Agricultural Nitrogen Management Affects Microbial Communities, Enzyme Activities and Functional Genes for Nitrogen Cycle Processes.

Improved understanding of nitrogen (N) cycling in agroecosystems is essential for increasing N use efficiency and sustainable food production. Availability of N from organic sources and fertilizers is the result of the enzymatic processes that comprise N mineralization, immobilization and nitrification. These transformations between organic N and inorganic N form a central part of the internal soil nitrogen cycle. A multi-year experiment was conducted in Utah to examine N-source effects on nitrification and mineralization in agricultural systems. N-sources include low and high levels of ammonium sulfate fertilizer (100 and 200 kg N/ha) and steer manure composts. We used a combination of enzyme activity, real-time quantitative PCR for target genes, amplicon pyrosequencing and metagenomic sequencing to examine functional changes in responsible organisms. Changes in bacterial communities, ammonia oxidizer communities, nitrite oxidizers, and ureolytic organisms in response to N treatments were found. Differential inhibition kinetics and quantitative PCR revealed that while ammonia-oxidizing archaea (AOA) gene counts were higher, ammonia-oxidizing bacterial populations were more dynamic and responsible for an equal or greater fraction of the ammonium oxidized. The combination of enzyme kinetic and metagenomic approaches has brought us closer to the goal of linking the capable organisms to the process rate and extent in the soil environment.