Soil Denitrifier Community Response to Residue Management in Biomass Cropping Systems.

Recently, interest in the use of plant based biomass energy has increased as a way to decrease dependence on fossil fuels.  When introduced, switchgrass and miscanthus were proposed as sustainable alternatives to annual crops for biomass production.  However, the effects of the repeated annual harvest (or residue removal) of perennial grasses (PGs), on soil quality indicators such as microbial abundance and activity are largely unknown. Our objective was to assess changes in microbial denitrifying community abundance and activity as influenced by residue management practices in biomass cropping systems.  Field trials were established in 2008 in a randomized complete design, comparing crop species (miscanthus, switchgrass, and a corn-soybean rotation), fertilization rates (0 and 160 kg N ha^-1) and biomass harvest dates (fall and spring in PGs) in Elora, Ontario, Canada. Additionally, subplots were established to investigate the effects of residue incorporation on denitrifying activity within spring-harvested PG plots. Soil was collected (0-15cm depth) from 2010-2012 from main plots and in summer 2011 from residue incorporation plots.  Quantitative PCR was used to enumerate the total bacterial communities (16S), and communities of denitrifiers by targeting nitrite reductase (nirS) and nitrous oxide reductase (nosZ) genes. Spring-harvested PGs had significantly higher nirS, nosZ, and 16S abundances over time than the fall-harvested annual rotation.  Additionally, direct residue addition to soils increased nosZ mRNA transcripts in PG plots over the summer of 2011, signifying a potential decrease in N₂O emissions due to complete denitrification occurring in these plots.  These results indicate that the residue management practices of leaving biomass standing until spring, or returning residues to soils, may influence transcription of denitrification genes and potentially decrease soil N₂O emissions.