B13C-0229 1340h
Assessing Impacts of Land Use Change on Carbon Dynamics via Soil Organic Matter Distribution and Stable Isotopic Composition
The conversion of vast amounts of land in Midwestern North America from native tallgrass prairie to agricultural uses promoted the loss of large amounts of soil carbon (C) from this region. Largely due to fire suppression in the region, much rural land not used for crops is comprised of successional forests; many other parcels consist of non-native, cool season grasses. In an effort to assess the status of soil C recovery following agricultural use on these lands, this study explores the C distribution and dynamics in soils supporting two land cover types - cool season grasslands and successional forests. Soil from the top 15 cm in the mineral profile were collected from eight sites representing the same soil type and similar land use histories at the University of Kansas' Nelson Environmental Study Area, within the Kansas Field Station and Ecological Reserves. Four of the sites support cool-season grasses that are maintained by mowing; the remaining four sites support successional forest that has developed for at least 35 y. Soils were subjected to size fractionation, and all fractions as well as bulk soil were analyzed for total C, total nitrogen (N), and C and N isotopic signatures. Soils also underwent long-term incubations, during which cumulative respiration and net N mineralization were assessed. There was no difference between land cover types in bulk soil C or N. The largest size fraction (212 to 2000 mm) from grassland soil had higher total C (45.5\pm8.67 vs. 29.4\pm3.36 mg g$^{-1}$) and higher total N (3.0\pm0.4 vs. 2.3\pm0.2 mg g$^{-1}$) than forested soils' largest fraction; for soil aggregates sized 63 to 212 mm, grassland soil exhibited a near-significant trend of lower total C (P=0.06) and significantly lower total N (2.3\pm0.2 vs. 3.0\pm0.3 mg g$^{-1}$) than the corresponding forest soil fraction. The smallest size fraction ($<$63 mm) exhibited no difference in total C and N between land cover types. The largest size fractions from both land cover types exhibited similar \delta$^{13}$C signatures, reflective of the current C3 vegetation at all eight sites. Smaller size fractions reflected more of a C4 signature in the grassland soils, suggesting that these sites have not experienced soil organic matter turnover to the extent that forested soils have. Net N mineralization data from the incubations are consistent with this suggestion. After 35 years of forest development at these sites, forested soils appear better able to process organic matter into more recalcitrant fractions than cool season grasslands. In conjunction with the larger stocks of biomass C in forests compared to grasslands, these results suggest that if land management decisions permit, allowing forest succession to take place on these soils may be a sound practice for sequestering relatively more atmospheric C.
B13C-0230 1340h
Response of Soil Organic Carbon to Woody Plant Encroachment: Evidence from Organic Matter Fractionations and Stable Isotopes
Soil organic carbon (SOC) storage has been shown to increase, decrease, or exhibit no net change following woody plant encroachment into grasslands. This variability in total SOC response to woody plant encroachment may be the result of climate, soil texture, and species characteristics interacting to impact particular pools of SOC in different ways. Among studies that have found an increase or no net change in SOC with woody plant encroachment, relatively fast cycling, particulate organic matter (POM) pools in shallow soils experience the largest alterations in C content and stable isotope composition. To assess whether this trend is supported where woody plant encroachment is accompanied by losses of SOC, we examined POM fractions from soils collected in sub-humid grassland and adjacent woodland at a Texas site where woody plant dominance has been shown to result in a $\sim$45% loss of SOC. Similar to previous work, the POM fraction $>$500 $\mu$m in size in the woodland contained more C at shallow depths (0-25 cm) than the grassland soil (730 and 290 g C m$^{-2}$ for woodland and grassland, respectively) and was isotopically similar (average $\delta$$^{13}$C = -27.2 %) to woody inputs at the site. At all depths in the woodland, POM pools and total SOC exhibited more negative $\delta$$^{13}$C values than adjacent grassland. Woodland POM pools $>$53 $\mu$m contained as much or more C than the grassland soil; however, the C content of the $>$53 $\mu$m SOC pool was significantly reduced. These results indicate that despite substantial incorporation of woody C inputs into SOC, loss of SOC with woody plant encroachment at this site results from decomposition of grassland derived organic matter residing in the smallest, most recalcitrant SOC pool.
B13C-0231 1340h
Compound-Specific d13C And dD Analyses Of Plant And Soil Organic Matter: Implications For Water Sources And C3-C4 Vegetation Change Studies
Here we present d13C and dD data of C27-C31 n-alkanes from C3 (trees) and C4 (grasses) plants and from the corresponding soils from a grassland-woodland vegetation sequence in central Queensland, Australia. Our data show that C4 species ({\it Iseilema} and {\it Astrebla}) from the grassland were consistently 13C-enriched relative to C3 tree plant materials ({\it Acacia} leaves and seedpods and {\it Atalaya} leaves) from the woodland and woody grassland. However, n-alkanes from the C4 grasses were \deltaD depleted (-77\permil) relative to the {\itAcacia} leaves and seedpods, but showed no difference in dD values when compared with C3 {\it Atalaya} leaves. This is contradictory to data from previous studies, showing that C4 plants were enriched in \deltaD relative to C3 plants (the same direction as the d13C values). This past observation has been ascribed to C4 plants accessing the more evaporation-influenced (D-enriched) surface water and tree roots sourcing more D-depleted deeper soil water. Our data, on the other hand, indicate that ecosystem characteristics (woody versus grassy) have a greater influence on the dD values of the vegetation than the type of photosynthetic pathway. Specifically, the differences in dD values from "woodland" trees ({\it Acacia}) compared with "woody grassland" trees ({\it Atalaya}) suggest that the dD of soil water in semi-arid climates is ecosystem-dependent. This concept is supported by d13C and dD analysis of the C31 n-alkane, a grass-specific biomarker, from woodland and grassland soils. The similar d13C values in the woodland (-25.2±0.34\permil) and grassland soil (-25.1±0.03\permil) confirmed that the C31 n-alkane is grass-derived. The dD values of the C31 n-alkane, on the other hand, were by 12\permil enriched in the soil under the woodland compared with the one under grassland, supporting that the dD values of the soil water profile are governed by the hydrological characteristics of the ecosystem, not by photosynthetic pathway. The C27 and C29 n-alkanes (tree-specific biomarkers) from the woodland soil were more enriched in dD than the C31 n-alkane and were very similar to the values of the Acacia leaves. A crossplot of the d13C and dD values of the long-chained n-alkanes from plants and soil organic matter indicates that isotopic values from soil organic matter n-alkanes faithfully record recent changes in vegetation (in this case C4 to C3-dominated) as well as changes in water sources as the ecosystem changed from grassy to forested. Thus, the data from this study provide insight into soil-water-plant dynamics in semi-arid climate soils and caution against the assumption that d13C and dD differences in C3 and C4 plants are independent of climate and water stress.
B13C-0232 1340h
Soil Carbon Accrual from C3 and C4 Plants Sources in a Restored Tallgrass Prairie Chronosequence
Rapid anthropogenic increase in atmospheric CO2 may have serious consequences for global climate and ecosystem function and integrity. Cultivation of natural areas to agriculture is responsible for substantial losses of carbon (C) from soil to the atmosphere through the alteration of water regimes, decreased annual primary production, and tillage which disrupts the soil structure that helps protect organic matter from decomposition. Due to their high belowground productivity and often well structured soils, prairie restorations are good candidates for restoring soil C to levels that existed prior to cultivation and thus can serve as a net C sink. After restoration, organic matter inputs generally outpace C release by respiration, resulting in a net C gain in the ecosystem. The cool season C3 and warm season C4 plants that compose prairies discriminate differently against the natural 13C stable C isotope of atmospheric CO2. The isotopic signature of soil C pools can therefore provide mechanistic understanding of dynamics of soil organic matter linked to prairie species composition. Soils were sampled from a prairie restoration chronosequence and a C3 pasture at Fermi National Accelerator Laboratory in Batavia, IL. In the prairies, soil gained C over a 15-year sampling interval, while the pasture remained in equilibrium with respect to soil C. Stable C isotopes were measured in whole soil from the C3 pasture and C3/C4 mixed prairies of different ages over a sampling interval to assess if accrued C is primarily from C3 or C4 vegetative sources, which will provide insight into C3-C and C4-C cycling in restored prairies and the mechanisms of C accrual in soil.
B13C-0233 1340h
Accumulation and $\delta$$^{13}$C Composition of Soil Carbon Across a Chronosequence of Dune Complexes at Mono Lake, CA
The amount of C sequestered and its permanence in some deserts could be higher than normally appreciated. Limited soil water availability and slow decomposition rates in desert soils may induce the long-term accumulation of soil organic C and coarse woody litter. We inventoried C in soils along a chronosequence of {\it Sarcobatus vermiculatus} shrub islands and interspaces at the Mono Basin Ecosystem Research Site, CA. Such shrub-island/interspace dune systems are widespread in basin habitats across the Great Basin Desert. We hypothesized that organic C stores would increase across the chronosequence (48, 84, $\sim$300, and 1800-3000 years since exposure by lake recession) and that $\delta$$^{13}$C values of soil organic C (SOC) would become enriched over time due to isotopic fractionation associated with C mineralization of leaf and root litter. C stores quantified in 0-50 cm soils included: SOC, soil inorganic C (SIC; i.e. carbonates removed by 12 M HCl fumigation), and C in partially decomposed woody and fine litter. The youngest dune system contains at least 13.6 Mg C ha$^{-1}$ and the oldest contains at least 37.9 Mg C ha$^{-1}$. Our data suggest slow turnover rates of SOC (C:N ratios $\sim$10) and substantial accumulation of organic C (coarse litter, fine litter, and SOC) in shrub islands across the chronosequence (islands at the youngest site = 8.0 g kg$^{-1}$ and islands at the oldest site = 24.0 g kg$^{-1}$. Large pools of SOC and C in woody debris are potentially protected in this shrub-dominated desert, especially in shrub islands of "old-growth" dune systems. Most of the C in the soil is SIC (94% in youngest dunes to 83% at the oldest dunes). The decrease in SIC proportion as the dune systems age is correlated with a decrease in pH across the chronosequence (10.6 at the youngest site and 9.7 at the oldest site). As dunes age, total soil C isotopic composition shifts from positive $\delta$$^{13}$C values (2.8 to 3.6 $\permil$), indicative inorganic processes, to slightly negative values (-1.2 to -3.7 $\permil$) as a result of organic C accumulation. Contrary to our hypothesis, however, SOC is not enriched in $\delta$$^{13}$C in older dunes. SOC $\delta$$^{13}$C values (-22.3 to -23.7 $\permil$) are similar to leaf litter inputs (-23.8 to -25.0 $\permil$), suggesting a stronger influence by physical weathering of accumulating litter and less influence by microbial C mineralization processes.
B13C-0234 1340h
Landscape variations in vegetation and soil: a moisture gradient at Bonanza Creek, Alaska
The boreal forests of Alaska contain large reserves of carbon that have the potential to act as sources or sinks of CO$_{2}$. To better understand how these reserves might respond to climate change, one must consider environmental differences such as moisture regime and plant community. Alaskan landscapes reflect a wide range of vegetation and soil types. Carbon cycling studies utilize information such as carbon content and isotopic composition to help partition the sources of CO$_{2}$ over various landscapes. In an effort to understand this partitioning, we examined $\delta$$^{13}$C and $\delta$$^{14}$C in plants, dissolved organic carbon (DOC) leached from soils, and CO$_{2}$ respired from soils. Four stations were established along a moisture gradient, which was located near Fairbanks, AK. Each station varied widely in height of water table, plant community composition, and soil depth. The wettest station had soils at or beneath the water table and was dominated by {\it Carex} and {\it Equisitum} spp. with a poorly developed bryophyte layer. The intermediate stations had well developed moss layers dominated by {\it Drepanocladus} and {\it Sphagnum} spp. The driest station had permafrost 65 cm beneath the moss surface and was dominated by willow, bog birch, and drier {\it Sphagnum} species. In July of 2004, we collected 14 samples from the top 5 cm of the moss and/or soil across this gradient. All samples were incubated in room air for approximately four weeks to determine potential CO$_{2}$ production rates. Preliminary data show that samples from the wettest and driest stations respire 3 times less CO$_{2}$ than those from the intermediate moisture stations. This suggests that microbial activity is limited at both high and low moisture contents in boreal organic soils. Future work will test whether fluctuating water levels at our intermediate moisture stations contribute to greater plant and microbial productivity at this site. In conjunction with respiration rates, $\delta$$^{13}$C and $\delta$$^{14}$C signatures of the bulk soil and DOC fractions will better assess the relationship between vegetation, soil respiration and substrate quality along this moisture gradient.
B13C-0235 1340h
Land Use Effects on Carbon Storage in Thailand Tropical Ecosystems
Measurements of stable isotopes of C have proved to be of value in estimating soil organic C turnover times and in partitioning soil organic carbon (SOC) from different sources. Typically, the contrast between sources and estimates of C turnover have been studied in ecosystems where C-3 photosynthetic plants such as hardwoods have been replaced by C-4 photosynthetic plants from agriculture such as corn or sugarcane. Here we report concentrations and stable C isotope ratios of SOC from Thailand coastal mangrove forests and intrusive coastal aquaculture in the form of shrimp and wastewater treatment ponds. There are clear changes in both magnitude and $^{13}$C/$^{12}$C of SOC at former mangrove sites which have been altered to make ponds for shrimp farming and wastewater treatment. For instance, total per cent C from 0-40 cm soil depth (average of four 10 cm layers at 2 sites) was 6.2$\pm$2.8% for mature mangrove, while it was only 0.5$\pm$0.4% for a 10-year old shrimp pond and 1.3$\pm$0.4% for an 8-year old water treatment pond. Previous studies of mangrove organic C balance have indicated that these inter-tidal forest ecosystems are a sink for C and that significant C is vested in both above- and below-ground biomass and stored in sediments. Mangrove forest disturbance by human activities clearly has the potential to affect C storage. Our data indicates that stable C isotope tracing will be of value in tracking changes in coastal forest-aquaecosystems just as it has been for forest-agroecosystems
B13C-0236 1340h
Detrital Controls on Dissolved Organic Matter in Soils: A Field Experiment
We established a long-term field study in an old growth coniferous forest at the H.J. Andrews Experimental Forest, OR, to address how detrital quality and quantity control soil organic matter accumulation and stabilization. The Detritus Input and Removal Treatments (DIRT) plots consist of treatments that double leaf litter, double woody debris inputs, exclude litter inputs, or remove root inputs via trenching. We measured changes in soil solution chemistry with depth, and conducted long-term incubations of bulk soils and soil density fractions from different treatments in order to elucidate effects of detrital inputs on the relative amounts and lability of different soil C pools. In the field, the effect of adding woody debris was to increase dissolved organic carbon (DOC) concentrations in O-horizon leachate and at 30 cm, but not at 100 cm, compared to control plots, suggesting increased rates of DOC retention with added woody debris. DOC concentrations decreased through the soil profile in all plots to a greater degree than did dissolved organic nitrogen (DON), most likely due to preferential sorption of high C:N hydrophobic dissolved organic matter (DOM) in upper horizons; %hydrophobic DOM decreased significantly with depth, and hydrophilic DOM had a much lower and narrower C:N ratio. Although laboratory extracts of different litter types showed differences in DOM chemistry, percent hydrophobic DOM did not differ among detrital treatments in the field, suggesting microbial equalization of DOM leachate in the field. In long-term laboratory incubations, light fraction material did not have higher rates of respiration than heavy fraction or bulk soils, suggesting that physical protection or N availability controls different turnover times of heavy fraction material, rather than differences in chemical lability. Soils from plots that had both above- and below-ground litter inputs excluded had significantly lower DOC loss rates, and a non-significant trend for lower respiration rates . Soils from plots with added wood had similar respiration and DOC loss rates as control soils, suggesting that the additional DOC sorption observed in the field in these soils was stabilized in the soil and not readily lost upon incubation.
B13C-0237 1340h
The influence of mixed forest litters on decomposition processes affecting soil organic matter dynamics
The interaction among diverse plant litters during decomposition can influence processes that dictate soil organic matter (SOM) dynamics. Forest plantation management practices typically include the planting of singletree species and the removal of understory shrubbery. Understory removal is generally seen as a beneficial practice as it tends to reduce competition for moisture and nutrients. However, the elimination of understory species creates a monospecific litter environment that may affect microbial diversity and SOM dynamics. To identify potential mechanisms, we conducted a laboratory microcosm study of soils from two research plantations of ponderosa pine with understory present (UP) and understory absent (UA). Litter amendments were applied as pine-only (P) and pine-ceanothus mixtures (PC), uniformly labeled with 13C and 15N in a reciprocal-label design. This designed allowed us to partition all litter- and soil-derived C and N pools. There were significant interactions between vegetation history and litter amendment in C and N mineralization patterns with the net turnover of both soil- and pine-derived C and N in the P treatments compared to the PC treatments. Vegetative history influenced microbial activity and litter degradation. UP soils showed increased microbial respiration and extractable N but decreased litter mineralization (13CO2) and DOC as compared to the UA soils. Litter additions also influence C and N cycling especially in UA soils. Mixed litter additions increased microbial respiration and litter degradation while decreasing extractable N as compared to single litter additions in the UA soil. This suggests that mixed litters can increase microbial incorporation of important nutrients like C and N as compared to monocultures. These results suggest that plantation management may be improved by modifying the traditional monoculture approach to incorporate more diverse litter inputs.
B13C-0238 1340h
On-Line Isotopic Analysis of Soil-Respired CO$_{2}$
Soils from a replicated long-term litter manipulation study were collected from a Douglas fir stand in central Oregon and from a sugar maple-black cherry stand in northwestern Pennsylvania to determine if processing of labile and recalcitrant organic matter could be detected through changes in the isotopic signature of density-fractionated soil. Soils were density- fractionated at 1.6 g cm$^{-3}$ using sodium polytungstate and incubated under controlled conditions (40% volumetric water content, 21\deg C) for 65 days. The isotopic measurement of soil-respired CO$_{2}$ was accomplished with a Finnigan Gas Bench II coupled to a Delta Plus XL Continuous Flow Mass Spectrometer; external precision was 0.06\permil. We found significant isotopic (\delta$^{13}$C) differences in respired CO$_{2}$ by density, but not among litter exclusion and addition treatments, even where the treatments have been in place for 11 years (PA). Microbially-mediated isotopic fractionation, expressed as the difference between substrate $^{13}$C and respired $^{13}$CO$_{2}$, was large and positive the day of wet-up, but fell to near zero by day 5. Isotopic composition and patterns of lability (measured as respiration rate per gram C) varied dramatically between the two sites. The deciduous forest soil (Alfisol) yielded the expected pattern of higher lability and lower \delta$^{13}$CO$_{2}$ values from the low density fraction. In contrast, the coniferous forest soil (Andisol) revealed a complicated pattern of higher lability in the high density fraction, suggesting organic matter composition, soil mineralogy, and microbiology are interacting in as yet unexplained ways in this soil. Overall, our findings suggest that density alone is not a clear indicator of the recalcitrance of soil organic matter, and that isotopes of respired CO$_{2}$ may yield information about the processing of labile and recalcitrant organic matter.
B13C-0239 1340h
Tracing C Fluxes From Leaf Litter To Microbial Respired CO2 And Specific Soil Compounds
Despite litter decomposition is one of the major process controlling soil C stores and nutrient cycling, yet C dynamics during litter decay are poorly understood and quantified. Here we report the results of a laboratory experiment where 13C depleted leaf litter was incubated on a 13C enriched soil with the aims to: i) partition the C loss during litter decay into microbial respired-CO2 and C input into the soil; ii) identify the soil compounds where litter derived C is retained; iii) assess whether litter quality is a determinant of both the above processes. Three 13C-depleted leaf litter(delta13C ca. -43), differing in their degradability, were incubated on C4 soil (delta13C ca. -18) under laboratory controlled conditions for 8 months, with litter respiration and delta13C-CO2 being measured at regular intervals. At harvest, Compound Specific Isotope Analyses was performed on soil and litter samples in order to follow the fate of litter-derived C compounds in the various pools of SOMn The delta13C of soils carbohydrates, alkanes and Phospho Lipids Fatty Acids (PLFA) were measured, and the mixing model approach used to quantify the contribution of litter derived C to the specific compounds.
B13C-0240 1340h
The Age and Amount of Carbon Released From Incubations of Permafrost Soil From Northeastern Siberia
Permafrost soils are important reservoirs of carbon (C) in arctic and boreal ecosystems. Rising global temperatures are expected to enhance decomposition of permafrost organic matter, and in turn, respired CO2 may cause a positive feedback to warming. Yedoma soils from northeastern Siberia represent a large and poorly understood reservoir of permafrost soil organic matter, and are estimated to contain up to 450 Gt of C frozen since the Pleistocene. These mineral soils from the ice-rich areas of northeastern Siberia accumulated as wind blown loess buried organic matter fragments that were subsequently frozen in permafrost. We conducted laboratory incubations of Yedoma soils in order to understand the lability and the temperature dependence of this frozen organic matter. We collected frozen Yedoma soils from four different locations in northeastern Siberia and incubated them at $5\deg$C, $10\deg$C and $15\deg$C to monitor CO2 flux from microbial decomposition. The four locations included two tundra sites close to the Arctic Ocean and two locations located in boreal forest along the banks of the Kolyma River near Cherskii, Siberia. At all sites, permafrost soils were collected from deep within the soil profile, at depths $>$10m. Additionally at one of the boreal forest sites, soils were also collected from the modern mineral soil surface, which is currently unfrozen in summer and at 2m depth, which is below the modern active layer. We found more than a four-fold range in the rate of CO2 flux among sites during the initial period of the incubation. This range in CO2 flux increased to more than a ten-fold range later in the incubation as the rates at some sites declined to very low levels. Incubation temperature had a positive effect on flux rates. The modern surface soil from the boreal forest had among the lowest fluxes, even though it receives current inputs of labile C from actively growing plant roots, dissolved organic C, and root exudates. The low fluxes from the modern soil relative to the permafrost soils demonstrates the lability of organic matter stored frozen in these Yedoma soils. In addition to fluxes, we measured C isotopes to determine the age and sources of respired CO2. Radiocarbon measurements showed that respired CO2 had 14C-ages ranging from 21,000 to 25,000BP across all sites, with the exception of the surface soil where modern `bomb' carbon was observed in respiration. Measurements of stable C isotopes ($\delta$13C) ranged from -23 to -29.7$\permil$ for all soils, and there was a significant relationship with 14C-ages, where younger respired C had more depleted $\delta$13C values. Our results indicate the potential for ancient C to fuel microbial respiration and C release once permafrost Yedoma soils are thawed.
B13C-0241 1340h
Average Age of Soil Carbon Pools Contributing to Soil CO2 in a Forest Exposed to FACE
Soils represent the largest C reservoir in terrestrial ecosystems and have been highlighted as potential sinks for the anthropogenic additions of CO$_{2}$ to the atmosphere. Soil organic matter (SOM) consists of several pools with different C turnover rates. Long-term terrestrial C storage with rising atmospheric [CO$_{2}$] requires that a substantial proportion of the additional C assimilated by plants is transferred to recalcitrant soil C pools and not quickly returned to the atmosphere through R$_{S}$. Soil CO$_{2}$ reflects oxidation of SOM occurring in soil C pools of different turnover rates and the age of soil CO$_{2}$ reflects the age of the C pools where the CO$_{2}$ originated. Soil CO$_{2}$ can therefore be used to infer the age of the C pools contributing to the return of soil C to the atmosphere through R$_{S}$. At the FACTS-1 research site, an intact forest ecosystem has been exposed to elevated CO$_{2}$ for 8 years. The $^{13}$C composition of the fumigation CO$_{2}$ functions as a continuous ecosystem label and the rate of incorporation of this label into SOM can be used to identify the soil C pools contributing to soil CO$_{2}$. We used the long-term record of the \delta$^{13}C of soil CO$_{2}$ to determine the age and relative contribution of the C pools contributing to soil CO$_{2}$ at different depths in this forest ecosystem.
B13C-0242 1340h
Dynamics of labile and recalcitrant soil carbon pools in a sorghum Free-Air CO2 Enrichment (FACE) agroecosystem
Experimentation with dynamics of soil carbon pools as affected by elevated CO2 can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable carbon isotopic (13C) analysis were used to investigate the labile and recalcitrant soil carbon pools and the translocation, among these pools, of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa Free Air CO2 Enrichment (FACE) experiment, in which elevated CO2 (FACE:560 ppmv) and ambient CO2 (Control: 360 ppmv) interact with well-watered (wet) and water-stressed (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO2 resulted in more soil organic carbon transferred into a slow-decay carbon pool. Also, isotopic mass balance analysis reveals that more new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in FACE wet and dry soil recalcitrant pools were 1.7 and 2.8 times those in Control wet and dry recalcitrant pools, respectively. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm increased from 17 \pm 5.2 years in the Control plots compared to 37 \pm 23.6 years in FACE plots, which was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to that in total SOC. Our results imply that terrestrial ecosystem such as agroecosystems may play a critical role in mitigating excess CO2 in the future atmosphere.
B13C-0243 1340h
Soil Organic Matter Stores and Dynamics in a Chaparral Ecosystem After Eight Years of Exposure to a Gradient of Atmospheric CO$_{2}$ Concentrations, From pre-Industrial Level to 750 ppm
Due to the continuously increasing concentration of atmospheric CO$_{2}$, it has become a priority to understand if soil organic matter (SOM) will act as a sink or a source of CO$_{2}$, under future environmental change. Although many studies have addressed the question, a clear answer, in particular on the long term response, is still missing. Here we report the results of an experiment where we quantified the soil C stores and investigated the dynamics of SOM, its aggregation and pool composition, in a Californian chaparral ecosystem, exposed to a gradient of atmospheric CO$_{2}$ concentrations. In the study site of Sky Oaks (Warner Springs, CA, USA), twelve closed chambers were installed in 1992, and for 8 years they were fumigated with different concentration of CO$_{2}$, ranging from pre-industrial levels (250 ppm) to 750 ppm CO$_{2}$, with step increments of 100 ppm. Fossil fuel-derived CO$_{2}$, depleted in $^{13}$C, was used to fumigate the chambers, thus allowed to trace the C input from the vegetation to the soil at all levels of CO$_{2}$ exposure. In January 2003, soil were sampled from each chamber and shipped to the SUN (Italy). Here, soil samples were separated by wet sieving into different classes of aggregates, namely, macroaggregates ($>$250$\mu$m), microaggregates (53-250 $\mu$m) and silt&clay ($<$53 $\mu$m). Within macroaggregates, we isolated three different structural and functional pools: the coarse particulate organic matter (POM), the microaggregates and the occluded silt&clay. Lastly, a density floatation with Sodium Polytungstate allowed the separation of light fraction contained in the microaggregates from the intra and inter-POM. The isotope mixing-model approach was used to quantify the net C input from the vegetation to the soil along the entire gradient of atmospheric CO$_{2}$ concentrations.
B13C-0244 1340h
Stable carbon isotope signatures of chloromethane in emissions from decaying organic matter and its implications for constraining the atmospheric chloromethane budget
Atmospheric chloromethane plays an important role in stratospheric ozone destruction, but many uncertainties exist regarding strengths of both sources and sinks and in particular the processes leading to formation of this naturally occurring gas. Previously, we have identified a new source of chloromethane (Hamilton et al. 2003), which can explain chloromethane formation in a variety of terrestrial environments. We have shown that chloromethane can be produced abiotically during decay of organic matter (e.g. leaf litter) at both ambient and elevated temperatures. A potentially useful tool in validating chloromethane emission flux estimates is comparison of the carbon isotope ratio of atmospheric chloromethane with those of chloromethane originating from various sources and sinks (Harper et al. 2001, Thompson et al. 2002, Harper et al. 2003, Keppler et al. 2004). We present carbon isotope signatures of chloromethane in emissions from various plant species. The effect of temperature ($40\deg$-$300\deg$C) on emission rate and isotope fractionation was investigated. In most cases $\delta$$^{13}$C values of chloromethane were extremely depleted relative to bulk biomass. The most depleted $\delta$$^{13}$C values were found for emissions of chloromethane released by C3 plants at the lowest temperature of $40\deg$ ($\delta$$^{13}$C values were in the range of $-110\permil$ to $-150\permil$). We suggest that, if decaying plants contribute to the bulk of atmospheric chloromethane, (which has an isotopic composition of around $-36\permil$, Thompson et al. 2002) there must be either a huge microbial sink for chloromethane in soil or a very large kinetic isotope effect associated with the reaction of chloromethane and OH radicals in the atmosphere. These observations are of potential biogeochemical significance in the application of carbon isotope ratios to constrain the atmospheric chloromethane budgets. REFERENCES Hamilton J.T.G., McRoberts W.C., Keppler F., Kalin R.M., Harper D.B., Science 301, 206-209 (2003). Harper D.B., Kalin R.B., Hamilton J.T.G., Lamb C., Environ. Sci. Technol. 35, 3616-3619 (2001). Harper D.B. et al., Chemosphere 52, 433-436 (2003). Keppler F., Kalin R.M., Harper D.B., McRoberts W.C., Hamilton J.T.G., Biogeosciences Discussions 1, 393-412 (2004). Thompson A.E., Anderson R.S., Rudolph J., Huang L., Biogeochemistry 60, 191-211 (2002).
B13C-0245 1340h
Dramatic $^{13}$C-depletion in the plant methoxyl pool and its global biogeochemical implications
Stable isotope analysis has become a powerful tool for environmental scientists, plant biologists, ecologists and geochemists studying global elemental cycles or past climatic conditions. Thus most plant species have been photosynthetically characterised as Calvin cycle (C$_{3}$), Slack-Hatch cycle (C$_{4}$) and Crassulacean acid metabolism (CAM) categories using carbon isotope signatures. Moreover variations in the carbon isotope composition (d13C) of compounds, produced and destroyed in the global carbon cycle, are often used to investigate biogeochemical cycles and global source-sink relationships, as well as the underlying mechanisms. Stable isotope techniques are increasingly applied to the study of atmospheric budgets of volatile organic compounds (VOCs). We report evidence that methoxyl groups in terrestrial plants (in esters and aromatic ethers) have a unique carbon isotope signature exceptionally depleted in $^{13}$C. Plant-derived C$_{1}$ volatile organic compounds (VOCs) are also highly depleted in $^{13}$C compared with C$_{n+1}$ VOCs. Our observations suggest that the plant methoxyl pool is the predominant source of C$_{1}$ compounds of plant origin in the biosphere such as methanol, chloromethane, bromomethane, iodomethane, and cyanomethane. Moreover this pool, which comprises approximately 2.5% of carbon in plant biomass and represents an important substrate for methanogenesis, is likely to be a significant source of highly depleted methane entering the atmosphere. The distinct $^{13}$C depletion of methoxyl groups in plants which is reflected in isotope signatures of C$_{1}$ VOCs may provide a helpful tool in constraining complex environmental processes. These isotope anomalies have a tremendous potential to improve our understanding of the global cycles of atmospheric trace gases and the biochemical pathways involved. Furthermore methoxyl groups could act as markers for biological activity in organic matter of terrestrial and extraterrestrial origin.
B13C-0246 1340h
Methane Climate Forcing and Methane Release in the Siberian Fresh-Water Systems and Marine Ecosystems.
Surface water is a significant part of the Arctic coastal plain landscape, comprising up to 50-80 percent of the land area. In general, Arctic coastal lakes and sea lagoons are thermokarst or thaw by origin. Zones of thawed permafrost called taliks underlie deeper lake and lagoon sediments. Thaw lakes have migrated across the coastal plains during the Holocene (and previous warm epochs), and their concentrations and depth has increased with warmer conditions. Warming increases permafrost thawing and vast organic reservoirs immobilized in permafrost become available for anaerobic destruction through lake growth, migration and developing into sea lagoons. Due to these processes a huge amount of methane releases from fresh-water and shallow marine ecosystems. It might contributes significantly in formation the maximum atmospheric methane over the Arctic exceeds that over Antarctica in wintertime by 8-10 percent. Our direct wintertime methane flux measurements from northern lakes demonstrate that wintertime income of methane provided by them is essential enough to be supposed as underestimated regional wintertime source of methane, because of far less sinks available to balance in winter. As taliks can be through permafrost, another source of atmospheric methane year-round can be associated with disturbance of the sub-sea hydrates. In this report we present the data concerning typical Arctic Siberian lakes, shallow zone of the Laptev and East-Siberian seas.
B13C-0247 1340h
Organic Nitrogen Cycling at the Duke Free Air CO$_{2}$ Enrichment Forest
A recent shift in the paradigm of nitrogen cycling suggests that amino acids provide an important source of plant-available nitrogen. Field experiments have documented intact amino uptake in boreal, alpine and wetland ecosystems. Our work shows that intact amino acid assimilation may contribute substantially to plant N demand in temperate pine stands with shallow organic horizons. By using universally labeled alanine and ammonium in field trials, we were able to demonstrate that pine trees assimilated amino acid-N at approximately half the rate of ammonium. Mean fine root $^{15}$N assimilation of alanine was 0.7 % compared to 1.8 % recovery of ammonium $^{15}$N (p= 0.008). On average the $^{13}$C: $^{15}$N ratio of fine roots in alanine tracer plots was 3:1, suggesting intact uptake occurred. Cycling of amino acids compared to inorganic N was calculated by tracer recovery in plant, microbial and soil pools. Microbes out competed trees for N, immobilizing up to 10% of the tracer N, while retention in SOM dominates the interim (weeks) distribution. Tracer recovery was greatest in the SOM, followed by microbes, extractable NH4 and roots. These data confirm that nitrogen mineralization assays do not accurately measure the pool of available N in temperate forests. These results advance the growing body of evidence for amino acid assimilation by quantifying its importance and bioavailability relative to inorganic N sources in the field.
B13C-0248 1340h
The Effects of N Addition on the Belowground C Cycle in two Temperate Forests
Human activities such as fossil fuel combustion, fertilizer-use, industrial ammonia and biomass burning have roughly doubled the amount of biologically active nitrogen entering ecosystems each year. N is essential for growth and is the limiting nutrient in many ecosystems. Additionally, N availability has been shown to affect plant, root and soil respiration. For several temperate forests, experimental addition of N is associated with a decline in soil CO2 efflux. This decline could be due to either (1) decreased allocation of C to root metabolism and growth because N demand of plants can be met with less energy expended belowground, or (2) decreased rates of organic matter supply or decomposition due to changes in leaf or root tissue chemistry, or to changes in the decomposer community. We use radiocarbon measurements in soil organic matter, heterotrophically respired CO2, and soil respiration to distinguish between these two hypotheses. Atmospheric 14C peaked in the 1960s due to atomic weapons testing and has subsequently been declining. Differences in 14C of soil organic matter and fine roots sampled in control versus N addition plots can be used to determine if turnover differs between these pools by treatment. In temperate forests heterotrophic respiration is distinguishable from autotrophic respiration by its 14C content, so radiocarbon measurements in respired CO2 can be used to estimate the contribution of root respiration to overall CO2 efflux We will report measurements made at two sites: (1) the Bear Brook watershed in eastern Maine, which consists of 2 10ha plots, a reference and another that receives 34 kg N ha-1 yr-1 with sections of hardwood and conifer stands in each plot, and (2) N amendment plots at the Harvard Forest in central Massachusetts, which consist of 6 0.09ha plots, a control, a plot receiving 50 kg N ha-1 yr-1, and one receiving 150 kg N ha-1 yr-1 in both conifer and hardwood stands. Data on root and litter/soil C dynamics on a series of timescales will be presented, together with implications for C storage in ecosystems subjected to anthropogenic N deposition.
B13C-0249 1340h
Nitrogen Transformation And Transport In An Arctic Watershed
Arctic ecosystems are strongly N limited, while a large amount of N exists in soil and a significant amount of N is lost as dissolved organic N (DON) in hydrological flow, bypassing plant uptake. Thus, the long-term N limitation of arctic ecosystems may be determined largely by storage and losses of N in forms that are unavailable for plant uptake. Understanding fundamental processes that determine the forms of N and the relationship between the forms of N and their availability, retention, and losses in soil is critical for modeling the N budget of arctic watersheds. For a better understanding of N dynamics, we studied the turnover and downslope transport of N and the forms of N lost to hydrological flow in a small arctic watershed in northern Alaska. In the early growing season, $^{15}$NH$_{4}$ (58.8mg $^{15}$N/m$^{2}$) was added at 4 different locations along a hill slope (crest, mid slope, foot slope, and riparian). Soils, plants, and soil water were collected from the treatment plots as well as downslope locations, and resin bags were deployed to determine the chemical forms and turnover of the added $^{15}$N. At the end of the first growing season most of the $^{15}$N ($\sim$80%) was found in soil, most of this in the top 3 cm of the moss layer. DON was the predominant fraction of total dissolved N in soil solution collected by lysimeters (60-80% of total), followed by NH$_{4}$ (20-35%) and NO$_{3}$ (4-7%) of near detection limit. The resin bag study showed that NH$_{4}$ was transformed to NO$_{3}$ or DON within all the plots during the growing season. The del-$^{15}$N values from the resin bag study suggest that $^{15}$N added as $^{15}$NH$_{4}$ moved down the slope up to 2 m within the first growing season and was detected as NH$_{4}$, NO$_{3}$ or DON. A previous fertilization study in the same watershed, in which both NH$_{4}$ and NO$_{3}$ were added in much higher doses, detected significantly higher N concentrations in plants at 6 m down the slope after 1 year of N application. The shorter downslope movement observed in current study may be due to the N addition in a trace amount. Alternatively, significant transport of N down the slope may occur during snowmelt rather than in a growing season. Potential transformation processes and the retention and losses of N from the system will be discussed.
B13C-0250 1340h
Soil Microbial $^{15}$N-Natural Abundance is Enriched Relative to Other Soil N Pools and Indicates Microbial C-Limitation
Soil microbial biomass is responsible for many of the nitrogen (N) transformations that occur between different soil organic matter pools, plants and atmosphere. For this reason, it is important to learn more about the $^{15}$N natural abundance of these organisms. The microbial biomass takes up organic carbon (C) and N and inorganic N for assimilation and respiration. Under C-limited conditions, organic N compounds are mainly utilized as C-source, and excess N leaves the cell (mineralization). The processes of N assimilation, N dissimilation, and export discriminate against the heavier $^{15}$N isotope. This leaves the cells $^{15}$N enriched compared to their supposed substrates, or total soil N. Selective uptake of enriched ammonium (as a result of nitrification) may contribute to the higher \delta$^{15}$N of the micro-organisms. We measured $^{15}$N natural abundances of the microbial biomass, using the chloroform-fumigation-extraction method, in grassland soils along an elevation gradient, a fire-disturbance gradient in Florida, a ponderosa forest restoration study, and a dung-gradient near a water source in desert grassland and found 0-12\permil difference between the soil extractable N and the microbial biomass. We also found that the enrichment is quantitatively dependent on C availability. We speculate that, in addition to an indicator for C availability to the microbes, microbial transformations may explain the $^{15}$N-enrichment of soil organic matter with depth.
B13C-0251 1340h
The Combined Role Of Manganese Oxides And Microbes In The Abiotic Uptake Of Amino Acid Nitrogen Into Litter And Soil Organic Matter
Soil organic matter (SOM) is the largest terrestrial C and N store. Microbial and abiotic processes that control the transformation of protein nitrogen in litter and soils into macromolecular humic materials play an important role in organic matter ystorage and soil productivity. There are major gaps, however, in our understanding of these processes and behaviors. Abiotic reactions of amines, phenols and sugars derived from forest leachates or present in detrital and litter organic matter are known to be key processes in the formation of complex organic nitrogen. We present here the results from a study designed to investigate how the inherent chemistry of lignin, leaf litter, and progressively advanced brown-rot wood decay impact the chemical reaction of amino acids with this organic matter. Additionally, experiments in the presence of birnessite (MnO2) were also conducted to investigate the role of mineral induced phenol oxidation on specific amino acid chemical humifcation processes. Solid and liquid state NMR, 13C-labelled tetramethyl ammonium hydroxide thermochemolysis and stable carbon and nitrogen isotope ratio mass spectrometry were used to track the alteration of litter material and document uptake of 13C and 15N labeled amino acids. Preliminary results from birnessite-containing experiments suggest that the metal-promoted oxidation of the lignin, leaf litter, and, in particular, demethylated brown rot wood residues, is necessary to convert the phenols to quinones of some type permitting amine addition. This relationship is particularly true for the production of soluble fractions after two and six weeks of reaction in the presence of the manganese oxides. Additionally, the production of leachable organic matter with incorporated N was promoted in the soluble fractions. Ongoing NMR studies will elucidate the nature of the chemical binding in these experiments.
B13C-0252 1340h
Molecular and Isotopic Comparison of Aquatic Organic Matter Fractions With Associated Stream and Floodplain Sediments
We conducted a pilot study to investigate the relationship between floodplain soils, and stream channel sediments with aquatic organic matter exported from a midwestern agricultural watershed. Similarities in source and relative state of degradation were assessed with molecular biomarkers (lignin) and bulk stable carbon isotope analysis. Sediments collected from the stream floodplain were characterized by greatest overall lignin yield and low relative degradation state. Molecular indicators and the relative state of degradation are similar between channel sediments and particulate organic matter suggesting a common source and degradation history. In contrast, colloidal and dissolved organic matter fractions are characterized by elevated yields and elevated relative degradation showing differences in either source or processing of these size fractions. Shifts in bulk stable carbon isotope values show that carbon-normalized differences in lignin yield are at least partly due to a dilution effect created by autochthonous production during low flow conditions when stream velocity is low and water clarity is high. Results from this work show that, for channel sediments and particulate organic matter in the water column, differences in terrestrial contributions (inferred from lignin yield) are mainly due to the presence/absence of autotrophic productivity in the stream. Floodplain and ditch deposits are most similar in molecular characteristics to agricultural fields; however, they are characterized by greater yield of lignin and are less degraded relative to the field soils. This may be due to selective mobilization of smaller particles during erosion events and subsequent protection from decomposition when deposited in the floodplain (removed from agricultural disturbance).
B13C-0253 1340h
Fatty Acids in Tubeworm-Associated Sediments in the Gulf of Mexico
Vestimentiferan tubeworms in the Gulf of Mexico are a visible and unique component of the macrofaunal community. Tubeworm distribution is controlled by microbially produced sulfide. Sulfide is absorbed by tubeworms from sediments through posterior extensions of their bodies, euphemistically called "roots." Diffusion of seawater sulfate into the sediments may, on its own, provide insufficient sulfate for sulfate-reducing bacteria to supply the sulfide required by a large, mature tubeworm aggregation. Tubeworms potentially overcome this shortfall by actively facilitating sulfate migration into anoxic sediments, possibly by excreting sulfate through their roots. Microbial biomass production would be stimulated by increased sulfate availability. In the summer of 2000, ten push cores were collected in close proximity to tubeworm aggregations using the submersible Johnson Sea-Link. Pore waters were analyzed for sulfur stable isotopes, and fatty acids from residual sediments were used to characterize the sediment microbial community. Lipid analysis shows a quantitive correlation between known bacterial biomarkers and the general biomass markers, hexadecanoate (16:0) and tetradecanoate (14:0). This correlation suggests that the extracted material is largely bacterial. Additionally, good correlations between biomarkers for sulfate-reducing bacteria, 13-methyltetradecanoate (ai-15:0) and 15-methylhexadecanoate (i-17:0), with hexadecanoate suggests microbial community composition does not change with respect to proximity of the tubeworm aggregation. Finally, microbial activity is largely controlled by labile organic matter inputs. Lipid distributions do not suggest tubeworms irrigate sediments with sulfate; however, this may be indicative of the small scale of sulfate diffusion from tubeworm roots rather than an absence of irrigation.
B13C-0254 1340h
Relating Pyrolysis GC-MS to Traditional Soil Organic Matter Characterization: a Comparative Study Across a Landscape.
Ecosystem scale soil organic matter studies commonly involve measurements such as physical or chemical separations to bulk soil elemental estimates, however, these measurements provide limited information about SOM structure and how it varies across landscapes. To bridge the gap between molecular-scale and pool-based understanding of SOM dynamics, we compared soil density fractionation and chemical digestion techniques to more detailed pyrolysis GC-MS measurements. Composite soil samples from 0-10 cm were collected from four sites with observably different soil characteristics along the Front Range in Colorado: aspen grove, spruce-fir forest, and alpine dry and wet meadows. Samples were separated into heavy, light, and water fractions (slow, intermediate, and fast SOM turnover pools, respectively) using a 1.8 g/mL density fractionation. Non-polar extractives, polar extractives, cellulose, and lignin abundances were determined using a standard method for organic matter digestion. Significant differences were absent between sites for all density derived components as well as the non-polar and polar extractives. The spruce-fir SOM, however, showed higher % lignin and lower % cellulose compared to all other sites. We measured lignin, polysaccharide, and long-chain aliphatic derivatives using pyrolysis GC-MS and then compared relative abundances using principal component analysis. Spruce-fir SOM was distinguished by the high relative abundance of lignin derivatives that parallels observed higher % lignin by digestion. The dry and wet meadow SOM was distinguished by the relative abundance of long-chain aliphatic compounds. While the density and chemical fractionations were unable to fully distinguish these soils apart, pyrolysis GC-MS provided more detailed information about the chemical composition of SOM across different ecosystems.
B13C-0255 1340h
Sorptive protection or chemical recalcitrance - how important are minerals for soil organic matter stabilization?
Stable soil organic matter (SOM) is important for many environmental issues, including soil-atmosphere carbon exchange and the fate of organic pollutants in soils. It is desirable to understand the chemical composition and the reasons for slow turnover of this organic matter pool. To discern the mechanisms responsible for OM protection and gain insight into the molecular composition of refractory compounds, we operationally divide stable OM into that protected by sorption to mineral phases and that protected by inherent (chemical) recalcitrance. Aggregation or physical protection is generally effective on shorter timescales (i.e., decades) than we are considering. While recalcitrance dominates the preservation of fossil OM in sedimentary rocks, the quantitative importance of intrinsic chemical resistance in soils is unclear. Our objectives were (i) to estimate the relative contribution of recalcitrant SOM to stable SOM; (ii) to characterize the gross chemistry of recalcitrant SOM, and (iii) to elucidate the contribution of lignin to recalcitrant SOM and to SOM stabilized by minerals. We studied 12 samples from acidic soils of varying mineralogical composition. A sequential oxidation-demineralisation treatment (NaOCl-HF) yielded quantitative estimates of the relative sizes of the sorptive protected and recalcitrant SOM pools. We found that the concentration of stable OM, at least in acid sub-surface soils, is governed by poorly crystalline minerals (oxalate-extractable Fe+Al). Stable OM was roughly 75% mineral-protected C and 25% chemically recalcitrant C (with large variations). When isolated, the recalcitrant OM fraction was older than sorptive protected OM and had uniform chemical composition dominated by aliphatic C moieties. Lignin was not an important component of the sorption-protected or chemically recalcitrant OM fractions.