Bioenergy by-products as soil amendments? Implications for carbon sequestration and greenhouse gas emissions

An important but little understood aspect of bioenergy production is its overall impact on soil carbon (C) and nitrogen (N) cycling. Increased energy production from biomass will inevitably lead to higher input of its by-products to the soil as amendments or fertilizers. However, it is still unclear how these by-products will influence microbial transformation processes in soil, and thereby its greenhouse gas (GHG) balance and organic C stocks. In this study, we assess C and N dynamics and GHG emissions following application of different bioenergy by-products to soil. Ten by-products were selected from different bioenergy chains: anaerobic digestion (manure digestates), first generation biofuel by-products (rapeseed meal, distilled dried grains with solubles), second-generation biofuel by-products (nonfermentables from hydrolysis of different lignocellulosic materials) and pyrolysis (biochars). These by-products were added at a constant N rate (150 kg N ha-1) to a sandy soil and incubated at 20 degrees C. After 60 days, > 80% of applied C had been emitted as CO2 in the first-generation biofuel residue treatments. For second-generation biofuel residues this was approximately 60%, and for digestates 40%. Biochars were the most stable residues with the lowest CO2 loss (between 0.5% and 5.8% of total added C). Regarding N2O emissions, addition of first-generation biofuel residues led to the highest total N2O emissions (between 2.5% and 6.0% of applied N). Second-generation biofuel residues emitted between 1.0% and 2.0% of applied N, with the original feedstock material resulting in similar N2O emissions and higher C mineralization rates. Anaerobic digestates resulted in emissions < 1% of applied N. The two biochars used in this study decreased N2O emissions below background values. We conclude that GHG dynamics of by-products after soil amendment cannot be ignored and should be part of the lifecycle analysis of the various bioenergy production chains.