The Effect of Litter Quality and Mineral Nitrogen On Microbial Functioning, Residue Decomposition and Soil Organic Matter Accumulation.

Can structural and physiological differences in microbial communities explain changes in residue decomposition and eventually soil organic matter (SOM) quantity and quality? This study aims to elucidate the link between organic residue (OR) quality, N amendments, and the function of microbial communities in SOM stabilization. This was achieved in a medium-term incubation experiment through an integration of a variety of microbial methods designed to target different physiological aspects of the microbial community. Microbial community function was assessed through analysis of microbial growth yield efficiency (GYE), extracellular enzyme activity (alpha-glucosidase, beta-glucosidase, cellobiohydrolase, xylanase, and L-leucine aminopeptidase), and respiration at five time points along the decomposition continuum. In the future, these analyses will be complimented by phospholipid fatty acid analysis and the measurement of SOM quantity and quality at each time point. Soils were collected in June 2009 from a long-term field site in Embu, Kenya. The incubation experiment is designed to mimic OR and mineral N application rates at the field site. Soils for each lab treatment, along with replicates within each treatment, were taken from their corresponding field plot in order to take full advantage of the long-term treatment history established at the site. The following residue treatments, with and without the addition of mineral N (100 mg N kg-1 soil), were applied to 200 g soil at a rate of 3.65 g C kg-1 soil and incubated at 25 degrees C for 180 days: Tithonia diversifolia leaves (l), stems (s), leaves + stems (l+s), and Zea mays. Furthermore, a glucose treatment was added to the incubation to test the effect of a very labile C source. An increased understanding of the role that microorganisms play in organic residue decomposition and SOM stabilization will help to improve predictive soil organic carbon models by considering microbial responses to agricultural management practices.