B23B-0412
Carbon and Nitrogen Storage in Glomalin-Related Soil Protein During Grassland-to- Woodland Succession
Glomalin is a hyphal glycoprotein produced by arbuscular mycorrhizal fungi that has been found to make a significant contribution to soil organic matter and to play a key role in the process of soil aggregation. However, little is known regarding the effects of land cover changes on glomalin storage in soil. To evaluate this, we quantified glomalin in soils along a grassland-to-woodland chronosequence in a subtropical mesquite savanna located in southern Texas. Soil cores (0-10 cm) were collected from remnant grasslands (age 0) and from adjacent woody plant stands (ages 14 to 86 yr). Glomalin-related soil protein (GRSP), operationally defined as Bradford reactive soil protein was extracted from soil as easily extractable glomalin (EE-GRSP) and as total glomalin (T-GRSP). EE-GRSP was extracted from 1 g soil with 8 ml of 20 mM citrate-buffer, pH 7.0 at 121 °C for 30 minutes. T-GRSP was extracted from 1 g soil with 8 ml of 50 mM citrate-buffer, pH 8.0 at 121 °C for 60 minutes; extractions were repeated up to 4 times. Extracts were purified by precipitation at pH 2.5, reconstituted in 0.1 NaOH, dialyzed against dH2O, freeze-dried, and analyzed for %C and N. EE-GRSP concentrations ranged from 1.0-1.4 mg/g in remnant grasslands, and from 1.7-2.3 mg/g in wooded areas. Similarly, T-GRSP concentrations ranged from 1.2-2.6 mg/g in remnant grasslands, and from 2.8-4.3 mg/g. Both GRSP fractions increased linearly during the first 40 years of woody plant encroachment, and then remained relatively constant at approximately 4 mg/g in woody clusters ranging in age from 50-90 years. Carbon and nitrogen concentrations in T-GRSP (C = 10-25%; N = 1-3%) were similar in both remnant grasslands and woody plant stands. C and N in T-GRSP accounted for 6% of total soil organic carbon (SOC) and 5% of soil total N in remnant grasslands, and 4% of both SOC and total N in wooded areas. Our results show that woody plant cover significantly affects GRSP concentrations, likely due to increased productivity of arbuscular mycorrhizal hyphae in wooded areas. In addition, glomalin represents a relatively constant but significant proportion of SOC and total N throughout the process of grassland-to- woodland conversion.
B23B-0413
Microbial Accessibility of Soil Organic Matter Following Woody Plant ýEncroachment Into Grasslands
Woody plant encroachment into savannas and grasslands is a global phenomenon that can ýhave profound impacts on soil organic matter (SOM) dynamics. Documented impacts ýinclude changes in plant chemical composition, SOM association with mineral particles, ýand microbial ecology. We used lab incubations to quantify the amount and δ13C of CO2 ýrespired from soils obtained along a successional chronosequence from grassland ýdominated by C4 grasses to woody patches dominated by C3 trees/shrubs in a subtropical ýsavanna parkland in southern Texas. Respired CO2 from remnant grasslands was 13C-ýdepleted with respect to bulk soil C by 1 to 4‰ during days 1-2, followed by ýprogressive 13C-enrichment out to 10 days such that CO2 was ultimately up to 2.5‰ enriched with respect to bulk. By day 29 all evolved CO2 was again depleted or ýequivalent to starting SOM values. The δ13C of evolved CO2 for clusters was more ývariable yet displayed the same pattern of δ13C depletion followed by enrichment, and in ýgeneral appeared to be controlled by cluster age. As time progressed younger clusters ýtrended to more depleted values, as did the grasses, while older clusters remained lightly ýenriched with respect to bulk soil C. It is unknown if the pattern of depletion followed by ýenrichment of respired CO2 during the first weeks of incubation arose from kinetic ýisotopic fractionation during decomposition, differential use of carbon sources with ývarying isotopic signatures, increasing contributions from δ13C enriched microbial ýbiomass, or preferential degradation of δ12C positions within compounds by ýmicroorganisms, as hypothesized in other studies. The two youngest clusters (14 and 23 ýyears) resembled the adjacent C4 grasslands rather than the more mature C3 woody ýclusters both in isotopic composition and CO2 respiration rates, signifying the use of ýprimarily older, C4-derived carbon. Upon day 21 the isotopic signature of respired CO2 ýfrom the two youngest clusters approached older clusters. In woody clusters >30 yrs old ýSOM dynamics appear to be altered as the fractional percentages of the free light fraction ýý(<1.0 g/cm3) and macroaggregates (>250 µm) within the soils approximately doubles ýwith respect to surrounding grasslands, which is reflected in a strong shift in the amount ýand δ13C of respired CO2. Continuing research will investigate respiratory kinetics of soil ýphysical fractions and characterize the microbial community that develops during decay. ýThis work will have important implications for understanding how this globally relevant ývegetation shift will influence soil organic carbon storage. ý
B23B-0414
Activities of Extracellular Enzymes in Soils Following Woody Plant Invasion of Grassland
Extracellular enzymes produced by microbes and immobilize in the soil environment are the principle means by which complex plant and microbial compounds are degraded. The concentration of these enzymes and their ability to interact with litter and soil organic matter contributes both to the stabilization and destabilization of soil carbon. We quantified the activities of three extracellular enzymes, B-glucosidase, B- glucosaminidase, polyphenol oxidase (PPO), and a general marker for hydrolytic activity through fluorescein diacetate (FDA) hydrolysis activity, in a subtropical savanna parkland in southern Texas where woody plants have invaded a once open grassland. Previous research has demonstrated that areas which have shifted to woody vegetation are accruing soil carbon, undergoing a dramatic shift in the chemistry of plant input, and increasing in hyphal biomass. Soils were obtained along a successional chronosequence from grassland dominated by C4 grasses to woody patches dominated by C3 trees/shrubs in Oct 2006 and stored immediately frozen until thawing for enzyme assay. Most enzymes, with the exception of PPO, show distinct behavior when comparing grassland and clusters in that grasslands exhibit far lower mass normalized activity than clusters and no activity trend with respect to age of the adjacent cluster. Both FDA and B- glucosaminidase activities are positively correlated with the age of the woody clusters and increase their activity by as much as 10-fold across the age gradient from 14 yr to 86 yr old clusters. The cellulose degrading enzyme, B-glucosidase, always exhibited greater activity (1.5 -4 fold) in woody clusters than in grasslands, but did not exhibit a trend with increasing cluster age. The PPO activity is anomalous in that there is no quantitative difference in mass normalized activity between grassland and cluster and no trend with cluster age. The results for the FDA and B-glucosaminidase assays are consistent with concurrent studies showing that there is a nearly 4 fold greater hyphal biomass, which is abundant in chitin, in wooded areas compared to remnant grasslands and that hyphal productivity rates increased linearly with woody plant stand age. These results have important implications for understanding how this globally important vegetation change will influence soil carbon cycling storage and release.
B23B-0415
Mycorrhizal Productivity Following Woody Plant Invasion of Grassland
Mycorrhizal fungi play an important role in soil carbon storage and dynamics through the production of recalcitrant organic compounds (e.g., glomalin and chitin), and through the production of hyphae which entangle and enmesh soil particles to form aggregates which physically protect organic matter from decomposer organisms. Despite these important functions, little is known regarding rates of mycorrhizal productivity and how these rates might be influenced by changes in plant community composition. We quantified mycorrhizal production in a subtropical savanna parkland in southern Texas where woody plants have invaded areas that were once open grassland. Mycorrhizal ingrowth bags (3 x 10 cm) were made from 50 μm nylon mesh, filled with sterile sand (200-600 μm particle size), and deployed in the field in triplicate in remnant grasslands (n=15), and in woody plant stands (n=13) ranging in age from 15 to 86 yrs. Ingrowth bags were installed in May and harvested in Oct 2007 after 156 days. Hyphae were isolated by flotation/filtration, cleaned thoroughly to remove sand, freeze-dried, and weighed. Microscopic examination indicated that nearly all hyphae recovered from ingrowth bags were from arbuscular mycorrhizal fungi. During the ingrowth period, nearly 4X more hyphal biomass accumulated in wooded areas (9.00 ± 3.84 g m- 2) compared to remnant grasslands (2.35 ± 0.56 g m-2). Hyphal productivity rates increased linearly with woody plant stand age (r = 0.89) from 15 ± 4 mg m-2 day-1 in grasslands (time 0) up to 58-98 mg m-2 day-1 in wooded areas >65 yrs old. When these productivity rates are annualized, we find that hyphal productivity represents approximately 4% of aboveground net primary productivity (ANPP) in wooded areas, and 2% of ANPP in remnant grasslands. These observations are consistent with concurrent studies showing that glomalin concentrations and chitinase enzyme activity both increase in soils with time following woody encroachment into grassland. Since woody plant invasion of grasslands is one of the most significant global land cover changes occurring today, these results have important implications for understanding how this vegetation change will influence soil carbon storage and dynamics.
B23B-0416
An Integrated Spatially Dynamic Disturbance and Forest Soil Carbon Model: Preliminary Results from Willow Creek Experimental Forest
Total forest carbon (C) storage is determined by forest succession, multiple interacting disturbances, climate and the edaphic properties of a site or region, including soil texture and depth. How these complex processes interact will determine forest carbon dynamics at landscape and regional scales. We have developed a new succession extension for the LANDIS-II forest landscape simulation model that incorporates the belowground soil C dynamics of the Century soil model. This extension simulates three primary soil organic matter (SOM) pools (fast, slow, passive), litter dynamics, and nitrogen (N) feedbacks to overstory production. The extension was validated against data from the Willow Creek experimental forest in Wisconsin, USA. We subsequently initialized the full model to simulate forest dynamics of 10,000 ha of the surrounding forest landscape. We simulated a representative harvest regime and a historic wind throw regime (50 year wind rotation period, including light, moderate, and extreme events), two common disturbances in mesic forests of the Lake States. We also simulated forest change and total C storage assuming no atmospheric N deposition and N deposition equivalent to 2008 rates. Our results indicate a strong feedback from harvesting to litter C and the fast and slow SOM pools. The passive SOM pool was not significantly altered. Wind disturbance had a negligible effect on all pools. Simulations without N deposition significantly underestimated contemporary forest productivity and the system was more sensitive to disturbances when N deposition was excluded. In conclusion, we have developed a robust model of above and belowground C and N cycling that can readily plug into an existing forest modeling framework to simulate landscape and regional scale forest dynamics and the interactions among forest disturbances, climate change, and soil processes.
B23B-0417
Mineral Control of Soil Carbon Dynamics in Forest Soils: A Lithosequence Under Ponderosa Pine
The role of soil organic carbon in regulating atmospheric CO2 concentration has spurred interest in both quantifying existing soil C stocks and modeling the behavior of soil C under climate change scenarios. Soil parent material exerts direct control over soil organic carbon content through its influence on soil pH and mineral composition. Soil acidity and mineral composition also influence soil microbial community composition and activity, thereby controlling soil respiration rates and microbial biomass size. We sampled a lithosequence of four parent materials (rhyolite, granite, basalt, limestone) under Pinus ponderosa to examine the effects of soil mineralogy and acidity on soil organic carbon content and soil microbial community. Three soil profiles were examined on each parent material and analyzed by X-ray diffraction, pH, selective dissolution, C and N content, and 13C signature. Soils from each of the four parent materials were incubated for 40 days, and microbial communities were compared on the basis of community composition (as determined through T-RFLP analysis), specific metabolic activity, biomass, δ13C of respired CO2, and cumulative amount of C mineralized over the course of the incubation. Soil C content varied significantly among soils of different parent material, and was strongly and positively associated with the abundance of Al-humus complexes r2 = 0.71; P < 0.0001, Fe-humus complexes r2 = 0.74; P = 0.0003, and crystalline Fe-oxide content r2 = 0.63; P = 0.0023. Microbial community composition varied significantly among soils and showed strong associations with soil pH 1:1 in KCl; r2 = 0.87; P < 0.0001, concentration of exchangeable Al r2 = 0.81; P < 0.0001, amorphous Fe oxide content r2 = 0.59; P < 0.004, and Al-humus content r2 = 0.35; P < 0.04. Mineralization rates, biomass and δ13C of respired CO2 differed among parent materials, and also varied with incubation time as substrate quality and N availability changed. The results demonstrate that within a specific ecosystem type, soil parent material exerts significant control over the lability and bioavailability of soil C and soil microbial community composition. We suggest that soil parent material and mineralogy are critical parameters for predicting soil C dynamics and recalcitrance of soil C stocks.
B23B-0418
Soil C Saturation is Evident in Soils Rich in Organic Matter
Recent studies suggest that mineral soils have a limit to their soil C stabilization capacity. We reasoned that C saturation will be most evident in soils that are already rich in soil organic C (SOC) and have been exposed to a broad range of C input. Therefore, we determined the modes of soil C saturation in an agricultural experiment located in Ellerslie, Canada, where organic matter-rich soils have been cropped to cereal grain for more than 20 years. In this experiment, soils are subject to a broad range of soil C input due to a combination of straw incorporation, N fertilization, and tillage treatments. We determined if C saturation is taking place in soil size fractions that are functionally different, namely the large macroaggregates (>2000 μm), small macroaggregates (2000-250 μm), microaggregates (250-53 μm), and silt plus clay fraction (<53 μm). Macroaggregates were further separated into coarse particulate organic matter (cPOM), microaggregates occluded within the macroaggregates, and silt plus clay fraction. The relationship between soil C input and SOC concentration in these fractions was examined. The soils were highly aggregated, with more than 80% of the soils consisting of macroaggregates. The SOC concentration associated with soil size fractions decreased from macroaggregates to silt plus clay fraction, showing a clear hierarchy of soil aggregates. The SOC concentration associated with whole soil and soil aggregates isolated from bulk soil did not increase with higher soil C input. Similarly, the soil fractions within macroaggregates did not stabilize extra SOC in response to greater soil C input. Thus, we conclude that C stabilization is governed by C saturation in this highly-structured soil. Our study suggests that soils that are farther away from C saturation potential will have greater C sink capacity than soils that are close to their C saturation capacity.
B23B-0419
Investigating extent of dissolved organic carbon stabilization by metal based coagulant in a wetland environment
This study is part of a larger project designed to investigate the feasibility of using metal-based coagulants to remove dissolved organic carbon (DOC) from island drainage water in the San Joaquin Delta and subsequently retaining the metal-DOC precipitate (floc) in wetlands constructed at the foot of levees to promote levee stability. Dissolved organic carbon is a constituent of concern as some forms of DOC can be converted to carcinogenic compounds during drinking water treatment. The focus of this work is to assess floc stability over time and to determine whether floc can be permanently sequestered as part of wetland sediment. Drainage water collected seasonally from Twitchell Island was coagulated with ferric sulfate and polyaluminum chloride at optimal and 50%-optimal dosage levels. Floc was incubated in the laboratory under anaerobic conditions for six weeks under various conditions including different DOC concentrations, microbial inoculants, and addition of nutrients. Preliminary results indicate the floc is a stable system; little to no DOC was released from the floc into the water column under incubations with native microbial inoculate. In addition, floc incubated with previously coagulated water appeared to remove additional DOC from the water column. Future work will involve field and laboratory studies using 13C labeled plant material to examine the effects of fresh plant matter and the effects of peat soil DOC on floc stability, in order to elucidate mechanisms behind carbon stabilization by metal-based floc.
B23B-0420
Spatiotemporal Variations in Soil Moisture With Elevation and Aspect in a Semi-arid Watershed; A Potential Control on the Soil Carbon Pool
Soils comprise the second largest terrestrial reservoir of carbon, yet the controls on this reservoir are not sufficiently understood to predict its response to global climate change. Investigations in the Dry Creek Experimental Watershed (DCEW) near Boise, ID seek to explain why the concentration of soil carbon increases by a factor of up to 7 over a 600m increase in elevation, and by a factor of 10 on north-facing slopes relative to south-facing slopes at a given elevation. We hypothesize that a key control on soil carbon storage in this environment is a topographically-driven difference in duration of soil moisture into the summer. Carbon production and soil respiration are temperature-limited during winter and moisture-limited during summer, but during a brief period in spring high soil moisture and temperature conditions combine to produce peak carbon fluxes. Changes to the duration or timing of these ideal conditions could strongly affect the carbon storage potential of soils in semi-arid environments. Soil moisture measurements made over the past year suggest that near-surface, lower-elevation soils dry to the wilting point (5% volumetric water content) about 2 weeks earlier than those at higher elevations, and that near-surface south-facing soils dry to the wilting point up to 2 weeks sooner than north-facing soils at a given elevation. Furthermore, measured soil respiration (carbon dioxide efflux) indicates that an increase in soil moisture resulting from a single precipitation event during the warm summer months can increase the annual carbon efflux from soils by about 25%. The measured trends indicate that both soil carbon storage and output processes are sensitive to soil moisture conditions. Additional potential controls on the soil carbon reservoir, including soil texture and vegetation productivity, are being examined for correlation with carbon storage patterns to allow an analysis of relative influences.
B23B-0421
Soil Organic Matter Storage and Dynamics Along Altitudinal Gradient in Bornean Tropical Forests: Preliminary Radiocarbon Results of Bulk Soils and Density Fractions
Density fractionation in combination with sonication is an effective approach isolating the soil organic matter (SOM) pools that differ in turnover rate and underlying stabilization mechanisms. For instance, low-density fraction (LF) is expected to have higher turnover rate and sensitivity to climate change than high-density fraction (HF). We examined SOM dynamics in undisturbed forest soils (top 10cm mineral soils) on metasedimentary parent material along an altitudinal gradient from 700m to 2700m in Mt. Kinabalu, Borneo. Soil was fractionated into the following pools: mineral-free LF (f-LF), mineral-associated LF (m-LF) which is liberated after sonication, and HF. We have previously shown a gradual increase in the mean residence time of surface soil C from 3-5 years at 700 m (MAT of 24°C) to 20-30 years at 2700 m (13°C), estimated from primary productivity and bulk soil C standing stock with steady-state assumption. The purpose of current study is to gain further insights on the turnover time of these soils and density fractions based on radiocarbon analysis. Initial analysis showed little difference in 14C content of bulk soils across altitudes (108-116 percent modern). The result suggests two possibilities. First, the same 14C content has two solutions for mean residence time, and thus upper-altitude SOM may turn over slower than lower-altitude SOM as expected from previous studies. Second, the 2700-m soil may have as fast turnover time as 700-m soil due to destabilization mechanisms that counteracts lower temperatures. For instance, significantly higher activity of earthworms at upper altitude on Mt. Kinabalu might enhance SOM turnover. The lowest 14C contents, and hence slowest turnover, were found in low-altitude, clay-rich, HF fractions, having surface area-normalized organic matter loadings of <1 mg C m-2. This result suggests that higher SOM loadings at higher altitudes are rich in relatively young organic matter, which is likely not as protected by clay-size minerals.
B23B-0422
Can Earthworm "mix up" Soil Carbon Budgets in Temperate Forests Under Elevated Carbon Dioxide?
The effects of global change on earthworms and their associated feedbacks on soil and ecosystem processes have been largely overlooked. We studied how the responses of a temperate deciduous forest to elevated carbon dioxide atmospheric concentrations (e[CO2]) influence earthworms and the soil processes affected by them. Our objectives were to: i) identify soil layers of active soil mixing under e[CO2] and current carbon dioxide atmospheric concentrations (c[CO2]) using fallout cesium (137Cs), ii) study how e[CO2] affects earthworm populations, iii) understand the relationship between soil mixing and earthworms at our study site, and iv) identify the implications of earthworm-mediated soil mixing for the carbon budget of a temperate forest. To study soil mixing, we measured vertical 137Cs activity in soil cores (0-24 cm depth) collected in replicated e[CO2] and c[CO2] sweetgum (Liquidambar styraciflua) plots (n = 2) in a Free Air CO2 Enrichment (FACE) ecosystem experiment at Oak Ridge National Laboratory. We measured earthworm density and fresh weight in the plots in areas adjacent to where soil cores were taken. Preliminary results on the vertical distribution of 137Cs in the c[CO2] treatments showed that higher 137Cs activity was located from 8-16 cm depth and no 137Cs activity was measured below 20 cm. In contrast, in the e[CO2] treatment, peak 137Cs activity was slightly deeper (10-18 cm), and 137Cs activity was still measured below 22 cm. Mean earthworm density was higher in e[CO2] than c[CO2] treatments (168 m-2 and 87 m-2, respectively; p = 0.046); earthworm fresh weights, however, did not differ significantly between treatments (32 g m-2 and 18 g m-2, respectively; p = 0.182). The 137Cs vertical distribution suggest that soil mixing occurs deeper in e[CO2] than in c[CO2] treatments, which is consistent with higher earthworm densities in e[CO2] than in c[CO2] treatments. Mixing deeper low carbon content soil with shallower high carbon soil may result in a dilution of net carbon inputs in forest soils exposed to e[CO2]. Vertical dilution of new carbon may explain why carbon accrual is detected only near the surface at this FACE site. By identifying the depths of active soil mixing and possible soil mixing mechanisms (e.g. earthworms), accounting of new organic carbon accrual could be more reliably determined for forest soils in response to e[CO2] conditions.
B23B-0423
Assessing Organic Carbon Stabilization in Chihuahuan Desert Soils Using Sequential Density Fractionation
Stabilization of organic matter on mineral surfaces strongly affects rates of soil organic matter (SOM) accumulation and turnover. Controls over SOM are of particular interest in arid and semi-arid systems where the abundance of woody plants has increased globally over the past century. This proliferation of woody plants may lead to significant soil organic carbon (SOC) accumulation, although a large degree of uncertainty exists in the direction and magnitude of SOC pool responses to woody encroachment. We hypothesized that SOC accumulation from woody encroachment would be primarily due to increased light fraction C pools and also that soil parent material would strongly influence SOC stabilization. Previous studies at mesic sites have used sequential density fractionation to separate soil particles based on mineralology and to explore C stabilization via organo-mineral complexes that might affect particle density. We explored mechanisms of SOM stabilization in arid soils by density fractionating four Chihuahuan Desert soils. The soils differed in parent material (igneous vs. limestone alluvium), landscape position (bajada vs. basin floor), and dominant vegetative cover (intact grassland vs. shrubland in former grassland). We used sodium polytungstate to separate soils into seven fractions with density cutoffs of 1.68, 1.87, 1.98, 2.18, 2.47, 2.66, and >2.66 g cm-3 (hereafter F1-F7, respectively). Concentrations of C and N generally decreased with increasing particle density. Similar to findings from mesic sites, C:N decreased with increasing particle density. While F1 accounted for a small proportion of total mass (0.29-2.61%), a large proportion of total C was present in this fraction (25.3-39.2% of total) due to the high [C] (21-38%C). Carbon in these light fractions is likely to be primarily recently-derived plant material that turns over rapidly and is not stabilized on mineral surfaces. The basin floor sites contained a large proportion of the total C in F5 (38 and 52% of total) despite relatively low [C] in this fraction (1.2 and 1.0%C). In contrast, the bajada sites had similar [C] in F5 (1.0 and 1.2%C), but a lower proportion of total C in this fraction (25.8 and 21.9% of total). Contrary to our expectations, C and N pools in both bulk soils and density fractions were largely consistent among soils within the same landscape position, while neither dominant vegetative cover nor parent material strongly influenced C and N distribution among density fractions.
B23B-0424
Labile organic carbon stocks and mineral-organic matter associations in soil: Role of anticipated changes in rainfall pattern
Anticipated changes in climate are expected to have significant effect on biogeochemical cycling of essential elements in the critical zone. In California, over the next 100 years, a significant shift in rainfall amount and timing is expected. There is currently a disagreement whether the anticipated increase in rainfall will fall as either additional winter rain or result in extension of the rainy season into late spring and early summer months. Here we present results from a field rainfall addition experiment that was setup to test the effects of increased amount of rainfall (20% over ambient) and timing (no addition = control, increased rainfall in the rainy season = winter addition, or increased rainfall during late spring and early summer = spring addition treatment). Specifically, in this presentation, we will discuss results from work looking into changes in amount and biochemical composition of free light soil organic matter fraction (fLF, <1.7g/cm3 in 0-5 and 5-10cm soil depths) and association of soil organic matter storage with Fe and Al oxides in soil (top 50cm soil). We found that the two treatments, winter vs. spring addition, have different effects on carbon storage and association of organic matter with iron and aluminum oxides in soil. Extension of the rainy season into late spring and summer months results in significant increase (20-40%) in the fraction of carbon that is stored as fLF, which was not observed in the winter treatment. Increase of rainfall amount during the already wet rainy season (winter addition treatment) leads to important changes in relationship of organic carbon with soil mineral that are critical for SOM stabilization. More than 35% of the variability in soil carbon storage in the control and spring treatments is explained by dithionite extractable (pedogenic) Fe, compared to <0.01% in the winter treatment plot. Similarly, more than 26% and 41% of the variability in carbon storage is explained by poorly crystalline iron and aluminum oxides in the control and spring treatments, respectively, compared to <5% in the winter treatment plots. Our results suggest that rapid shifts in rainfall patterns could have significant implications for not just soil carbon storage but also its stabilization.
B23B-0425
Response of Organic Carbon Fractions in Tropical Soils to Land Use Changes: Evidence from 13-C Natural Abundance
Tropical soils store about one third of the global soil organic carbon. Quantitative knowledge of stabilization and decomposition processes is necessary to understand, assess and predict effects of land use changes on storage and stability of soil organic carbon (SOC). We analyzed the effects of land use (natural forest, pasture, secondary forest) on carbon storage in different organic matter fractions of topsoils developed on different parent material (marine Tertiary sediments and volcanic ashes) in the humid tropics of northwest Ecuador. Soil density fractionation was combined with 13-C analysis to determine the origin and stability of SOC under pasture and secondary forest. Stocks of mineral-associated carbon and particulate, light organic carbon were greater in volcanic ash soils than in sedimentary soils. Conversion of forest to pasture reduced total SOC stocks and it decreased the relative contribution of the light fraction to total SOC storage in both parent materials. Relative changes in SOC stocks were more pronounced in the light fraction than in the total soil carbon. The 13-C abundance in soil carbon fractions revealed that recently incorporated pasture-derived carbon was less stabilized in the volcanic ash soils than in the sedimentary soils. Most of the pasture-derived SOC which accumulated during about 20 years of pasture use was rapidly mineralized under secondary forest in sedimentary soils. The mineral-associated, pasture-derived C was the fraction with the highest stability. In the volcanic ash soils decomposition of pasture-derived SOC was even faster. It was completely lost after 10 to 20 years of secondary forest growth. The results show that the stabilization of recently incorporated SOC depends on the soil type and on the associated mineralogy. They also indicate that recently accumulated SOC stocks in the analyzed topsoils represent rather labile SOC pools.
B23B-0426
Carbon Stabilization in Wet Tropical Forest Volcanic Soils
Volcanic soils, particularly Andisols, have high carbon storage capacities due to the accumulation of highly reactive, non-crystalline minerals. Previous research along a pedogenic chronosequence on volcanic lava in Hawai'i found that soils in the intermediate weathering stage, dominated by allophane, contained the largest soil C stocks with slowest turnover rates. Potential mechanisms for long-term soil C stabilization include an accumulation of chemically recalcitrant C, microenvironmental conditions unfavorable for decomposition, and strong sorption of soluble and otherwise labile C to mineral and/or metals. In well-drained soils in wet climates, dissolved organic matter (DOM) is a likely main pathway for the transport of C from the zones of highest microbial activity to deeper mineral horizons. To address the production, transformation, and fate of dissolved organic matter (DOM), we have installed tension and zero tension lysimeters throughout sequentially deeper organic and mineral horizons in an intermediate aged soil (ca. 350k years) under wet (ca. 3000 mm mean annual rainfall) native tropical forest in Hawai'i. The soils are characterized by thick O horizons and Bh horizons 20-30 cm deep, followed by mineral horizons showing redoximorphic features. Bulk soil carbon to nitrogen ratios increase with soil depth, matching that of DOM in the surface organic horizons at 40-50 cm depth. Low pH does not seem to explain this accumulation of C-rich, N-depleted OM, as soils become less acidic with depth. Soil C:N are positively correlated with alumina, oxalate-extractable Al, and dithionite citrate-extractable Al. The greatest source of DOC is the forest floor (Oie), followed by the Oa horizon, and concentrations decrease significantly in the mineral horizons. DOC concentrations increase with total dissolved Al and Fe in the Oie horizon, and with total Fe in solution in the Bg horizon. In the Bh horizon, DOM C:N are negatively correlated with total Al and Fe in solution. Metals appear to be implicated in the mobilization of C in solution and its stabilization in mineral horizons. The formation of cracks along large peds facilitates macropore flow and downward delivery of carbon.
B23B-0427
Distinct Litter Stabilization Dynamics Pathways for Decomposition of Pine Needle and Fine Root Within Soil
The chemical composition of litter imparts a strong control on the initial rates of microbial decay but it is unclear how plant chemistry influences the ultimate stabilization of soil organic matter (SOM) and the nature of the products stabilized. We determined the concentration and 13C enrichment of lignin phenols and substituted fatty acids (SFA) in SOM fractions from an experiment in which 13C- and 15N-labeled needles or fine roots were added to the mineral soil in a Ponderosa pine (Pinus ponderosa) forest in the Sierra Nevada, CA, USA. 1.5 y after litter addition, we analyzed bulk soil (< 2 mm), free light fraction (LF, mean residence time (MRT) ~5 y) and alkali/acid insoluble humin (MRT ~270 y) fractions. Needles contained nearly 2 and 3x the lignin and SFA content per organic carbon unit as did roots. Lignin and SFA decreased from the free LF to the bulk soil to the humin fraction; and molecular properties were more similar within a SOM fraction regardless of the litter source. However, LF and humin from the root addition contained more lignin than from the needle addition. Based upon the relative movement of litter-derived 13C and 15N into SOM fractions during 1.5 y, it was proposed that the 13C accumulation in the humin fraction for needles was derived from high C/N, needle-derived biopolymer molecular fragments that are surficially associated with particles. In contrast, the root-derived material entering SOM fractions was much lower in C/N and was likely from microbial by-products. Consistent with this hypothesis, both lignin and SFA in the LF and humin fractions amended with enriched needles were highly enriched (+ 30-60 permil) with respect to the SOM fractions from soils amended with roots. These differences were large even considering the lower concentration of SFA and lignin in root material. Although the chemistry and MRT of LF and humin were dramatically different, the extent of 13C-enrichment among lignin and SFA were comparable for the needle experiment while most lignin phenols for the humin from the root addition had greater 13C content than SFA. This indicates that molecular fragments of plant biopolymers can readily associate with both labile and stabilized SOM fractions. At the same time, these results suggest that distinct decomposition and stabilization pathways exist for litters, such as needles vs. roots, of different chemical quality.
B23B-0428
Impacts of Soil Temperature and Moisture Change on Soil Carbon Dynamics in the Alaskan Yukon River Basin ¡§C From the Perspective of Vertical Distribution
Boreal ecosystems are experiencing rapid climate change and land surface disturbances (e.g., fires and insect outbreaks), which have triggered substantial changes in ecosystem structure and functions, biogeochemical cycle, and land surface processes. These changes in turn have major implications to the changes of regional and climate systems. There is an urgent need to develop robust process-based land surface modeling systems that can simulate the responses of many poorly understood but fast-changing soil processes in the region. In this study, we improved the soil physics module in the Erosion-Deposition-Carbon Model (EDCM), mainly following the algorithms in the Integrated BIosphere Simulator (IBIS), to simulate historical and future changes of soil temperature, moisture, active layer thickness, permafrost depth, and their impacts on organic layer and soil carbon dynamics from two boreal forest sites in the Alaskan Yukon River Basin. We used a multi-snow-soil-layer model structure to represent the vertical profiles of soil properties and processes. Our results showed more soil carbon emission than the single-soil-layer models predicted under future climate change scenarios. A multi-soil-layer model has to be used in the northern high latitudes to predict the fate of deep soil organic matter, organic layer thickness, and degradation of permafrost. At the site scale, major uncertainties for model development and characterization of the responses of vegetation and soils to future climate change include vegetation succession, transitional vegetation rooting depth change, root mortality, and fire intensity and frequency. Additional intensive site studies and remote sensing research have to be conducted to address some of the uncertainties in order to apply the model at the regional scale.
B23B-0429
Carbon storage in frozen loess and soils of the mammoth tundra-steppe biome
During the Last Glacial Maximum (LGM), atmospheric CO2 concentration was 80-100 ppmv lower than in pre- industrial times. At the time of LGM steppe-tundra was the most extensive biome on Earth. Some estimates of the C storage in that biome assume that it was similar to cold desert and that the terrestrial carbon (C) reservoir increased at the Pleistocene-Holocene transition by 400-1300 Gt, requiring that the world oceans be a large C source. To estimate C storage in the entire steppe-tundra biome we used data of C storage in soils of this biome that persisted in permafrost of Siberia and Alaska and developed a model that describes C accumulation in soil profiles and in permafrost. The model shows a slow but consistent C increase in soil when permafrost appears. At the Pleistocene-Holocene boundary tundra-steppe soils became a C source of greater than 1000 Gt to the atmosphere. The implications of these model results are that the ocean was not a source of carbon but absorbed several hundreds of gigatons of C at that time. The model results also show that restoring the tundra-steppe ecosystem in northern Siberia would enhance soil C storage.
B23B-0430
Primary Energy Production via Light Reactions With Humic Substances.
Humic substances (HS) represent a ubiquitous and structurally diverse form of complex organic matter in the environment. Often considered recalcitrant to microbial degradation, HS nevertheless support both oxidative and reductive energy-generating microbial respiratory reactions. For example, microorganisms can oxidize reduced functional groups within HS (such as hydroquinones) providing electrons capable of supporting anaerobic respiratory processes. In this study we investigated the ability of HS to mediate the capture of light energy and its subsequent conversion into bioavailable chemical energy that can be used to support microbial growth. Aged acid iron mine drainage systems were used as a model environment for this metabolic scheme. Acidic solutions containing 5mM concentrations of the model humic quinone 2,6-anthraquinone disulfonate (AQDS) and ferric chloride were illuminated with both UV and visible light. Illumination led to the production of Fe(II) beyond that observed in either dark controls or in illuminated controls lacking AQDS. Likewise, HS isolated from marine, swamp, and lake sources were capable of enhancing Fe(II) production under the same illumination conditions. Fe(II) generated in these co-illuminated photoreactions was readily oxidized by the aerobic Fe(II)-oxidizing archeon, Ferroplasma acidarmanus Fer1T, indicating the bioavailability of photogenerated Fe(II). No oxidation of Fe(II) was observed in the absence of cells. Cell counts indicated that the oxidation of approximately 2.0mM photo-produced Fe(II) by Fer1T was coupled with cell growth. Oxic and anoxic illumination of 5mM AQDS solutions alone with UV or visible light led to the production of reduced organic species, whose reducing equivalents could be scavenged as Fe(II) in a subsequent dark reaction by the addition of Fe(III). Likewise, HS from a variety of environments could be converted to more reduced species via UV illumination under anoxic conditions or even by exposure to the radioactive decay products of a Cs-137 source (gamma rays and beta particles). Fe(II) produced from dark reactions with the photoreduced HS or AQDS products was also bioavailable, and could be rapidly oxidized under aerobic conditions by Fer1T. Again, no Fe(II) oxidation occurred in the absence of cells. Alternatively, the photoreduced AQDS could be directly oxidized as an electron donor in the absence of the iron shuttle by model denitrifying microorganisms under anoxic conditions. These data suggest that light interactions with HS support primary production by converting light energy into bioavailable reducing equivalents such as reduced humic material and Fe(II) that can be used to support microbial chemotrophic growth.
B23B-0431
Reduced Soil Tillage Affects the Concentration, Production and Stabilization of Microbial Biomass
Soil microbial communities dominated by fungi have been associated with reduced N losses and increased soil aggregation. Moreover, fungal residues have been found to degrade slower than bacterial residues. For these reasons, fungi-dominated communities may be more conducive to ecosystem C storage. In agricultural systems, a shift towards a fungal decomposition pathway might help to regain some of the soil C that was lost due to cultivation. However, measurements on standing microbial biomass alone do not fully reveal fungal and bacterial contributions to SOM dynamics. Therefore, we compared the effect of reduced and conventional tillage on both the growth and concentration of fungal and bacterial biomass, by using leucine and acetate incorporation techniques and epifluorescence microscopy. We also measured the concentration of fungal and bacterial residues, by quantifying amino sugars glucosamine, galactosamine and muramic acid. Soil samples were collected at two different depths from spring barley field plots that were under conventional vs. reduced tillage management for 7.5 growing seasons. Reduced tillage strongly increased both fungal and bacterial biomass in the top soil layer. However, microbial growth rates only showed small responses, suggesting a slower turnover of microbial biomass under reduced tillage. Across soil depths and tillage treatments, total amino sugar contents ranged between 440 and 560 mg C per kilo soil. Fungal derived amino sugars increased under reduced tillage, whereas bacterial residues remained unaffected. These results suggest that reduced tillage enhances the fungal contribution to SOM dynamics both by stimulating fungal growth and stabilization of fungal biomass.
B23B-0432
Major Factors Influencing Soil Organic Carbon Storage in Temperate Forests, USA: A model perspective
Terrestrial soil organic carbon (SOC) plays an important role in the carbon cycle and global climate change, and temperate forests are a large component of total SOC. In order to explore the major factors affecting SOC storage in temperate forests we developed a Windows-based CENTURY SOC model that allows rapid simulation of multiple parameter permutations. Three long-term forest carbon cycle research sites were selected to perform sensitivity tests on factors influencing SOC storage: Willow Creek in north-central Wisconsin, Pellston in Michigan, and Propect Hill Tract in north-central Massachusetts. We tested environmental variables, such as air temperature, precipitation, soil texture, and model parameters such as carbon/nitrogen ratios, lignin contents, decomposition and mortality rates, and parameters for forest growth. The results indicate that temperature significantly impacts tree growth and SOC storage. Soil N deposition also had a large influence on SOC storage. C/N ratios of tree compartments demonstrated different effects on SOC pools: changes of C/N ratios of tree compartments had more impact on the fast pool than on the slow and passive pools. SOC changes caused by lignin content, tree mortality, and decomposition rate were negligible in our simulations. In the model, SOC is proportional to clay content and simulations of SOC were in agreement with the field measurements at three sites. We also simulated forest management and natural disturbance with the revised model. Regular harvesting (once every 50 years) was predicted to significantly reduce SOC storage through depletion of above- and belowground biomass. However, natural fire disturbance (medium fire with the revisit time of 100 years) may maintain SOC at a relatively low but stable level. In this study, modeling was very helpful to identify key parameters driving SOC and the uncertainties associated with SOC estimates.
B23B-0433
Above and belowground controls on litter decomposition in semiarid ecosystems: effects of solar radiation, water availability and litter quality
The integrated controls on soil organic matter formation in arid and semiarid ecosystems are not well understood and appear to stem from a number of interacting controls affecting above- and belowground carbon turnover. While solar radiation has recently been shown to have an important direct effect on carbon loss in semiarid ecosystems as a result of photochemical mineralization of aboveground plant material, the mechanistic basis for photodegradative losses is poorly understood. In addition, there are large potential differences in major controls on above- and belowground decomposition in low rainfall ecosystems. We report on a mesocosm and field study designed to examine the relative importance of different wavelengths of solar radiation, water availability, position of senescent material above- and belowground and the importance of carbon litter quality in determining rates of abiotic and biotic decomposition. In a factorial experiment of mesocosms, we incubated leaf and root litter simultaneously above- and belowground and manipulated water availability with large and small pulses. Significant interactions between position-litter type and position-pulse sizes demonstrated interactive controls on organic mass loss. Aboveground decomposition showed no response to pulse size or litter type, as roots and leaves decomposed equally rapidly under all circumstances. In contrast, belowground decomposition was significantly altered by litter type and water pulses, with roots decomposing significantly slower and small water pulses reducing belowground decomposition. In the field site, using plastic filters which attenuated different wavelengths of natural solar radiation, we found a highly significant effect of radiation exclusion on mass loss and demonstrated that both UV-A and short-wave visible light can have important impacts on photodegradative carbon losses. The combination of position and litter quality effects on litter decomposition appear to be critical for the formation of soil organic matter and an integration of the relative importance of these processes could provide in the potential for carbon sequestration in arid and semiarid ecosystems.
B23B-0434
Microorganisms in small patterned ground features and adjacent vegetated soils in the High Arctic, Canada
We compared 1) microbial biomass C (MBC), 2) number of bacteria (MPN), 3) diversity of fungal isolates (FI), and 4) the composition of the bacterial community in patterned ground features (PGFs) and adjacent vegetated soils (AVS) in study sites from three islands (Ellef Ringnes, Prince Patrick and Banks Islands) located along a bioclimatic gradient in the High Arctic of Canada. Soil samples were collected from PGFs and AVS located along transects in zonal (mesic) sites, and within a range of topographic conditions (drier and wetter areas at each island). FI and MPN inoculates were grown at two temperatures (7 vs. 25°C). MBC at the mesic position was greatest in Green Cabin, intermediate in Mould Bay, and lowest in Isachsen. MBC was higher in AVS than in PGFs in Green Cabin and Mould Bay but when we compared other topographic position within the study sites (islands), we found that microbial biomass C was also higher in AVS than in PGF in the dry position in Isachsen and Green Cabin. MPN were different among study sites and locations but not between incubation temperatures. MPN were higher in AVS than in PGF. FI were greater in AVS than in PGF but not different among sites. We conclude that MBC decreased with increasing latitude in these High Arctic islands. In addition, heterotrophic bacteria, some fungal genera and the number of TRFLP phylophytes vary along the gradient. Further, microbial activities are lower in PGFs as compared to AVS; and this effect could be decoupled by the effect of topographic position and temperature.
B23B-0435
Contribution of species-specific chemical signatures to soil organic matter in Kohala, HI.
Soil organic matter (SOM) inherits much of its chemical structure from the dominant vegetation, including phenolic (lignin-derived), aromatic, and aliphatic (cutin and wax-derived) compounds. The Hawaiian fern species Dicranopteris decomposes more slowly than the angiosperm, Cheirodendron due to high concentrations of recalcitrant C compounds. These aliphatic fern leaf waxes are well-preserved and may comprise a large portion of the recalcitrant organic matter in these soils. Our objective was to determine the chemical signature of fern and angiosperm vegetation types and trace the preservation or loss of those compounds into the soil. We collected live tissue, litter, roots, and soil (<53 μm) from five dominant vegetation types including two angiosperms Cheirodendron and Metrosideros, two basal ferns Dicranopteris and Cibotium and a polypod fern Diplazium in Kohala, HI. We characterized them via TMAH-pyrolysis-gas chromatography-mass spectrometry. We found distinct chemical differences between angiosperm and fern vegetation; angiosperm contained more G- and S-derived lignin structures and the fern species contained greater relative abundances of P-derived lignin and tannin-derivatives. There was a general decrease of lignin-derived phenolic compounds from live to litter to soils and an increase in more recalcitrant, aromatic and aliphatic C. Recalcitrant fern-derived cutin and leaf waxes (alkene and alkanes structures) were evident in the soils, but clear species differences were not observed. Although ferns contain distinct lipid and wax-derived compounds, soils developed under fern do not appear to accumulate these compounds in SOM.
B23B-0436
Soil Carbon Dynamics Along the Pathway From Diverse Microbial Carbon to Humus in a Temperate and Tropical Forest
This research investigates the importance of microbial biochemistry to humification pathways in two climatically different forest ecosystems; Blodgett forest (BF), a temperate forest in the Sierra Nevada and Luquillo forest (LF), a tropical forest in Puerto Rico. Non-living 13C enriched temperate and tropical microorganisms from four biochemically contrasting microbial groups (fungi, actinomycetes, bacteria gram (+), and bacteria gram (-)) were separately added to soil at both sites in a reciprocal transplant experiment. Decomposition rates were substantially greater at LF than BF for all microbial inputs. Although there were initial differences in microbial C turnover and recovery within the soil microbial biomass and dissolved organic carbon pools for unique microbial C inputs at both sites, over time treatment differences converge within each site and the quality of input microbial C becomes less important to C remaining and maintained within these soil C pools. Physical soil fractionation revealed important trends which illustrate the role of the soil mineral matrix to protect and stabilize C in soil. Results indicate different C turnover rates associated with the light, aggregate- occluded, and mineral-associated soil fractions at both sites. At BF input C recovered within the light and mineral-associated fractions decreased substantially over time (1 to 13 months), while C occluded within aggregates only slightly decreased. Similarly, LF soils exhibit only a slight decrease in aggregate-occluded C over time (0.5 to 3.5 months), while C recovered within the light fraction decreased substantially; however, unlike BF, LF soils exhibited only a slight decrease in C recovered within the mineral fraction. The distribution of total C among these physical soil pools differs substantially for either site, suggesting differences in the relative importance of the mineral matrix to protect and stabilize C. Preliminary compound-specific isotope analyses employing pyrolysis gas chromatography mass spectrometry and isotope ratio spectrometry (Py-GC-MS/IRMS) for temperate BF soils treated with 13C enriched temperate fungal residues indicates a substantial enrichment of low molecular weight (MW) compounds from microbial additions after 1 month in the field; however, after 5 months in the field the 13C enrichment shifts to higher MW compounds. These trends suggest higher MW compounds are formed through humification as synthesis or condensation products, which highlights the importance of monitoring biogeochemical transformations of unique sources of C over time. Future and ongoing work examines specific compounds associated with these high 13C enrichment values in an effort to understand the link between microbial C quality and humification products.
B23B-0437
Organic matter abundance controls the development of microbial zonation
Current biogeochemical models predict that microbial functional groups segregate into discrete zones in the environment, a phenomenon called microbial zonation. To understand the origin of discrete microbial zones, we developed a numerical model that couples sediment diagenesis with microbial kinetics. This model considers simultaneously physical and biogeochemical processes in the sediments, including solute transport, microbial catabolism, and microbial growth and maintenance. According to our modeling results, where organic matter content is low, the rates of microbial catabolism are limited, which diminishes the sizes of microbial populations and gives rise to the discrete zones. On the other hand, where organic matter is abundant, no signification competition occurs among functional groups and, as a result, no discrete zones develop in the environment. In this case, different microbial metabolisms occur simultaneously without excluding each other from the environment. Previous study has focused on microbial zonation in oligotrophic environments. We thus took the sediments of Upper Klamath Lake in Oregon, USA as an example and investigated the occurrence of microbial zonation in eutrophic environments. Results of our analysis showed that the sediments of the lake contained abundant organic matter. As suggested by the chemical analysis and in situ rate measurements, microbial iron reduction, sulfate reduction, and methanogenesis occurred simultaneously at depths close to the water/sediment-interface. Our modeling results and field observations thus suggest that the availability of organic matter controls significantly the development of microbial zonation in the environment.