Influence of Nitrogen Sources and Soil pH On Soil Microbial Communities In a Long-Term Crop Rotation System.

Agricultural management practices such as crop rotation, nitrogen fertilization, and lime application may cause changes in soil microbial community structure. The objective of this study was to examine effects of nitrogen sources and soil pH on soil microbial communities in a long-term crop rotation system. The field experiment, Cullars Rotation, consisting of a three-year rotation of cotton, corn, wheat, soybean and clover, was established in 1911 on a Marvyn loamy sand soil. Soil samples were collected in June and October of 2008 and February of 2009 at two depths (0-5 and 5-15 cm). Soil pH values for treatments without fertilization or lime were much lower than other treatments. Analysis of variance of soil microbial biomass carbon and basal respiration showed that nitrogen sources, soil depth, and the interaction effects were significant (P=0.001). Multivariate analyses of phospholipid fatty acid and automated ribosomal intergenic spacer profiles showed that changes in soil microbial communities were associated with nitrogen sources and soil pH. Fungal biomarker (18:2ω6) was higher in the surface soil and the stress indicator ratio was higher in the subsurface soil for all treatments. Fungal biomarker and fungi/bacteria ratio were positively correlated to soil organic carbon content. These results indicate that changes in soil microbial community structure were influenced by changes in soil properties due to management practices, such as nitrogen fertilization, crop rotation with winter legumes and lime application.