Agricultural practices of perennial energy crops affect nitrogen cycling microbial communities

Abstract

Production of perennial bioenergy crops on farmland that is suboptimal for food crops may mitigate nitrous oxide emissions and reduce nutrient loss while producing bioenergy feedstocks. The ability of bioenergy crops to provide these multiple ecosystem services depends on interactions between the soil N cycle and the bioenergy production system, but our understanding of these interactions is limited. The objective of this study was to determine the effects of perennial energy crop management practices, including grass species (switchgrass, prairie cordgrass, Miscanthus × giganteus, and a grass mixture), harvest timing (at anthesis and after a killing frost), and N application rate (0, 56, and 112 kg N ha−1) on populations of N-cycling soil microorganisms. We quantified the abundances of genes encoding N cycling enzymes involved in ammonia oxidation (amoA), nitrite ammonification (nrfA), denitrification (nirK, nirS, norB, and nosZ), and N fixation (nifH). We found that N application significantly affected the abundance of N-cycling genes. The abundance of bacterial amoA (AOB) increased as N fertilization increased, thereby increasing the proportion of AOB compared to archaeal amoA (AOA). The nitrite reductase genes, nrfA and nirS, were more abundant in the plots with 56 kg N ha−1 applied; however, the nirK nitrite reductase did not differ across N applications. Overall, AOA, nrfA, and norB were correlated with each other as were AOB, nirK, and nosZ. Our data imply that management practices of perennial grasses, especially N application, could have significant impacts on functionally important soil microbial communities within the soil.