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Colorado State University (2002)

Biogeochemical response of United States Great Plains grasslands to regional and interannual variability in precipitation

McCulley, Rebecca Lynne

Titre : Biogeochemical response of United States Great Plains grasslands to regional and interannual variability in precipitation

Auteur : McCulley, Rebecca Lynne

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (PhD) 2002

Résumé
Current climate change scenarios predict increasing variability in both the amount and timing of rainfall for the Great Plains region of North America. In this region, aboveground production is tightly linked to both long-term average and interannual precipitation patterns, suggesting that future changes in climate may have significant consequences for grassland ecosystem function. However, aboveground production accounts for only 50% of the carbon input into these ecosystems, and little is known about the belowground production response or biogeochemical consequences of interannual variability in precipitation. Biogeochemical processes, such as nitrogen mineralization, determine the amount of resources available for plant growth and have shown sensitivity to alterations in water availability. Thus, interannual variability in precipitation is likely to have direct and indirect effects on plant production by influencing water availability and by altering biogeochemical processes. In this dissertation, I address the influence of regional, seasonal, and interannual variability in precipitation on nitrogen (N) and carbon (C) cycling and microbial biomass and community composition in grassland ecosystems of the Great Plains. At 5 sites spanning a 500 mm mean annual precipitation gradient and encompassing, from west to east, shortgrass steppe, mixed grass prairie, and tallgrass prairie plant community types, I measured monthly in situ net N mineralization and soil respiration rates and annual above- and belowground net primary production and litter decomposition rates during the 1999-2001 time period. To quantify variability in the microbial biomass and community composition I analyzed the phospholipid fatty acid content of soil samples taken in October 2000 and June 2001 from these 5 sites. Carbon cycling rates and microbial biomass increased from semi-arid shortgrass steppe to sub-humid tallgrass prairie. At each site, C cycling rates were responsive to interannual variability in precipitation and this responsiveness varied across grassland community types. There were no significant regional, seasonal, or interannual trends in N cycling rates. Microbial biomass was larger during the growing season than in the fall, and microbial community composition was different for each of the 3 grassland types but was not significantly different across landscapes (uplands or lowlands) or between seasons at any of the sites.

Mots clés : Grasslands — Great Plains Biogeochemistry — Rain and rainfall

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