). Two lysimeters were placed at 15-cm depth (below the A horizon) and two at 70-cm depth (below the B horizon) in each plot for a total of eight samplers and four instrumented plots per site. Sampling of soil solutions was event based. A total of 16 events were measured during the 2 yr.

Two zero-tension lysimeters were also installed under the O horizon. Unfortunately, they only rarely collected water and were therefore omitted from the data analysis.

All lysimeter solutions were analyzed for solution-phase total organic C using conventional combustion analysis (TOC-5050 analyzer, Shimadzu Corp., Tokyo). The DOC samples for 14C analysis (a total of 244) were sent to the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratory (LLNL) in the form of 5- to 10-mL solutions that were preconcentrated at 95°C by evaporation to provide sufficient C in that volume for 14C analyses. At CAMS, they were further concentrated to ~1 mL by vacuum centrifuging before being pipetted into combustion tubes for final drying in vacuo. Samples were combusted in vacuo in batches in sealed quartz tubes at 900°C with CuO oxidizer plus Ag powder to scavenge impurities such as S and Cl. Samples were measured by accelerator mass spectrometry on the Van de Graaff FN accelerator at CAMS using standard techniques. Splits of combusted sample were taken for 13C analysis for correction of mass-dependent fractionation in the reported radiocarbon values, and all radiocarbon values are presented as {Delta}14C ({per thousand}) according to Stuiver and Polach (1977). Samples collected in 2001 were contaminated with 14C during preparation in an ORNL laboratory previously associated with radiocarbon scintillation counting, and therefore only data from 2002 and 2003 are presented here.

Calculation of Dissolved Organic Carbon Fluxes
To estimate C fluxes, DOC solution concentration data were combined with data on water flux. Storm-driven percolation data from all four sites were shown to be consistent at daily or longer time steps (Fig. 2 ), allowing us to use a single (albeit depth-specific) estimate of hydrologic percolation for the calculation of DOC movement within the soils. Hydrologic flux associated with DOC transport in these soils was estimated from a modified version of the water budget model PROSPER (Huff et al., 1977) parameterized within the stand-level model INTRASTAND (Hanson et al., 2004, 2005b). INTRASTAND is an hourly time step model designed for use in the interpolation of measured physiological data over time for the calculation of daily and intra-annual forest stand C and water budgets. The model is coded using Stella modeling software (isee systems, Lebanon, NH). Published results for the water budget can be found in Hanson et al. (1998, 2001, 2004, 2005b) and Johnson et al. (2002). Calculated total water fluxes were 725 and 953 mm at 15 and 70 cm, respectively, in 2002. In 2003, the fluxes were 893 at 15 cm and 1125 mm at 70 cm.


Figure 2
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  		Fig. 2. Measured patterns of soil water content at two depths within the soil profile of all four study sites (thin lines) along with the predicted water content for each depth (heavy line). Differences in the magnitude of the water content at a given depth for each site and their respective field capacities (flat portions of these curves) are dependent on location-specific rock content and probe calibration biases.

 
Environmental Monitoring
A single solar-powered, weather station with data logger was placed at each research site for the collection of hourly measurements of air temperature, relative humidity, throughfall, soil water content, and other environmental variables used for modeling of water fluxes (Hanson et al., 2005a).

Data Analysis
A two-compartment stable-isotope mixing model was used to evaluate the contribution percentage of litter-derived C to DOC at 15 and 70 cm:

Formula 1[1]
where Litter% is the contribution of new litter to the DOC, DOC enriched and DOCbackground are the average DO14C concentrations at enriched and near-background sites, respectively, and Litterenriched and Litterbackground are the 14C concentrations of the added litter.

Dissolved organic C from litter was assumed to have a 14C signal identical to that in the added litter. The 14C in the non-litter-derived compartment is then calculated as

Formula 2[2]
For a 3-yr increment of time, radioactive decay (1/8267 yr) is negligible and is ignored in the calculations.

Analysis of variance using data from all lysimeters was used to test for differences in DOC concentration and DOC- {Delta}14C abundance. The DOC concentration data, but not DO14C data, were log-transformed before analysis. To account for temporal dependence of successive measurements close in time, a repeated measurements analysis, assuming compound symmetry covariance structure, was performed. One three-way ANOVA per depth was run with treatment, soil type, and position within the ORR (east vs. west) as main effects. In addition, one ANOVA was run per site and depth to check for differences in DO14C between treatments within each site. Finally, an ANOVA including all four sites was run to check for differences in DOC concentrations between sites. All statistical analyses were conducted using SAS 9.1.3 (SAS Institute, Cary, NC).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Concentrations and Fluxes of Dissolved Organic Carbon
The average DOC concentration at the four different sites at 15- and 70-cm depth were 7 and 4 mg L–1, respectively. Although there were differences in DOC concentrations between the sites, no consistent differences in DOC concentrations (Fig. 3 ) or fluxes (Table 2) between the two different soil types were found. Haw Ridge (an Inceptisol) had higher measured DOC concentrations at 15-cm depth than the other three sites (P < 0.05), and Pine Ridge (the other Inceptisol) had the lowest DOC concentrations of all four sites, although not significantly different from the two Ultisols. The same pattern repeated at 70 cm, where Haw Ridge had significantly (P < 0.05) higher DOC concentrations than TVA and Pine Ridge (P < 0.05) and also a tendency for a higher concentration than Walker Branch (P = 0.18). The Haw Ridge DOC levels in lysimeter solutions were consistent with its larger Oe and Oa horizon C pool (Table 1). We do not, however, know the concentrations directly beneath the O horizon and we therefore do not know the amounts of DOC retained in the upper 15 cm of the mineral soil. Seasonal patterns at the four sites were all similar (Fig. 3).


Figure 3
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  		Fig. 3. Measured concentration of dissolved organic carbon (DOC) in lysimeter samples collected at 15- and 70-cm depth at each of the four study sites (Haw Ridge [HR], Pine Ridge [PR], Walker Branch [WB], and Tennessee Valley Authority [TVA]) throughout 2002 and 2003.

 

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  		Table 2. Estimated annual flux of dissolved organic carbon (DOC) below 15 cm, below 70 cm, and the net sorption or consumption between 15 and 70 cm for 2002 and 2003. Standard errors (n = 4) are based on the differences in plot-to-plot DOC concentrations only.

 
The DOC concentrations at 15 cm were low compared with other studies in temperate forests, whereas the concentration at 70 cm was more typical of other published data (Michalzik et al., 2001). The low concentration of DOC at 15 cm implies that, despite relatively large water fluxes, the net transport of DOC from the upper part of the mineral soil down to the B horizon was relatively small compared with observations for forest soils (Michalzik et al., 2001). Multiplication of site-specific DOC concentrations interpolated throughout 2002 and 2003 by the estimated percolation rates allowed the calculation of daily DOC flux for the 15- and 70-cm measurement locations (e.g., Fig. 4 ). Table 2 summarizes the calculated annual flux of C below 15 and 70 cm at all four sites and the net retention in the 15- to 70-cm zone. The flux of organic C through Haw Ridge soils was two to three times greater than for Walker Branch, Pine Ridge, and TVA, and the fluxes for 2002 and 2003 were comparable. Estimates of DOC inputs in the B horizons of the Walker Branch and TVA Ultisols and the Haw Ridge and Pine Ridge Inceptisols showed that these horizons, as expected, received more C than they lost. The net amount of DOC retained between 15 and 70 cm varied between 2 and 10 g m–2 yr–1 (Table 2), with variations occurring mainly between the different sites, and with only small variations between the two different years for each individual site.


Figure 4
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  		Fig. 4. The derivation of dissolved organic carbon (DOC) flux below the A horizon (below 15 cm) or through the B horizon (below 70 cm) throughout 2002 and 2003 for the Haw Ridge soils. Point-in-time measures of DOC concentrations ([DOC])were assumed to apply equally during the time period between measurements (Fig. 1) when percolation flow down through the soil profile was greater than zero. Similar data are available for the Walker Branch, Pine Ridge, and Tennessee Valley Authority sites.

 
Radiocarbon Concentrations in Dissolved Organic Carbon
Mean radiocarbon concentrations in DOC at the 15-cm depth were between 100 and 400{per thousand} for each site, with the lowest values at the near-background sites having near-background litter additions and the highest values at the plots where enriched litter was applied (Table 3). At 70 cm, the DO14C values were lower than at 15 cm, with average values per site ranging from just above 0{per thousand} to nearly 300{per thousand} (Table 3).


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  		Table 3. Dissolved organic 14C (DO14C, mean ± SE) measured at 15 and 70 cm at the four sites in 2002 and 2003. Data for each year and the multiyear average are derived from multiple collections within the respective time period.

 
No statistically significant (P < 0.05) differences between soil types were found for the 14C abundances of the DOC at 15 or 70 cm (Table 3), which corresponds to the DOC concentration data. In theory, the Ultisols should, due to higher Fe oxide content, have a higher capacity of DOC retention than the Inceptisols (Jardine et al., 2006) and therefore both lower DOC fluxes and lower DO14C signal, but this could not be verified with these data.

A clear response in 14C concentration of the DOC to the addition of 14C-enriched litter was found both at 15- and 70-cm depths (Table 3), but the difference was small considering the large difference in 14C between the two types of added litter. There was, on average, a tendency for a slight increase from 2002 to 2003 in the difference between the enriched and near-background sites (Table 3), but overall there was no dramatic change in the treatment effect with time. The average DOC at 15 cm from the plots receiving near-background litter had a 14C abundance on average 128{per thousand} lower than the DOC from plots with enriched litter (Table 3). Differences were largest at the eastern (near-background) sites with, on average, 154 and 112{per thousand} in the DO14C at the plots receiving near-background litter and 314 and 253{per thousand} at the plots with enriched litter for Haw Ridge and Walker Branch, respectively (Table 3). At both of these sites the difference was statistically significant (P < 0.05). In contrast, the differences in DO14C at 15 cm between the treatments at the western (enriched) sites were not statistically significant (i.e., P > 0.05). The DO14C values were 259 and 305{per thousand} for the near-background plots and 331 and 399{per thousand} at the enriched plots for Pine Ridge and TVA, respectively. When data were combined across sites, the treatment effect of the litter manipulations overall was strongly significant at 15 cm with P < 0.0001.

At the 70-cm depth, there was a statistically significant, albeit small, effect of the litter additions on the DO14C signal (P < 0.01 in the three-way ANOVA including all sites). Only at Haw Ridge was the difference between the treatments statistically significant on its own (139 and 265{per thousand} for near-background and enriched plots, respectively, with P = 0.019, Table 3) At the other three sites, the difference was not significant (42 vs. 89{per thousand} at Walker Branch, 126 vs. 250{per thousand} at Pine Ridge, and 114 vs. 257{per thousand} at TVA for near-background and enriched treatments, Table 3).

Contribution from Fresh Litter to Dissolved Organic Carbon at the Two Depths
The DO14C differences between treatments were not large enough to conclude that fresh litter was a major source of DOC at 15 cm; in fact, the opposite may be the case. The difference in DO14C between the treatments with enriched and ambient litter was much smaller than the difference in 14C for the two different types of litter. The enriched litter had a 14C signal of 1000{per thousand}, but the DO14C for the plots receiving enriched litter at 15-cm depth on average was just slightly above 300{per thousand} (Fig. 5 ). If the litter additions had made a major impact on DOC at 15 cm, the DO14C should have been much higher for those plots where 14C-enriched litter was applied. According to a simple two-compartment mixing model, only about 14% (range 3–28%) of the DOC at 15 cm was derived directly from the added litter (Table 4).


Figure 5
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  		Fig. 5. Carbon-14 content in bulk soil (solid bars) and dissolved organic carbon (DOC; patterned bars) for both treatments at all four sites averaged for 2002 and 2003. Error bars represent standard errors.

 

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  		Table 4. The percentage of dissolved organic C (DOC) derived from the added litter and the 14C in DOC derived from sources other than added litter, according to a two-compartment mixing model.

 
At 70 cm, the differences between the treatments show that recent litter was not a dominant source of DOC at depth. During 2002 to 2003, the difference in 14C signal between DOC from the treatments with enriched and near-background litter was, on average, 134{per thousand}. The two-compartment mixing model results demonstrated a 14% contribution (range 5–23%) from added litter to total DOC at 70 cm (Table 4). It can be argued (e.g., Hanson et al., 2005a) that the mixing model should not be applied to the sites where the difference is insignificant; however, the difference between the {Delta}14C in litter and in DO14C at 15 cm is striking and supports the conclusion is that added litter contributes only to a limited degree to the DOC at 15 cm. Furthermore, insignificant differences in DO14C must be interpreted as being indicative of only small DOC contributions from recent litter additions. The estimates have large uncertainties, but nevertheless suggest that fresh litter at the surface makes only a limited contribution to the DOC leaching at 15- and 70-cm depth.

The somewhat higher DO14C values in 2003 than 2002, especially at the western two sites (Table 3), suggest a higher 14C concentration in non-litter-derived DOC in 2003 than 2002 at both depths (Table 4). This increase with time implies a gradual accumulation of 14C-enriched water-soluble organic matter from the 1999 14C pulse in the soil at the western two sites that cannot be attributed to the litter manipulation experiment.

Limited DOC inputs to the mineral soil from recent litter reported here, complements other recent studies where C isotopes have been used to trace the origin of DOC in mineral soils. Using 13C, Hagedorn et al. (2003) found that 90 to 95% of DOC collected with suction cups at 5- to 10-cm soil depth was older than 4 yr. Using bomb14C, Michalzik et al. (2003) observed that DOC in the mineral soil was mainly derived from humified organic matter. Furthermore, DO14C data from Karltun et al. (2005) suggest that DOC in the upper mineral soil was mainly derived from old C. Using 14C, Fröberg et al. (2006) also found that DOC collected in two podzol B horizons was, to a large extent, derived from the mineral soil itself and not from the O horizon.

Even though there was only a small fraction of DOC derived from recent litter at the 70-cm depth, the greater 14C signals in DOC from this depth compared with the respective bulk soil signatures suggests that a significant fraction of DOC was either derived from organic matter in the upper 15 cm of the soil or from younger C deeper in the soil profile. The DO14C signal did not change much between 15 and 70 cm, despite low bulk 14C signals in the mineral soil below 15 cm (Fig. 5). The decrease with depth was, on average, little more than 100{per thousand} (range 15–285{per thousand}), from 207{per thousand} at 15-cm depth to 105{per thousand} at 70 cm for the near-background plots and from 324 to 179{per thousand} for the enriched treatments (Table 3). Therefore, the DO14C did not approach the 14C of the bulk soil, which on average was about 70{per thousand} at 15 to 30 cm, close to 0{per thousand} at 30 to 60 cm, and slightly negative at 60 to 90 cm (Fig. 5). A significant fraction of the DOC at 70 cm must therefore be derived from the soil above 15-cm depth or from soil fractions at depth with higher 14C signal than the bulk soil (e.g., roots or previously retained DOC).

Root-derived C represents an alternative source of DOC within the soil profile. The elevated atmospheric 14CO2 at the western sites increased 14C of the roots at Pine Ridge and TVA, suggesting a potentially interesting comparison of these two sites with the eastern sites (Haw Ridge and Walker Branch). Unfortunately, the ANOVA revealed a statistical interaction between the soil types and the west–east root "treatments," limiting the application of such comparisons. That is, at both the 15- and 70-cm depths, there was a large difference between west and east in the Ultisols (TVA vs. Walker Branch), but there was no corresponding significant effect in the Inceptisols (Haw Ridge vs. Pine Ridge).

Carbon inputs from litter-derived DOC between 15 and 70 cm could also be compared with the input of root-derived C to the mineral soil. The stock of live fine-root biomass at the EBIS sites is about 60 g C m–2 between 15- and 60-cm depth, with turnover times in the range of several years (Joslin et al., 2006) but with a fraction of roots having much shorter turnover times. For DOC, the mixing model suggests that only about 14% of the C was derived from the added litter, which implies that <2 g C m–2 from the annual addition of 250 g C m–2 of fresh litter each year found its way below 15 cm. By comparison, the C stocks between 15 and 60 cm were 1300 to 1700 g m–2, implying that yearly inputs of DOC from litter correspond to about 1{per thousand} or less of the mineral soil C stocks in this depth increment. It is therefore not surprising that the labeled litter additions did not change the 14C signal in the bulk mineral soil during the 3 yr of litter manipulations (Fig. 5).

This study demonstrates that DOC leaching from recent litter inputs to an upland oak forest does not represent a major input of C to the soil below the 15-cm depth. It should not, however, be concluded that other DOC derived from the forest floor Oe and Oa horizons will not contribute significantly to the C stocks in the mineral soil. The 14C signal from the added Oi horizon litter had not yet been incorporated into the Oe and Oa layers, which may be more important sources of DOC (Park et al., 2002; Park and Matzner, 2003; Fröberg et al., 2005). The DOC from the Oe and Oa horizons has been shown to turn over slowly when adsorbed to the mineral soil (Kalbitz et al., 2005) and DOC derived from aboveground sources may therefore, in a longer perspective, be important for soil C stocks.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The 14C-labeled DOC used in this study provided a unique opportunity to isolate recent litter-derived DOC from the overall pool of mobile DOC migrating through the soil profile. The data suggest that rapidly leached materials from fresh litterfall are largely retained or consumed in the near-surface soils, but there remains a small fraction of litter-derived DOC that moves through the soil without retention. Future studies will address the downward migration of the particulate litter C and its contribution to long-term mineral soil C pools.


   ACKNOWLEDGMENTS
 
Funding for the EBIS project was provided by the U.S. Department of Energy (DOE), Office of Science, Biological and Environmental Research, as a part of the Terrestrial Carbon Processes Program. Oak Ridge National Laboratory (ORNL) is managed by UT-Battelle, LLC, for the DOE under contract DE-AC05-00OR22725. The EBIS project participants appreciate access and use of Tennessee Valley Authority (TVA) land on Chestnut Ridge near the Oak Ridge Reservation allowed under Contract no. 105906 between TVA and ORNL. Work at Lawrence Livermore National Laboratory (LLNL) was performed under the auspices of the DOE by the University of California, LLNL, under Contract no. W-7405-Eng-48.

Received for publication May 16, 2006.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome