B51C-0954 INVITED 0800h
Remote Estimation Of Net Ecosystem Carbon Dioxide Exchange In Crops: Principles, Algorithm Calibration And Validation
Accurate estimation of spatially distributed CO2 fluxes is of great importance for regional and global carbon balance studies. Tower-based instruments provide flux data from a small footprint area and scaling beyond the footprint to the region is quite challenging. We developed a technique that relates tower-based mid-day CO2 exchange data with remotely sensed reflectances in the near infrared and either the green (around 550 nm) or the red-edge (near 700 nm) spectral ranges, to accurately estimate net ecosystem CO2 exchange (NEE) in commodity crops. The technique, which is solely based on remotely sensed data, was tested for mid-day NEE estimation in irrigated and rainfed maize and soybean during three seasons (2001 through 2003). The technique provides accurate estimations of mid-day NEE in crops, explaining more than 88% of NEE variation in maize and 86% in soybean, and shows great potential for remotely tracking crop NEE. The technique was validated by an independent data set; root mean square error in predicting mid-day NEE in the range 0-2.5 mgCm-2s-1 was 0.3 mgCm-2s-1 using NIR and red-edge bands and 0.38 mgCm-2s-1 using NIR and green bands. The developed technique will improve our understanding of how to retrieve crop ecosystem CO2 exchange synoptically. By improving the accuracy of retrievals, we will advance the understanding of regional and global carbon dynamics, reducing the uncertainties attendant to NEE estimation in crops.
B51C-0955 0800h
CO2 Exchanges Between The Atmosphere And An Alpine Meadow Ecosystem On The Qinghai-Tibetan Plateau
We measured the net ecosystem CO2 exchange (NEE) in an alpine meadow ecosystem (lat $37\deg$29-45$^{'}$N, long $101\deg$12-$23$^{'}$E, 3250 m a.s.l.) on the Qinghai-Tibetan Plateau in China, in the 2001 and 2002 by the eddy covariance method to examine the carbon dynamics and budget on this unique plateau. The maximum CO$_{2}$ uptake and release rates derived from the diurnal course of CO$_{2}$ flux were -10.8 $\mu$ mol m$^{-2}$ s$^{-1}$ and 4.4 $\mu$ mol m$^{-2}$ s$^{-1}$, respectively, indicating a relatively high net carbon sequestration potential as compared to subalpine coniferous forest at similar elevation and latitude. The largest daily CO$_{2}$ uptake flux was 3.9 g C m$^{-2}$ d$^{-1}$ on 7 July 2002, which is almost half less than those reported for lowland grassland and forest at similar latitudes. The daytime CO$_{2}$ uptake flux was lineally correlated with the daily photosynthetic photon flux density in each month. The nighttime average CO$_{2}$ effluxes increased exponentially with the increment of soil temperature measured at the depth of 5cm with a Q$_{10}$ value of 3.7. The nighttime average CO$_{2}$ flux, however, showed a negative linear correlation with the soil water content. Diurnal changes in gross primary production (GPP) and ecosystem respiration (R$_{e}$), derived from the regression curves of the nighttime R$_{e}$ to the soil temperature, showed that an afternoon increase of NEE was highly associated with an increase of Re. Seasonal changes in GPP corresponded well to changes in the leaf area index and daily photosynthetic photon flux density. The alpine ecosystem exhibited lower GPP (575 g C m$^{-2}$ y$^{-1}$) than, but similar net ecosystem production (78.5 g C m$^{-2}$ y$^{-1}$) as, subalpine forest ecosystems. The results suggest that the alpine meadow behaved as a CO$_{2}$ sink during the 1-year measurement period, but apparently sequestered a rather small amount of C in comparison with similar alpine ecosystems.
B51C-0956 0800h
Effects of Future Climate Shifts on CO2 Exchange of a Grassland Ecosystem
Increases in atmospheric CO2 concentration not only affects climate variables such as precipitation, water vapor concentration, and air temperature, but also affects intrinsic ecosystem physiological properties such as the maximum carboxylation capacity and stomatal conductance. De-convolving these two effects remains uncertain in biosphere-atmosphere water and carbon cycling. Using a simplified analytical net ecosystem CO2 exchange (NEE) model, tested with recently collected flux measurements in a humid grassland ecosystem in Ireland, we assess how much projected climate shifts affect net canopy photosynthesis (A) without physiological adjustments and contrast those findings with physiological adjustments already reported for several grassland ecosystems. Our analysis suggests that the intrinsic grassland ecosystem physiological adjustment of A is about forty five times more important than the resulting climatic forcing shifts from the IS92a scenario (and a double of atmospheric CO2 concentration). Implications to afforestation policy and future experimental efforts to quantify the carbon sink from humid grassland ecosystems are also discussed.
B51C-0957 0800h
Isotopic Disequilibrium Between Carbon Fixed and Released in a Rice Paddy Ecosystem as Influenced by Methanogenesis From CO$_{2}$ Under Anaerobic Conditions
Stable carbon isotope ratios of various ecosystem components and ecosystem respiration (\delta$_{R}$) were measured in a Japanese rice paddy. An automated air sampling system was used to collect nighttime air samples to estimate \delta$_{R}$ by means of Keeling plot. Throughout the growing season in 2003, significantly (3\permil to 4\permil) higher \delta$^{13}$C values were observed in \delta$_{R}$ than those observed in plant tissue samples, indicating a strong decoupling process for carbon assimilated and respired in the ecosystem. It is well known that production of methane from CO$_{2}$ exhibits a larger isotope fractionation than that can be found in equilibration of CO$_{2}$ with soil water. CO$_{2}$ entrapped in soil showed 5.5\permil to 7.5\permil higher \delta$^{13}$C values than \delta$_{R}$. Given these isotopic differences, we partitioned total ecosystem respiration into plant respiration and soil (including root) respiration components with an assumption that there is no isotope fractionation associated with respiratory processes of rice plant. The estimated proportion of soil respiration to total ecosystem respiration was about 30% under flooded conditions, but increased to about 40% by floodwater drainage. The partitioned respiratory fluxes from soil contributed to reducing the discrepancy between measured plant biomass increase and accumulated net ecosystem exchange (NEE) for the entire growing season. Partitioning NEE into photosynthetic assimilation and ecosystem respiration based on the isoflux approach revealed that floodwater drainage increased daytime respiratory fluxes greater than the estimated respiratory fluxes from an exponential relationship between nocturnal NEE and air temperature.
B51C-0958 0800h
Using Biome-BGC to estimate production in annual crops - A study in Nebraska
The Biome-BGC ecosystem process model (Version 4.1.2) has been used successfully in many ecosystems, but was not developed for use with agricultural crops. Therefore, program modifications are needed for use with crops, including the addition of carbon allocation to fruiting and the inclusion of springtime planting. The program has been modified and tested using both C3 (soybean) and C4 (maize) vegetation. Results from the Biome-BGC model runs were validated using AmeriFlux tower eddy CO$_{2}$ flux-based estimates as well as two years of biomass and yield estimates at the University of Nebraska Agricultural Research and Development Center (ARDL) near Mead, NE. The model was also used to obtain tower site and regional estimates of NEE, GPP and NPP. Preliminary results indicate that the model works well in estimating both productivity and yield of both maize and soybean. These results are combined and scaled to a 7 x 7-km area equivalent to that of the MODIS subset (resolution = 1 km$^{2}$) centered on the research farm and available from Fluxnet and the Oak Ridge National Laboratory (http://www.fluxnet.ornl.gov/fluxnet/modis.cfm). The comparisons provide a means to test the ability of the MODIS algorithms to capture seasonal variations and agricultural carbon dynamics. The results of this study will be used in the future for spatial extrapolation to scales from 1 - 20,000 km$^{2}$ to evaluate relative accuracies of MODIS GPP/NPP regional data and provide estimates of the regional carbon balance for the larger 20,000 km$^{2}$ area within the National Institute for Global Environmental Change (NIGEC) Great Plains and Midwestern study regions.
B51C-0959 0800h
Estimating the Isotope Ratio of Ecosystem Respiration Using the Keeling Plot and the Flux Ratio Method
Stable carbon isotope analyses have been used in identifying global carbon sources and sinks and in partitioning ecosystem CO$_{2}$ exchange into component fluxes. The isotope ratio of ecosystem respiration ($\delta^{13}$C$_{r}$) is a critical parameter in applying stable isotope techniques to carbon cycle problems. The commonly used Keeling plot method in estimating $\delta^{13}$C$_{r}$ has limitations related to: 1) insufficient range of CO$_{2}$ mixing ratio; 2) high sensitivity to curve-fitting techniques; and 3) extrapolation of CO$_{2}$ mixing ratio beyond observations. In this study, the Keeling plot method was examined and compared with the flux ratio approach using continuous measurements of the mixing ratios of $^{12}$CO$_{2}$ and $^{13}$CO$_{2}$ over an extensive corn canopy during the 2003 growing season. The seasonal variation of $\delta^{13}$C$_{r}$ estimated from both methods harmonized with the ecosystem phenology. The $\delta^{13}$C$_{r}$ started to increase (became more positive) from mid June and peaked in early August, followed by a decrease into October. The Keeling plot method agreed well with the flux ratio method in the seasonal pattern of $\delta^{13}$C$_{r}$, but tended to give lower values (more negative). The discrepancy between the two approaches was significant in July and August (about 5 per mil) and relatively small in June and September (about 1 to 2 per mil). We examined this discrepancy with respect to wind direction/advection and measurement footprints. In addition, our analysis of high-frequency data (every two minutes) using the flux ratio method indicates that $\delta^{13}$C$_{r}$ may vary significantly at short time-scales (e.g., hourly), which could have significant implications for flux partitioning studies.
B51C-0960 0800h
Altered Seasonality and Magnitude of Rainfall Affects Soil Respiration and Nitrous Oxide Fluxes in California Annual Grassland
Currently, climate models do not agree on how rising concentrations of CO$_{2}$ and other greenhouse gases will affect rainfall in California. Changes in moisture regime will likely alter rates of carbon (C) loss via soil respiration, as well as fluxes of N$_{2}$O. Moisture availability can also affect plant productivity in highly seasonal environments. We examined the consequences of wetter conditions in an annual grassland in the Sierra foothills of northern California by extending the duration of the wet season by about 5 weeks and augmenting total annual rainfall by approximately 50 %. Discrete wet-up events took place prior to the onset of natural rains (early October 2003) and early in the drought period (May 2004). Soil respiration, N$_{2}$O and CH$_{4}$ effluxes, N mineralization, and above- and belowground plant production were measured in treatment and control plots over a one-year period. Soil CO$_{2}$ fluxes for the first treatment year, though large, were not statistically different between wet and control plots (1078 \pm148 g C m$^{-2}$ and 1006 \pm138 g C m$^{-2}$, respectively). The combined wet-up events comprised 17 % of the soil respiration over the 12-month period in treated plots, about twice as much C released by control plots during the same time interval. Aboveground biomass was similar between wetted and control plots (415 \pm45 g m$^{-2}$ y$^{-1}$ and 374 \pm36 g m$^{-2}$ y$^{-1}$, respectively), while root biomass increased significantly with wetting during the first year of treatment (179 \pm23 g m$^{-2}$ y$^{-1}$ and 111 \pm13 g m$^{-2}$ y$^{-1}$ for treatment and control plots, respectively). The additional biomass C gained in treatment plots (53 g C m$^{-2}$) partly offset the greater losses from respired C observed in treatment plots (72 g C m$^{-2}$). Nitrous oxide emissions were low to negligible during the year with the exception of the time directly following wet-up, when N$_{2}$O emissions averaged over 78\pm13 ng N cm$^{-2}$ h$^{-1}$. Our first year of water manipulation in annual grasslands suggests that increased water availability via early and late rainfall events releases large pulses of CO$_{2}$, increases belowground C inputs, and increases N$_{2}$O emissions.
B51C-0961 0800h
Effects Of Temperature On The CO2 Flux In An Alpine Meadow Ecosystem On The Qinghai-Tibetan Plateau
Alpine meadow ecosystem on the Qinghai-Tibetan Plateau, the highest ecosystem in the world, is characterized by strong solar radiation and low temperature. The ecosystem CO2 exchange (NEE) between the atmosphere and ecosystem is controlled by cool temperature, even in growing period. We hypothesized that low temperature may limit the NEE at the alpine ecosystem. We tested this hypothesis by using the CO2 flux data measured by eddy covariance method and other climate variables during the growing season at the alpine meadow ecosystem on the northern edge of the Plateau (lat 37͒36'N, long 101͒18'E, alt. 3250 m). The effect of temperature on the carbon budget of ecosystem was evaluated in the late growing periods for two years. The optimum temperature for the NEE under light saturation conditions was estimated to be around 15>|C. The annual NEE estimated from August 9, 2001 to December 31, 2003 was about 290 g CO2 m-2 year-1 despite of the low ecosystem temperature. We found that NEE from August 9 to September 10 in 2001 with low average temperature was greater than that in 2002 with high average temperature during the same period, but ecosystem gross primary production (GPP) was similar during the two periods. Further analysis showed that ecosystem respiration was significantly higher in 2002 than 2001 based on the estimation from the relationship between temperature and ecosystem respiration during nighttime. The ratio of ecosystem respiration (Re) to GPP (Re/GPP) was lower for 2001 than 2002, indicating a higher proportion of ecosystem respiration in GPP in 2002, which may be partly due to the high temperature in 2002 if we assume the temperature dependence of respiration was the same. The results suggest that low temperature controlled the NEE mainly through the influence of temperature on the ecosystem respiration. Keywords: Alpine meadow, Ecosystem CO2 exchange, Ecosystem respiration, Temperature, Qinghai-Tibetan Plateau
B51C-0962 0800h
Interannual Variability in Growing Season Carbon Budget at a Customarily Cultivated Rice Paddy Field in Central Japan
Rice paddy fields in eastern Asian countries are irrigated during most of the growing season. Since flooded paddy fields are favorable for methanogenesis, it is necessary to measure both carbon dioxide flux and methane flux in order to estimate carbon budget of the paddy fields. As one of AsiaFlux network sites we continue long-term measurements of carbon dioxide and methane fluxes at a customarily cultivated paddy field in central Japan. At the study site rice is transplanted in early May and harvested in early or middle September. We are measuring carbon dioxide flux by the eddy covariance method and methane flux by the gradient method. In 2003 we had the coolest summer in these 10 years, while the 2004 summer was the third warmest since 1946: Air temperatures averaged from June to August in 2003 and 2004 were 0.6 $\deg$C below and 1.3 $\deg$C above the 30-year average, respectively. Carbon dioxide flux showed distinct seasonal variation with rice growth. In 2004, net daily carbon dioxide uptake by the ecosystem became obvious in late May, increased to the maximum daily uptake of 11 gC m$^{-2}$ d$^{-1}$ in July, and then decreased to nearly zero at the end of August. In 2003, carbon dioxide uptake showed temporary decrease in mid-growing season because of small amount of solar radiation and low temperature. The sum of carbon dioxide uptake by the ecosystem from the transplanting to the end of August was 443 gC m$^{-2}$ in 2003, and 553 gC m$^{-2}$ in 2004. Methane emission in 2004 started a month after transplanting, increased gradually and reached midsummer peak of 331 mgC m$^{-2}$ d$^{-1}$ in late July with a flush of methane at the final drainage. The seasonal variation of methane flux in 2003 was generally similar to that in 2004, but flushes of methane at midsummer drainages as well as final drainage were observed more distinctly than in 2004. Despite small amount of methane flushes, seasonal methane emission in 2004 was greater than that in 2003 by 2 gC m$^{-2}$. Higher soil temperature and larger supply of photosynthate in 2004 presumably enhanced methane production in soil and resulted in larger methane emission. The ratio of methane emission to carbon dioxide uptake was about 4% and showed little difference between 2003 and 2004.
http://ecomdb.niaes5.affrc.go.jp/
B51C-0963 0800h
Trace Gas Emissions and Soil C and N Transformations Following Moisture Pulses in Sagebrush: Effects of Invasive and Native Companion Plant Species
Simulating water pulses is an important tool for understanding biogeochemical processes in semi arid environments. Global change triggered shifts in plant species composition exert significant control over belowground C and N transformations. They also affect the ecosystem resiliency and its ability to withstand exotic weed invasion. We monitored effects of water additions on trace gas emissions and soil C and N in sagebrush soils, both canopy and shrub interspace, on sites dominated by either native bunchgrass, western wheatgrass, or an exotic annual, cheatgrass. Our results indicate that long-term cheatgrass establishment affects not only soil under its own thatch, but also soil under shrubs within cheatgrass stand. Overall, soil total N and total organic C on cheatgrass sites were lower than these of western wheatgrass. Trace gas measurements of non-wetted soils showed greater N2O and smaller CH4 fluxes compared to western wheatgrass sites. Upon water pulse, cheatgrass soils demonstrated greater CO2 production rates, relative to pre-wet conditions, greater N2O flux per unit soil total N, and more rapid soil microbial biomass C and dissolved organic C response compared to western wheatgrass. Possible mechanisms include faster turnover of microbial biomass and greater nitrification potential of cheatgrass soils.
B51C-0964 0800h
Effect of Grazing on Soil Temperature and Moisture and Subsequent Implications
Cattle grazing, a common form of land use on grasslands, may affect transpiration and evaporation from soil by defoliation and, in turn, could alter the climate at the surface. This study focused on whether physical landscape changes associated with grazing could have a significant impact on soil temperature and moisture, and thereby affect the microclimate. Objectives were to analyze how soil temperature and moisture vary with simulated grazing treatments. Climatological data were collected at a USDA shortgrass steppe in northeastern Colorado. Eight (1 x 1 meter) plots were selected to represent variations in the fraction of bare ground, while two (1 x 1 meter) plots were used to measure the impact of the arrangement of bare ground. Soil temperature and soil moisture measurements were recorded under a vegetated and bare area in each plot. Additionally, the eddy covariance method was used in the recommended practice of moderate grazing (40% reduction in above-ground biomass). Results from the plots were used to discuss implications for the effect of different grazing densities on the microclimate and water budgets of moderate grazing management and the accuracy of remote sensing images (using large pixels). In addition, the results were used to suggest potential impacts on the summer nesting habitat for the Mountain Plover ({\it Charadrius montanus}), an endangered bird found at the site.
B51C-0965 0800h
Downward CO$_2$ flux observed over a bare field by the eddy covariance method using an open-path infrared gas analyzer
Improvement of fast response infrared gas analyzers enabled us to carry out long-term measurement of CO$_2$ exchange between terrestrial ecosystem and the atmosphere. However, CO$_2$ flux measurement by the eddy covariance method still has several problems to be solved before we estimate annual net CO$_2$ exchange based on the method. One of the problems the authors are facing now is downward CO$_2$ flux often observed at a paddy field in dormant season. In order to understand why those unrealistic fluxes are observed, we made an intensive field experiment at a ploughed paddy field from March to April 2003. The study site was flat and homogeneous, and little vegetation was found during the experiment. Through-flow chamber measurement showed emission flux between 0.5 and 1.0 $\mu$molm$^{-2}$s$^{-1}$. CO$_2$ concentration measured at 1.1 m and 3.8 m also showed gradients indicating upward transport. However, CO$_2$ flux measured by the eddy covariance method using an open-path infrared gas analyzer showed downward transport in the daytime even after the density correction was applied when sensible heat flux was typically between 150 and 200 Wm$^{-2}$. Contrary, the eddy covariance method using a closed-path infrared gas analyzer showed emission from the soil and the magnitude of the flux was similar to that by the chamber measurement. If we assume mass balance of dry air, an increase of the observed sensible heat flux by 50% or pressure flux as much as -30 to -40 Pa ms$^{-1}$ is needed to compensate the differences between open- and closed-path eddy covariance measurements. The possibility of underestimation of sensible heat flux density measured with sonic thermometry can be excluded because it agreed well with the sensible heat flux measured with a fine-wire thermocouple. Although we did not measure pressure fluctuation directly, the pressure flux estimated from the turbulent kinetic energy budget equation as well as from literatures were smaller than the required in the correction by one order of magnitude. Those curious downward fluxes have significant influence on annual budget of CO$_2$ at the paddy field because during the dormant season about 20% of the observed fluxes by the open-path eddy covariance method were downward.
B51C-0966 0800h
Impacts of Canopy Structure on Water, Energy and Carbon Exchange in a Loblolly Pine Forest in Southeast USA
Forest ecosystems play a key role in water, energy and carbon exchange between the terrestrial ecosystems and the atmosphere. One of the major factors that govern the rate of exchange of these fluxes is canopy structure. However, most ecosystem models simulating water, energy and carbon exchange of forest ecosystems with the atmosphere are based on a simplified canopy structure, where the forest canopy is assumed to be a turbid medium with leaves uniformly distributed within the canopy as particles of infinitesimal size, and light propagates through the canopy following an exponential decay. In reality, there are gaps of various sizes within the forest canopy. Lights passing through the gaps are not attenuated creating sunflecks on the forest floor. In this study, we represent the canopy of a loblolly pine stand as an assembly of individual crowns with gaps between and within the crowns. Gaps in the canopy are estimated based on statistics of canopy structure. Such a representation of canopy structure relinquishes the need for the sizes and locations of each tree as are needed in three-dimensional radiation transfer models, making it possible to account for the landscape canopy structure in ecosystem models. The loblolly pine stand is located in the Blackwood Division of Duke Forest, and is an AmeriFlux site where water, energy and carbon exchanges with the atmosphere have been monitored with eddy-covariance instruments since 1997. We simulated the exchanges of these scalars using the RHESSys model with gaps in the canopy estimated from the statistics of canopy structure. Comparision with data from the eddy-covariance instruments shows that replacing a turbid medium canopy with a gapy canopy in RHESSys significantly improved simulation of these fluxes through the forest ecosystem on a daily and weekly time scale. The results identify improper canopy representation in models as a source of uncertainty in estimates of regional water, energy and carbon cycles of the terrestrial ecosystems and suggest a convenient approach to improve the accuracy of these estimates.
B51C-0967 0800h
Monitoring Rangeland Health With MODIS Vegetation Index Data
Rangelands cover approximately one third of the land area of the conterminous U.S. These lands supply much of the forage for the U.S. cattle industry. Large area monitoring of these vast expanses of range has proved challenging since most of these lands are in the western U.S., are relatively sparsely populated, and are not well covered by meteorological weather stations. Improvements in the spatial and temporal precision of rangeland health information would be useful both for the cattle industry and for scientific studies of soil erosion, water runoff, ecosystem health, and carbon cycling. Optical multispectral remote sensing data from satellites are an objective source of synoptic, timely information for monitoring rangeland health. The objective of this study is to develop and evaluate a method for measuring and monitoring rangeland health over large areas. In the past, data collected by the Advanced Very High Resolution Radiometer has proved useful for this purpose, however the basic 1 km spatial resolution is not ideal when scaling up from ground observations. This study assesses MODIS 250 meter resolution vegetation index data for this purpose. MODIS data not only have finer spatial resolution and improved geolocation, but they also exhibit enhanced vegetation sensitivity and minimized variations associated with external atmospheric and non-atmospheric effects. Ground data collected over 51 sites in western South Dakota over four years are used as training for regression tree models of range health. Range health maps for the growing season derived from the models are presented and evaluated.
B51C-0968 0800h
Carbon Dioxide Exchange in Live Oak-Ashe Juniper Forest and C4 Grassland Ecosystems on the Edwards Plateau, Texas (Texas Hill Country)
A recent ecoregion-scale analysis of the Ameriflux network indicated that ecosystem/environment combinations in central and south Texas were not represented by the current network of eddy flux towers. The Edwards Plateau (Texas Hill Country) is a large (93,000 km2), distinct ecoregion in south and west central Texas that is a biological crossroads for three major biomes of North America (grassland, desert, deciduous forest). Extensive portions of the Plateau are dominated by live oak-Ashe juniper savannas underlain by C4 grasslands. Currently, grasslands are disappearing due to encroachment by juniper. A long-term comparative study was begun in 2004 to quantify fluxes, sources, and sinks of atmospheric CO2 on the Edwards Plateau, and to determine how encroachment by Ashe juniper alters CO2 fluxes and CO2 source/sink relationships. Meteorological towers for eddy covariance measurements of CO2 exchange were installed on C4-dominated grassland and live oak/Ashe juniper forest sites, representing non-encroachment and full-encroachment conditions. Preliminary results of CO2 exchange in the two ecosystems during the summer and fall of 2004 will be presented and compared with an emphasis on the response of the two ecosystems to rainfall and water deficit conditions.
B51C-0969 0800h
Annual Methyl Halides Fluxes and Isotopic Signatures of Methyl Chloride from Irish Soil Ecosystems
Methyl halides (CH3X, where X = Cl, Br or I) provide a significant portion of reactive halide radicals to the atmosphere, where the radicals are responsible for ozone catalysis within the stratosphere and decreasing the oxidative capacity of the troposphere. One of the least understood reservoirs within the methyl halide budgets is the soil, or subsurface, ecosystem. Here we present an annual study of methyl halide emissions and consumption in two Irish ecosystem soils, forested and agricultural pastureland. Pastureland is a net consumer of methyl chloride but consistently produces methyl iodide. Forested soils are highly variable, with general overall production of methyl chloride and consumption of methyl iodide. Methyl bromide is slightly produced in both ecosystems. Isotopic analysis of methyl chloride from forested soil sampling chambers gives significantly enriched values for methyl chloride d13C. Isotopic analysis of methyl chloride from pastureland soils shows no significant deviation from atmospheric values. Further tests on soil cores from forested soils confirm the enrichment of methyl chloride d13C by biological processes. Addition of water to soil cores appears to drive a biological cycle (presumably bacterial) that consumes methyl chloride and methyl bromide more rapidly than under dry conditions. Abiotic production and consumption of methyl halides in the forested ecosystem is very small compared to biological processes.
B51C-0970 0800h
Rainfall regulates the impact of elevated atmospheric CO2 on carbon balance in Chesapeake Bay wetland and Florida Scrub Oak.
Elevated atmospheric CO2 (Ca) is expected to stimulate ecosystem carbon assimilation and reduce stomatal conductance and transpiration in native ecosystems but the interaction between these effects and water and nutrient supply is highly uncertain. We report results of measurements of Net Ecosystem CO2 Exchange (NEE) from an 18 year study of the effects of elevated Ca on a Chesapeake Bay Maryland wetland and a seven year study in a scrub oak ecosystem Cape Canaveral, Florida exposed to ambient or elevated (normal ambient plus 350 ppm) Ca in open top chambers which were also used to measure NEE. Although acclimation of photosynthesis was commonly observed, elevated Ca often caused a stimulation of photosynthesis of 35-100 percent. Elevated Ca stimulated NEE was the same or greater than NEE in sites at normal ambient Ca but inter-annual variation in the effect was large. Stimulation of NEE by elevated Ca was linearly correlated with rainfall. These results suggest that the inter-annual variation in the effects of elevated Ca on ecosystem carbon balance were due to the availability of water and not to other factors such as nitrogen supply.
B51C-0971 0800h
Stabilization of eroded soil organic carbon in two types of depositional basins
Processes of soil erosion and sedimentation redistribute 30-100 Gt (Gt= 1015g) soil yr-1 (Gt= 1015g) along with ~5 Gt (C)yr-1/yr, of which 70-90% is deposited in different depositional basins within the same or adjacent toposequence; it is not quantitatively delivered to the ocean as was assumed in past C models. Burial of eroded soil and associated soil organic carbon can promote C-sequestration locally if it removes organic C from more active components (plant biomass and topsoil) and stores them it in potentially more passive stable reservoirs, where the C is offered physical protection away from near-surface environments. We characterized the rates of input, amount of storage, and relative stability of soil organic matter in a hollow (terrestrial depression) and a flood plain to determine understand the fate of eroded soil organic carbon (SOC) after sedimentation and its potential preservation in the different depositional basins in a California coastal scrub-grassland watershed. We found that the hollow and floodplain contain 13 and 27 percent more carbon, and 14 and 25% more nitrogen, compared to the eroding hillslopes, respectively. Density fractionations followed by 13-NMR and selective dissolutions of Fe and Al (with citris dithionite, ammonium oxalate and pyrophosphate) show that that the organic matter in the depositional basins is not necessarily more stablized than that on the eroding slopes. Thus we conclude that depositional basins can comprise a significant sink term in the global carbon budget, the sustainability of their sink is dependent on a variety of environmental and human (disturbance) factors.
B51C-0972 0800h
The role of low soil moisture in mitigation of Water and Carbon exchange at a Bahia Grass ({\it Paspalum notatum}) Pasture in Central Florida.
Measurements of latent (LE) and sensible (H) heat fluxes (June 2000-February 2004) and carbon exchange (February 2003-February 2004) were made to determine the role soil water content (SWC) plays in water and carbon exchange over a pasture in central Florida. Maximum evapotranspiration rates (ET) were $<$ 5 mm day$^{-1}$ and occurred during the wet season (June to September) of each year. Average SWC in the rooting zone varied between 0.065 and 0.5 m$^{3}$ m$^{-3}$ and was shown to be an important determinant of energy partitioning and carbon exchange. Bowen ratio values (H/LE) were below 1 at high SWC but increased sharply to above 2 when SWC decreased below a critical value of 0.15 m$^{3}$ m$^{-3}$. The decoupling coefficient ($\Omega$) was calculated to assess the degree to which physical (radiation) versus biological (surface conductance) controls regulate ET. When SWC was relatively high, $\Omega$ generally exceeded 0.5 and ET was predominantly driven by radiation. During the driest periods of each year when SWC fell below 0.15, $\Omega$ ranged between 0.15 and 0.3, indicating that ET was strongly controlled by surface conductance. Peaks of maximum daily net ecosystem carbon exchange (NEE) (4.8 to 6.2 g C m$^{-2}$) were a carbon gain and occurred between June and July 2003. This ecosystem behaved as a carbon source during periods of low SWC. Carbon loss from ecosystem respiration (Re) increased with air temperature (Ta) since end of February until May and then generally remained constant at approximately 11 g C m$^{-2}day$^{-1}$) until September 2003. However, daily values of gross primary productivity (GPP) increased continuously from 4 g C m$^{-2}$) in February to 14 g C m$^{-2}$) in August 2003 when both SWC and leaf area index where near an annual maximum. The ecosystem was a net carbon sink during the year of carbon exchange measurements, accumulating a total of about 260 g C m$^{-2}$. Although Florida has a subtropical climate, changes in soil moisture play an important role in regulating plant production and energy partitioning in these pastures.
B51C-0973 0800h
Carbon Cycling Dynamics in Mixed-Grass Prairie Following Coupled Changes in Winter Snow and Summer Precipitation
Changes in timing or amount of precipitation may be of great consequence for carbon cycling in the western U. S. Mixedgrass Prairie, partly due to the fact that both production and decomposition are tightly coupled to soil water properties. In addition, the Mixedgrass Prairie constitutes the largest grassland type in North America. The objective of our project is to quantify how net ecosystem C flux (NEE), ecosystem respiration (Re) and gross photosynthesis (GPP) respond to experimental changes in winter and summer precipitation in a Mixedgrass Prairie. Our study is conducted at the USDA-ARS High Plains Grasslands Research Station, west of Cheyenne, Wyoming. We installed three replicated 50 m snow fences to increase winter snow on the lee ward side of the snow fence and experimentally manipulated summer precipitation by either increasing or decreasing precipitation amounts. Snow additions showed no significant effects in the spring or early summer of 2003 due to the wet spring conditions. However, the snow treatments had greater fluxes of NEE, GPP and Re values in the late summer of 2003 compared to ambient conditions. Rates of ecosystem carbon flux in the snow addition plots were many times larger than ambient fluxes in the spring and summer of 2004. Additionally, snow addition plots in 2004 had almost twice the flux rates as snow addition plots in 2003, compared to lower fluxes in 2004 for ambient plots. The large fluxes in the 2004 snow plots were due to a combination of wet soils from snow packs melting and from hot and sunny spring and early summer. Summer precipitation treatments had much less of an effect on ecosystem carbon cycling than the winter precipitation treatment. Increasing summer precipitation generally had a slight increase in ecosystem carbon fluxes. This was most noticeable in the late summers. Decreasing summer precipitation tended to lower ecosystem carbon fluxes. Reductions in fluxes were most visible in GPP and Re measurements in the late summers. However, the reduction in GPP and Re were of similar magnitude so there were little changes in NEE. This research is important for predicting changes in soil carbon storage and plant production due to climate change in the largest grassland type in North America.
B51C-0974 0800h
Seasonal Variations in Sugar Contents and Microbial Community Behavior in a Ryegrass Soil
Soil is a complex mixture of numerous inorganic and organic constituents that vary in size, shape, chemical constitution and reactivity, and hosts numerous organisms. Total sugars have been estimated to constitute 10% (average) of soil organic matter, occurring in living and decaying organisms, as well as in extracellular materials. The role of sugars in soils is attributed to their influence on soil structure, chemical processes, plant nutrition and microbial activity. The sources of sugars in soils are: a) plants (the primary source); b) animals (the minor source), and c) microorganisms (fungi, bacteria, algae), which decompose the primary plant and animal material, and synthesize the major part of soil carbohydrates. A particular soil sample provides a momentary glimpse into a dynamic system (continuous addition, degradation and synthesis) that might, except for seasonal variations, be in equilibrium. The purpose of this study is to identify and quantify the major sugars in a grass soil and characterize the relationship between their concentration variations and soil microbial behavior over an annual cycle. Soil samples were collected monthly in a ryegrass field close to Corvallis, Oregon, and analyzed by gas chromatography-mass spectrometry as total silylated extracts for sugar composition, and by gas chromatography-flame ionization as fatty acid methyl esters derived from phospholipids and neutral lipids (PLFA and NLFA, respectively). The preliminary results of the first six-month experiment (from January to June, 2004) show that as the ambient temperatures increase the sugar concentrations (glucose, fructose, sucrose and trehalose) also tend to increase in the soil. A decrease is observed in March when precipitation was low during the whole month. The same trend is observed for the active biomass of fungi and bacteria estimated by their fatty acids derived from phospholipids. Fatty acids 18:2$\omega$6c and 18:3$\omega$6c are used as fungal biomarkers. Branched (15:0i, 15:0a, 16:0i) and monounsaturated fatty acids (16:1$\omega$7c) are used as biomarkers for gram-positive and gram-negative bacteria, respectively. The contents of 18:2$\omega$6c and 18:3$\omega$6c from neutral lipids, which are used as an index of fungal storage, have a significant increase in June, similarly to the disaccharide trehalose. This increase in fungal lipid storage may have occurred in response to the large input of detrital carbon into the soil from cutting the grass early in that month.