The Impact of Perennial Grass Bioenergy Production On Soil Microbial Communities.

Switchgrass and miscanthus biomass crops are proposed as more sustainable for energy production compared to corn and soy.  However, repeated annual harvest of these perennial grasses may have negative effects on soil quality.  Removal of aboveground biomass decreases natural carbon and nitrogen inputs to the soil and may influence important nutrient cycling processes performed by soil microorganisms.  The objective of this study is to assess changes in key microbial communities involved in carbon and nitrogen cycling as influenced by biomass removal and biomass production strategies.   

Field trials were established in 2008 comparing crop species (miscanthus, switchgrass, corn and soy), nitrogen fertilization rates (0 and 160 kg N ha^-1) and biomass harvest dates (fall and spring) in multiple field sites across Ontario, Canada.  Experimental plots were set-up in a randomized complete block design, specifically as a split-split block design with three field replicates per site.  The main plot factor is crop species, the split-block factor is fertilizer rate and the split-split block factor is harvest timing. Soil sampling was conducted at contrasting field sites in Elora (Guelph silt loam) and Ridgetown (Brookston clay loam), Ontario in October 2010 and May-June 2011.  Soil cores were taken for the 0 - 15cm depth and immediately transported back to the lab.  DNA was extracted from soils using Powersoil kits (Mobio, Carlsbad, CA, USA) within 48 hours of sampling.  Soils were analyzed for organic carbon and mineralized nitrogen.  Quantitative PCR (qPCR) was used to enumerate the total bacterial communities (rpoB gene), and communities of denitrifiers and cellulose degraders through nirS, cel5 and cel48 genes.

Preliminary results indicate microbial communities associated with denitrification and cellulose degradation are influenced by biomass production strategies in these agroecosystems.  Through an improved understanding of carbon and nitrogen cycling, we may be able to more accurately assess the environmental footprint and sustainability of agroecosystems for biomass production.