2006 Annual Report
This research addresses several goals of the action plan for the Global Change National Program - NP 204. These include Goals I.1 and I.4 (Quantify rates of soil C storage with different land use practices and of CO2 fluxes of different ecosystems), I.5 (Determine effects of CO2 enrichment on mechanisms of soil C and N interactions), III.1 (Determine global change effects on the ecology, hydrology, and productivity of North American rangelands), and IV.2 (Assess the changing water requirements of agricultural crops under CO2 enrichment).
Less than half of the C that humans release into the air each year stays in the atmosphere. Part of the C appears to vanish into land, likely taken up by plants and soils. Understanding where this C is going, how much additional C can be stored, and how sensitive C cycling is to changes in climate, atmospheric CO2 concentration, and vegetation is directly or indirectly relevant to every citizen.
FY 2006 Milestone: Measure effects of CO2 concentration, precipitation, and soil type on plant water dynamics.
FY 2006 Milestone: Measure effects of CO2 concentration, precipitation, and soil type on leaf gas exchange.
FY 2006 Milestone: Measure effects of CO2 concentration, precipitation, and soil type on nitrogen (N) mineralization in soil and plant tissue N concentration.
FY 2007 Milestone: Prepare publications describing effects of CO2, precipitation, and soil type on plant water dynamics, leaf gas exchange, and soil and plant nitrogen (N) status.
FY 2007 Milestone: Measure root biomass of forbs and grasses of tallgrass prairie using soil cores collected to 1.5 m in each of 80 monoliths along a controlled environment chamber. Compile data on aboveground biomass of grasses and forbs.
FY 2007 Milestone: Determine effects of CO2 enrichment, anticipated changes in rainfall, and soil type on evapotranspiration rates and other components of ecosystem water balance using soil water measurements on each of 80 monoliths along a controlled environment chamber.
FY 2008 Milestone: Prepare publications describing effects of CO2, rainfall, and soil type on root biomass of forbs and grasses of tallgrass prairie. Transfer findings to producers.
FY 2008 Milestone: Prepare publications describing effects of CO2, rainfall, and soil type on evapotranspiration rates and other components of the water balance of tallgrass prairie. Transfer result to policy makers.
FY 2008 Milestone: Sample soil from tallgrass prairie exposed to different CO2 and rainfall treatments to determine treatment effects on C balance. Analyze measurements of soil respiration and C content and plant production to assess treatment effects on C cycling.
Objective 2: Determine how history of cultivation and density and biomass of invasive woody plants affects the vertical distribution and sizes of pools of organic C in mesic grasslands.
FY 2005 Milestone: Collect soil samples to 2 m depth from grasslands with different densities of woody plants.
FY 2005 Milestone: Harvest aboveground biomass of herbaceous species in grasslands with different densities of woody plants.
FY 2005 Milestone: Record woody density and the sizes of individuals of each woody species in each study plot.
FY 2006 Milestone: Remove roots from soil and begin to measure concentrations of organic carbon and nitrogen in soils collected from grasslands with different densities of woody plants.
FY 2006 Milestone: Measure the carbon concentration in tissues of herbaceous species. Estimate the biomass and carbon content of woody plants growing in grassland plots by felling and measuring similarly-sized trees located offsite.
FY 2007 Milestone: Complete chemical analyses on soils and roots collected from grasslands with different densities of woody plants. Complete C analyses on wood.
FY 2008 Milestone: Collate C analyses of soils and plants from grasslands with different densities of woody plants. Prepare manuscripts describing woody effects on rangeland C balance. Transfer results to policy-makers.
Objective 3: Determine whether climate change (temperature, precipitation) effects on net ecosystem exchange of C (NEE) from western rangelands may creditably be predicted from the response of NEE to seasonal and inter-annual variation in temperature and precipitation.
FY 2006 Milestone: Assemble data of CO2 fluxes (NEE), temperature, and precipitation from rangeland sites in the western U.S.
FY 2007 Milestone: Test relationships between annual means of CO2 fluxes and means of climate variables. Apply statistical tests (homogeneity of slopes model) to multi-year flux data from each of 8 rangeland sites in the western U.S. to determine effects of variability in precipitation and temperature on annual variability in CO2 fluxes.
FY 2008 Milestone: Complete statistical analyses of the sensitivity of rangeland CO2 fluxes to variability in temperature and precipitation.
FY 2009 Milestone: Prepare manuscripts describing the sensitivity of CO2 fluxes to variability in temperature and precipitation at rangeland sites in the western U.S.
Photosynthesis and growth responses to rainfall variability in two C4 grasses in tallgrass prairie: Future climate scenarios project increased precipitation variability, but it is not clear how such a change will impact critical pools and fluxes of carbon in grasslands. In particular, will leaf-level photosynthesis respond similarly to shifts in soil moisture when rainfall is more variable, or will increased precipitation variability change the photosynthesis-soil moisture relationship? An ARS scientist at the Grassland, Soil & Water Research Laboratory in Temple, TX, with collaborators from Kansas State University measured leaf photosynthesis and numerous other leaf attributes of Andropogon gerardii and Sorghastrum nutans, both C4 tallgrasses, in field plots under rain exclusion shelters receiving either ambient levels of precipitation quantity and variability (ambient treatment) or increased variability in rainfall (longer dry intervals and larger rain events, but the same total precipitation as the ambient treatment). Increased precipitation variability altered the photosynthesis-soil moisture relationship in S. nutans, but not in A. gerardii, demonstrating for the first time that morphologically and systematically similar species may respond differently to shifts in soil moisture variability. Species composition of grassland communities must be considered when considering climate change impacts on carbon uptake. Research addresses Component IV (Changes in Weather and the Water Cycle at Farm, Ranch and Regional Scale) of the Global Change National Program (NP 204) and Objective 3 of the current Action Plan (Determine whether climate change effects on ecosystem C balance of western rangelands may creditably be predicted from the response of C cycling processes to seasonal and inter-annual variation in temperature and precipitation).
CO2 enrichment magnifies intra-specific variation in seedling growth of an invasive shrub: The invasive shrub honey mesquite reduces grass production and the utility of grasslands for grazing. The continuing rise in atmospheric carbon dioxide (CO2) concentration could stimulate mesquite growth by increasing the biochemical rate at which mesquite plants convert atmospheric CO2 to plant carbon, but CO2 effects may not be expressed uniformly among mesquite genotypes. ARS scientists at the Grassland, Soil & Water Research Laboratory in Temple, TX, measured effects of doubling CO2 concentration on seedling growth of 14 mesquite genotypes collected from across the shrub's geographic distribution in the southwestern U.S. Seedling biomass one month following emergence was 3% to 75% greater at elevated than ambient CO2 depending on genotype, but CO2 enrichment did not favor the largest or fastest-growing genotypes at ambient CO2. CO2 enrichment could accelerate mesquite encroachment on grasslands by increasing seedling growth, but CO2 effects on mesquite are not predictable from seedling size or growth rate at the current concentration. Research addresses Component III (Agricultural Ecosystem Impacts) of the Global Change National Program (NP 204) and Objective 2 of the current Action Plan (Determine how invasive woody plants affect pools of organic carbon in mesic grassland).
The capacity of any ecosystem to store C depends both on the rate at which C is captured or fixed via plant photosynthesis (carbon input) and on the average period during which fixed C is retained before being respired by plants or soil microorganisms or otherwise lost (residence time of C). To date in the current project, we have shown that soil C in grassland was unchanged at elevated CO2 because CO2 enrichment reduced the residence time of C sufficiently to almost completely offset a large enhancement in C fixation. Respiration rates of grassland (plants plus soil microorganisms) were greater at elevated than pre-Industrial (subambient) CO2 levels partly because CO2 enrichment increased respiration per unit of carbon input. Losses of formerly stored (old) C offset increases in new C at elevated CO2 as soil N was transferred from old soil fractions with high C:N ratios to newly produced plant tissues with low C:N ratios at elevated CO2. This net transfer of N from soil to plants likely increased the response of plant production to higher CO2, but also moderated any increase in net sequestration of C at elevated CO2 by reducing the amount of C in older soil organic matter and stimulating grassland respiration. Our results indicate that little of the additional C fixed by grassland plants at elevated CO2 may be sequestered in soil if N is depleted from older soil organic matter to meet the nutritional demands of more rapidly growing plants.
Climate models predict increased precipitation variability for the Central Plains, but it is not clear how such a change will impact critical pools and fluxes of carbon on grasslands. To date in the current project, we have show that increased variability in rainfall alters the relationship of leaf C uptake (photosynthesis) to soil moisture for some, but not all, dominant grasses of tallgrass prairie, indicating that species composition partially regulates the response of grassland carbon cycling to rainfall variability. We also have shown that rainfall event sizes and intervals between rainfall event may be more important than the total annual quantity of rainfall in regulating key components of the carbon cycle on tallgrass prairie, including aboveground production and soil respiration. The potential impact of our research for public policy is enormous.
This research accomplishment is aligned with two components of the Global Change National Program (NP 204) - Carbon Cycle and Carbon Storage and Changes in Weather and the Water Cycle at Farm, Ranch and Regional Scale - and with Objectives 1 and 3 of the current Action Plan (Determine how atmospheric CO2 enrichment affects plant production and other components of the carbon cycle on tallgrass prairie; Determine whether climate change effects on ecosystem C balance of western rangelands may creditably be predicted from the response of C cycling processes to seasonal and inter-annual variation in temperature and precipitation).
This research also addresses Goal 5 of the ARS Strategic Plan (Protect and enhance the nation's natural resource base and environment) and is particularly relevant to Objective 1 of Goal 5 (Provide science-based knowledge and education to improve the management of forest, rangelands, and pastures) and Performance measure 5.1.1 (Develop ecologically based information, technologies, germplasm, and management strategies that sustain agricultural production while conserving and enhancing the diverse natural resources found on rangelands and pasture lands).
Technology transfer included.
Oral presentation of the Unit's global change research program to a global change class from Southwestern University (April 2005; 15 persons).
Polley, H.W., Wilsey, B.J., Derner, J.D., Johnson, H.B., Sanabria, J. 2006. Early-successional plants regulate grassland productivity and species composition: a removal experiment. Oikos. 113:287-295.
Polley, H.W., Tischler, C.R., Johnson, H.B. 2006. Elevated atmospheric CO2 magnifies intra-specific variation in seedling growth of honey mesquite: an assessment of relative growth rates. Rangeland Ecology and Management. 59:128-134.
Gill, R.A., Anderson, L.J., Polley, H.W., Johnson, H.B., Jackson, R.B. 2006. Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2. Ecology. 87(1):41-52.
Wilsey, B.J., Martin, L.M., Polley, H.W. 2005. Predicting plant extinction based on species-area curves in prairie fragments with high beta richness. Conservation Biology. 1835-1841.
Derner, J.D., Schuman, G.E., Jawson, M.D., Shafer, S.R., Morgan, J.A., Polley, H.W., Runion, G.B., Prior, S.A., Torbert Iii, H.A., Rogers Jr, H.H., Bunce, J.A., Ziska, L.H., White, J.W., Franzluebbers, A.J., Reeder, S.J., Venterea, R.T., Harper, L.A. 2005. USDA-ARS global change research on rangelands and pasturelands. Rangelands. 27(5):36-42.
Polley, H.W., Derner, J.D., Wilsey, B.J. 2005. Patterns of plant species diversity in remnant and restored tallgrass prairies. Restoration Ecology. 13(3):480-487.
Collins, S.L., Calabrese, L., Smith, M.D., Fay, P.A. 2005. Climate drivers and rate of change in mesic and arid grasslands. In: Ecological Society of America Proceedings, August 7-12, Montreal, Canada. 2005 CDROM.
Kaufman, D.M., Harper, C.W., Nippert, J.B., Fay, P.A., Blair, J.M. 2005. Ecosystem responses to rainfall quantity and variability in model grassland assemblages. In: Ecological Society of America Proceedings, August 7-12, 2005, Montreal, Canada. 2005 CDROM.
Fay, P.A., Smith, M.D., Travers, S.E., Nippert, J.B., Blair, J.M., Garrett, K.A. 2005. Ecological responses to precipitation quantity and frequency in grasslands: pattern and process from the gene to the ecosystem. In: Ecological Society of America Proceedings, August 7-12, 2005, Montreal, Canada. 2005 CDROM.
Porporato, A., Vico, G., Fay, P.A. 2006. Superstatistics of hydro-climatic fluctuations and interannual ecosystem productivity. Geophysical Research Letters. 33:15402-15405.