LB 38-317 - Linking N-transforming microbes to N2O emissions in Southeastern US conventional and organic agroecoecosystems

Nitrogen is the most limiting nutrient for plant productivity in most terrestrial ecosystems, agroecosystems in particular. Conventional agriculture is characterized by high inputs of synthetic nitrogen (N) fertilizers. These applications increase crop yields, but fail to maximize the N reservoir within soil organic matter. Excessive and inefficient use of N fertilizer jeopardizes human health and the environment through nitrate leaching and nitrous oxide (N2O) emissions. Animal and crop production account for ~70% of all anthropogenic N2O emissions. Our research takes advantage of long-term farming systems at Center of Environmental Farming Systems (CEFS) that have undergone 20 years of periodical measurements for soil and microbial properties in the Southeastern US agroecosystems to answer one big question: how do alternative agricultural practices affect microbial N transformations and plant nitrogen-use-efficiency?The goals of this study were to provide a holistic understanding of how long-term farming practices and biochar affect N-transforming microbes (nitrifies and denitrifiers in particular), N transformations and N2O emissions, and plant nitrogen use efficiency (NUE).Our approach to addressing our goals involved field and greenhouse studies. These experiments were arranged in a randomized complete block (RCBD) and subject to mixed model analyses. We utilized a myriad of microbial and biogeochemical analyses, including qPCR quantification of nitrifiers and denitrifiers, mycorrhizal hyphae-ingrowth techniques and stable 15N tracing, to unearth direct linkages between the effects of alternative agroecosystems on N-transforming microbes (i.e., bacteria and archaea), soil N transformations, and N2 emissions. Here we show that functional gene abundances of N-cycling microbes did not predict N2O emissions. Despite possessing significantly higher abundances of N-cycling microbes, N2O emissions from organic agroecosystems were ~40% lower than conventional agroecosystems. Long-term farming practices did not significantly affect the relative composition of the denitrifier community as characterized by functional genes (i.e., the ratios of nirk-, nirS- and nosZ-denitrifiers). However, biochar addition and symbiotic associations (arbuscular mycorrhizal fungi) were found to have significant effects on gas emissions and denitrifier gene abundances. In conclusion, our results show that the relationship between the abundances of N-cycling microbes and N2O emissions was differentially affected in conventional and organic agroecosystems. This study suggests that short-term management practices such as biochar additions have the capacity to offset the effect of different long-term farming practices on denitrifiers and N2O emissions.