B44B-01 16:05h
Observed Variation in Carbon and Water Exchange Across Crop Types, Seasons, and Years in Un-irrigated Land of the Southern Great Plains
Accurate prediction of the regional responses of carbon and water fluxes to changing climate, land use, and management requires models that are parameterized and tested against measurements made in multiple land cover types and over seasonal and inter-annual time scales. In particular, modelers predicting fluxes for un-irrigated agriculture are posed with the additional challenge of characterizing the onset and severity of water stress. We report results from three years of an ongoing series of measurement campaigns that quantify the spatial heterogeneity of land surface-atmosphere exchanges of carbon dioxide, water, and energy. Eddy covariance flux measurements were made in pastures and dominant crop types surrounding the US-DOE Atmospheric Radiation Measurement Program central facility near Lamont, Oklahoma (36.605 N, 97.485 W). Ancillary measurements included radiation budget, meteorology, soil moisture and temperature, leaf area index, plant biomass, and plant and soil carbon and nitrogen content. Within a given year, the dominant spatial variation in fluxes of carbon, water, and energy are caused by variations of land cover due to the distinct phenology of winter-spring (winter wheat) versus summer crops (e.g., pasture, sorghum, soybeans). Within crop and yearly variations were smaller. In 2002, variations in net ecosystem carbon exchange (NEE), for three closely spaced winter wheat fields was 10-20%. Variations between years for the same crop types were also large. Net primary production (NPP) of winter wheat in the spring of 2003 versus 2002 increased by a factor of two, while NEE increased by 35%. The large increase in production and NEE are positively correlated with precipitation, integrated over the previous summer-fall periods. We discuss the implications of these results by extracting and comparing factors relevant for parameterization of land surface models and by comparing crop yield with historic variations in yield at the landscape scale.
B44B-02 INVITED 16:20h
Similarities and Differenes in Carbon and Energy Fluxes Between Grassland and Agricultural Ecosystems
Grasslands and managed agricultural ecosystems comprise a significant fraction of the terrestrial biosphere. Compared to forested ecosystems, the growing season of grasslands and agricultural systems (periods of carbon sinks) are generally short and are closely linked to the water cycle due to shallow rooting zones. Measurements of carbon and energy fluxes, along with spectral signatures which provide information on the leaf area index, are used to explore the similarities and differences in seasonality of these fluxes and the critical environmental factors that affect the overall net exchange of carbon and energy. Comparisons will not only focus on the growing season when both carbon fixation and transpiration are at their peak values, but the analysis will include the spring and fall periods when the ground is covered with dead plant material that acts as an insulating mulch layer, and the impact this layer has on the fluxes.
B44B-03 INVITED 16:35h
Carbon, Water Vapor, and Energy Fluxes of Grazed and Ungrazed Tallgrass Prairie
To determine the impact of seasonal steer grazing on annual CO2 fluxes of annually-burned native tallgrass prairie, we used conditional sampling (relaxed eddy accumulation) on adjacent pastures of grazed (GR) and ungrazed (UG) tallgrass prairie from 1998 to 2001 and eddy correlation from 2002 to 2004. Fluxes of CO2 were measured almost continuously (24 hr) from immediately following burning through the burn date the following year (365 d). Aboveground biomass and leaf area were determined by clipping biweekly during the growing season. Carbon lost due to burning was estimated by clipping immediately prior to burning and collecting residual surface carbon after the burn. Soil CO2 flux was measured biweekly at midday each year using portable chambers from 1998 to 2002 and diurnally by large autochambers from 2002 to 2004. Steers were stocked at twice the normal season-long stocking rate (0.81 ha steer-1) for the first half of the grazing season (~May 1 to July 15) and the area left ungrazed the remainder of the year. That system of grazing is termed "intensive early stocking" and is commonly used throughout the Kansas Flint Hills. During the early growing season, grazing reduced net carbon exchange relative to the reduction in green leaf area, but as the growing season progressed on the grazed area, regrowth produced younger leaves that had an apparent higher photosynthetic efficiency. Despite a substantially greater green leaf area on the ungrazed area, greater positive net carbon flux occurred on the grazed area during the late season. Nighttime carbon losses were greater on the ungrazed area in the early season, but were greater on the grazed area late in the season. During the peak growth period, an amount equivalent to ~80% of the carbon fixed on a clear day was lost each day through soil CO2 flux and plant respiration. Soil CO2 flux followed a definite diurnal pattern during the growing season with daytime fluxes twice that of nighttime. During the dormant season, daytime and nighttime fluxes were similar. Both grazed and ungrazed tallgrass prairie appeared to be carbon storage neutral except in 2002, an exceptionally dry year (1998: UG -31 gC m-2, GR -5 gC m-2; 1999: UG -40 gC m-2, GR -11 gC m-2; 2000: UG +66 gC m-2, GR 0 gC m-2; 2002: UG -82 gC m-2, GR -4 gC m-2 2003: UG +27 gC m-2, GR +33 gC m-2).
http://spuds.agron.ksu.edu/GUFluxG%20entire2.htm
B44B-04 16:50h
Seasonal and interannual variations in carbon and oxygen isotopes of atmospheric CO$_{2}$ observed over a C$_{4}$-dominated tallgrass prairie in central Kansas, USA
We conducted weekly measurements of carbon (\delta$^{13}$C) and oxygen (\delta$^{18}$O) isotopes in atmospheric CO$_{2}$ over a C$_{4}$-dominated tallgrass prairie in 2002, 2003 and 2004. Air samples above and within canopies were collected using 100-ml flasks for both day- and nighttime periods. A two-source mixing line approach estimated isotope ratios of ecosystem respired CO$_{2}$ for both carbon (\delta$^{13}$C$_{R}$) and oxygen (\delta$^{18}$O$_{R}$). In general, values of \delta$^{13}$C$_{R}$ showed a significant shift from $\sim$ -20 $\permil$ in early spring to $\sim$ -12 $\permil$ in mid-summer for all 3 years, reflecting the dominance of C$_{4}$ photosynthesis in the $\it{wet}$ and warm environment. Precipitation in the spring has a profound impact on the seasonal variations in \delta$^{13}$C$_{R}$ values and net ecosystem exchange (NEE) CO$_{2}$ fluxes. Variations in \delta$^{13}$C$_{R}$ corresponded with NEE fluxes on both weekly and interannual time scales; more positive \delta$^{13}$C$_{R}$ values (C$_{4}$ dominance) were observed with greater NEE fluxes under well-watered conditions. When C$_{4}$ photosynthetic uptake of atmospheric CO$_{2}$ decreased, values of \delta$^{13}$C$_{R}$ reflected an increased impact of C$_{3}$ forbs and nearby C$_{3}$ cropland. The coupling between photosynthetic fluxes and respired \delta$^{13}$C suggests that a significant portion of recently fixed carbon was returned to the atmosphere through autotrophic respiration within days. Measuring oxygen isotopes of ecosystem CO$_{2}$ provides a means to further separate total ecosystem respiration into contributions from above- and belowground components. Our measurements showed that values of \delta$^{18}$O$_{R}$ ranged from $\sim$22 to $\sim$35 $\permil$ (VSMOW scale) within a season. These variations were a result of respired CO$_{2}$ equilibrated with two isotopically distinct ecosystem water pools: \delta$^{18}$O values in leaf water are more positive relative to soil water owing to the evaporative enrichment during the day. \delta$^{18}$O values of leaf and soil water will be modeled to constrain \delta$^{18}$O$_{R}$ measurements in order to partition respiratory fluxes in this tallgrass prairie.
http://ecophys.biology.utah.edu/Research/DOE_TCP/index.html
B44B-05 17:05h
Does Plant Diversity Affect Carbon and Water-Use Efficiency in Grasslands? Evidence from a Biodiversity and Ecosystem Functioning Experiment
In view of rapid species loss on a global scale it is imperative that we shed more light on how this loss of diversity will affect the functioning and functions of ecosystems. Previous research on biodiversity and ecosystem functioning has shown that plant biodiversity often has a positive relationship to net primary productivity (NPP), and/or net ecosystem productivity (NEP), particularly in temperate grassland systems (Hector et al 2002). One of the big ensuing questions is whether plant diversity also affects nutrient cycling and biogeochemical cycles in ecosystems. A more detailed look at the dynamics of carbon and water-use changes caused by plant diversity has only recently occurred (Caldeira et al 2001) but there is still much work to be done in this area. A large-scale grassland experiment entitled "The Jena Experiment" was started in spring 2002 in Germany in order to investigate the effect of plant diversity on ecosystem functioning, focussing mainly on element cycling and trophic interactions. Out of a total species pool of 60 species mixtures of 1 to 16 species and one to four functional groups were randomly selected and seeded as newly established communities on 82 plots of 20 x 20 m. The four functional groups consist of grasses, small and tall herbs, and legume species. With the use of natural abundance stable isotope ratios (δ13C and δ 15N) in aboveground plant material, the relationship between plant diversity (both functional and species-driven) and productivity, water-use efficiency and nitrogen cycling was investigated. Results so far, show increased above-ground productivity with increasing species diversity as well as with increasing functional diversity of plants in a system. At community level, both carbon and nitrogen concentrations as well as δ13C in plants remained similar across the diversity gradient. In contrast we found a decrease in δ 15N values with increasing plant diversity. This suggests that bigger total carbon pools in more diverse systems can be attributed only to higher biomass, whereas some form of fractionation of δ 15N seems to be occurring during N uptake or in the soil in the most species-rich systems. Soil carbon also did not tend to respond to plant diversity levels. At the whole system scale there was no evidence for changes in water-use efficiency over the diversity gradient. Implications for the functioning of future grassland systems with reduced diversity will be discussed. References Caldeira, M. C., R. J. Ryel, et al. (2001) Mechanisms of positive biodiversity-production relationships: insights provided by delta C-13 analysis in experimental Mediterranean grassland plots. Ecology Letters 4(5): 439-443. Hector, A., E. Bazeley-White, et al. (2002). Overyielding in grassland communities: testing the sampling effect hypothesis with replicated biodiversity experiments. Ecology Letters 5(4): 502-511.
B44B-06 17:20h
Evidence of Shifted Soil Moisture Sensitivity of Soil Respiration Under Warming in a Tallgrass Prairie
Although recent studies have demonstrated that climate warming has shifted temperature sensitivity of soil respiration, it is still not clear how climate warming will influence the soil moisture sensitivity of soil respiration. In this study, we quantify the threshold values of soil moisture sensitivity in a four year warming experiment and provide evidence that climate warming changes soil moisture sensitivity of soil respiration. The shifted soil moisture sensitivity could allow more carbon released from soils under warming, compensate warming-induced thermal acclimation, and add a positive feedback to global carbon cycling. Uncertainty analysis further confirms the shift in distributions of soil moisture sensitivity of soil respiration. Considering global warming will change the precipitation amounts, spatial distributions, and seasonal patterns, likely inducing more summer drought in the temperate areas, the decreased soil moisture sensitivity of soil respiration under warming could be critical in accurately predicting feedback effects of terrestrial ecosystems on climate warming.
B44B-07 INVITED 17:35h
Incorporating representation of agricultural ecosystems and management within a dynamic biosphere model: Approach, validation, and significance
At the scale of individual fields, crop models have long been used to examine the interactions between soils, vegetation, the atmosphere and human management, using varied levels of numerical sophistication. While previous efforts have contributed significantly towards the advancement of modeling tools, the models themselves are not typically applied across larger continental scales due to a lack of crucial data. Furthermore, many times crop models are used to study a single quantity, process, or cycle in isolation, limiting their value in considering the important tradeoffs between competing ecosystem services such as food production, water quality, and sequestered carbon. In response to the need for a more integrated agricultural modeling approach across the continental scale, an updated agricultural version of a dynamic biosphere model (IBIS) now integrates representations of land-surface physics and soil physics, canopy physiology, terrestrial carbon and nitrogen balance, crop phenology, solute transport, and farm management into a single framework. This version of the IBIS model (Agro-IBIS) uses a short 20 to 60-minute timestep to simulate the rapid exchange of energy, carbon, water, and momentum between soils, vegetative canopies, and the atmosphere. The model can be driven either by site-specific meteorological data or by gridded climate datasets. Mechanistic crop models for corn, soybean, and wheat use physiologically-based representations of leaf photosynthesis, stomatal conductance, and plant respiration. Model validation has been performed using a variety of temporal scale data collected at the following spatial scales: (1) the precision-agriculture scale (5 m), (2) the individual field experiment scale (AmeriFlux), and (3) regional and continental scales using annual USDA county-level yield data and monthly satellite (AVHRR) observations of vegetation characteristics at 0.5 degree resolution. To date, the model has been used with great success to quantify the impact of nitrogen fertilizer management and climate variability since 1950 on nitrate export in the Mississippi Basin, the consequence of historical land-cover changes in the U.S. on the hydrologic cycle, and most recently, the potential consequence of springtime warming since the 1950s on farm management and crop yields across the Corn Belt. The power of this modeling approach is that representations of the key ecological processes interact with the important drivers of environmental change including climate, atmospheric carbon dioxide, and maybe most importantly, humans.
B44B-08 17:50h
An Inter-comparison of Vegetation Greenness From Satellite Observations and a Terrestrial Ecosystem Model
Terrestrial ecosystem models simulate the structure and functioning of vegetation as well as the exchanges of energy, water, and nutrients between components of the land surface and the atmosphere. While these models use numerical methods that are based on a wealth of observations, the accuracy of a model in simulating ecosystem processes at the regional scale is difficult to test because evaluation has traditionally relied on {\it in situ} measurements made at point locations (on the order of several m$^{2}$ in area). Daily satellite observations may provide a means for better model evaluation through the sensing of ecosystems at regional to global scales; however, there are several challenges to this method of evaluation. Satellite measurements may suffer from signal corruption from the earth's atmosphere, sensor and solar geometry issues, and sensor calibration problems. In addition, most of the quantities of interest in model evaluation must be derived from the reflectances detected by the sensor, which increases the uncertainty in these variables, and are usually given to the community after downgrading the daily values to monthly average values. In this study, we compare twenty years of Pathfinder Advanced Very High Resolution Radiometer (AVHRR) monthly-averaged measurements of the Normalized Difference Vegetation Index (NDVI), the fraction of photosynthetically active radiation absorbed by the vegetation canopy (FPAR), and the leaf area index (LAI) with output from the Integrated Biosphere Simulator (IBIS) over grasslands, croplands, and forests within the United States. Because two variables, FPAR and LAI (secondary, or derived quantities), have different relationships with NDVI (primary quantity), this three-variable evaluation may provide a method of assessing uncertainty in both simulated and observed (derived) quantities. Results show that IBIS captures the observed seasonality and magnitude of NDVI over all biomes, although FPAR and LAI are underestimated in croplands and forests compared with the AVHRR observations. Recent satellite observations have shown an apparent `greening' of certain ecosystems through an increasing trend in the derived quantity of net primary productivity (NPP). Results from the 20-year IBIS simulation appear to capture the satellite-observed increasing trend in NPP over some North American biomes. Because IBIS is forced with observed climate, these results may act to validate the satellite observations.