Biochar Alters Manure's Effect on Nitrogen Cycling and Greenhouse Gas Emissions in a Calcareous Soil.

Few multiyear field studies have examined the impacts of a one-time biochar application on net N mineralization and greenhouse gas emissions in an irrigated, calcareous soil; yet such applications are hypothesized as a means of sequestering atmospheric CO₂ and improving soil quality.  We fall-applied four treatments: stockpiled dairy manure (42 Mg ha^-1 dry wt.); hardwood-derived biochar (22.4 Mg ha^-1); combined biochar and manure; and no amendments (control).  Nitrogen fertilizer was applied in all plots and years based on treatment’s pre-season soil test N and crop requirements, and accounting for estimated N mineralized from added manure.  From 2009 to 2011 we measured greenhouse gas fluxes using vented chambers, net N mineralization using buried bags, corn yield, and N uptake, and in a succeeding year, root and shoot biomass and biomass C and N concentrations.  Both amendments produced persistent soil effects.  Manure increased seasonal and 3-y cumulative net N mineralization, root biomass, and root:shoot ratio 1.6-fold, CO₂-C gas flux 1.2-fold, and reduced the soil NH₄:NO₃ ratio 58% relative to no-manure treatments.   Relative to all other treatments on average, biochar-only produced 33% less cumulative net N mineralization, 20% less CO₂-C and 50% less N₂O-N gas emissions, 35% less root biomass, and increased the soil NH₄:NO₃ ratio 1.8-fold.   These long-term effects suggest that biochar slightly impaired nitrification and N immobilization processes, and were likely caused by enduring biochar porosity and surface chemistry characteristics that influenced N-transformation processes, alter microbial populations, and sequester soil NH₄.  While the biochar-only treatment demonstrated a potential to increase corn yields and minimize CO₂-C and N₂O-N gas emissions in these calcareous soils, biochar also caused decreased corn yields under certain soil nutrient conditions.  Combining biochar with manure effectively utilizes these soil amendments as it eliminated potential yield reductions and maximized manure net N mineralization potential.