Impacts of surface-applied residues on N-cycling soil microbial communities in miscanthus and switchgrass cropping systems

The production of biomass crops such as the perennial grasses (PGs) miscanthus (Miscanthus spp.) and switchgrass (Panicum virgatum L.) has increased considerably. The repeated annual harvest of aboveground PG biomass removes organic inputs from the soil and may influence soil health and soil microbial communities, which drive terrestrial nitrogen (N) cycling, influencing the ecosystem services provided by these feedstock systems. Our objective was to assess soil bacterial N-cycling communities as influenced by the return or removal of aboveground plant biomass (residues) to soils in PG biomass feedstock systems at different N fertilization (0 or 160 kg N ha−1) rates. Soil was collected from a field trial and quantitative PCR was used to enumerate bacterial 16S rRNA and denitrifying (nirS and nosZ) genes and transcripts. Denitrifier gene expression (nirS and nosZ) was significantly higher in N-fertilized compared to unfertilized plots, indicating that applying fertilizer in these systems may shift the activity of the denitrifying populations and possibly lead to associated N losses with no return in yield. Returning biomass residues resulted in significantly higher nosZ transcripts than in soils with residues removed but did not influence nirS gene expression. The removal of plant residues in these systems may influence the activity of the nitrous-oxide reducing microbial community, resulting in potential changes in the ecosystem services moderated by soil microbial communities, which may need to be incorporated into future soil health assessments of bioenergy feedstock systems.