HR: 0800h
AN: B31A-0070    [Abstracts]
TI: Linking N Cycling to Microbial Function Within Soil Microenvironments in Cover Crop Systems
AU: * Kong, A Y
EM: aykong@ucdavis.edu
AF: University of California, Davis, One Shields Avenue, Davis, CA 95616, United States
AU: Scow, K M
EM: kmscow@ucdavis.edu
AF: University of California, Davis, One Shields Avenue, Davis, CA 95616, United States
AU: Hristova, K
EM: krhristova@ucdavis.edu
AF: University of California, Davis, One Shields Avenue, Davis, CA 95616, United States
AU: Six, J
EM: jwsix@ucdavis.edu
AF: University of California, Davis, One Shields Avenue, Davis, CA 95616, United States
AB: Cover crops have emerged as a crop management strategy to achieve agricultural sustainability and maintain environmental quality. Thus, fundamental knowledge of microbial-mediated C and N cycling is vital to understanding soil organic matter (SOM) dynamics in cover cropped agroecosystems. We investigated the effects of short-term cover crop-C input on N processing by microbial communities within SOM microenvironments and in bulk soil, across a gradient of organic to conventional crop management. We hypothesized that cover crop C and N inputs promote soil aggregation, which increases the abundance of ammonia oxidizing bacteria (AOB) and stimulates greater microbial cycling of N within soil microenvironments, thereby leading to potential increases in N stabilization coupled with decreases in N loss. Our hypothesis was tested on the long-term organic, low-input, and conventional maize-tomato rotations at the Center for Integrated Farming Systems experiment (Davis, CA). We collected soil samples (0-15cm) across the cover crop and subsequent maize growing seasons and then isolated three SOM fractions soil: coarse particulate organic matter (cPOM; >250um), microaggregates (53-250um), and silt-and-clay (<53um). Total C and N were measured on both bulk soil and SOM fractions. Real-time polymerase chain reaction (PCR) using primers for the functional genes, amoA and nosZ, were employed to quantify AOB and denitrifier population sizes, respectively. We also measured gross ammonification and nitrification rates in short-term 15N-incubations of the bulk soil to link cover crop induced N cycling to N-transforming bacteria. Total soil C and N concentrations and soil aggregation were higher in the organic than conventional and low-input systems. The amoA and no Z copy numbers g-1 dry soil were highest in the microaggregate fraction and similar between the cPOM and silt-and-clay fractions, among all cropping treatments. Abundances of AOB and denitrifiers were lower in bulk soil from the conventional and low- input than organic system. Our study indicates that long-term, annual cover crop inputs to the organic system lead to greater aggregation and development of microaggregate structures. Consequently, the abundance of nitrifiers and denitrifiers as well as the rates of ammonification and nitrification are augmented in the organic system compared to the conventional, which does not receive a cover crop, and the low-input system, which receives cover crops only in alternate years. These results shed light on the specific mechanisms governing short-term N stabilization versus losses under long-term crop management.
DE: 0402 Agricultural systems
DE: 0465 Microbiology: ecology, physiology and genomics (4840)
DE: 0469 Nitrogen cycling
SC: Biogeosciences [B]
MN: 2007 Fall Meeting