Thursday, August 9, 2007

PS 56-24: Soil respiration and N2O emissions under patches dominated by C3  and C4 grasses, and forbs in the short-grass steppe: Influence of fungal and bacterial activity

Eliana E. Bontti, Graduate Degree Program in Ecology Colorado State University and Indy C. Burke, Natural Resources Ecology Lab. (NREL), Colorado State University.

Microbial nitrogen transformations via nitrification and denitrification contribute about 70% of the annual global emissions of N2O. Nitrification is the main source of N2O in the short grass steppe, where water contents do not exceed 24%, and it could be carried out by autotrophic bacteria or heterotrophic microorganisms. Previous work in grasslands and semiarid soils has shown that N2O emissions were related to heterotrophic activity, but the source has been unknown in shortgrass steppe. Our objectives were to estimate respiration rates of fungi and bacteria in patches dominated by different plant functional types (C3 and C4 grasses, and invasive forbs) in plots under control and resource addition treatments. These resource addition treatments included nitrogen + water addition plots, which were fertilized and watered 30 years ago.  The second objective was to evaluate N2O emissions during lab incubations, and determine whether fungal or bacterial biomass was the most important source.  We incubated soils collected monthly during the growing season. Respiration and N2O emissions were lower in June, when the precipitation was relatively low, and they increased during consecutive months when precipitation increased. Field trace gas fluxes were variable in time and space. We did not find consistent differences between plant functional types or resource additions. In general, fungal inhibition significantly decreased respiration and N2O emissions. Carbon dioxide and N2O emissions were higher under higher precipitation. Future analyses will help determine whether N2O emissions were mainly determined by nitrification or denitrification.

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