Cycling Biochar Affects Crop Production and Water Quality.

Pyrolysis of energy crop biomass generates a by-product, biochar, which can be recycled to sustain nutrient and organic C concentrations in biomass production fields. Research is needed to evaluate biochar effects on soil properties, nutrient balance, biomass production and water quality at rates determined by the biomass yield harvested and pyrolyzed. Three replications of eight sorghum establishment treatments were installed in box lysimeters (45.5 x 33 x 15 cm) under greenhouse conditions. Treatments included soil with increasing rates (0, 1.5 and 3.0 Mg ha^-1) of topdressed or incorporated biochar return supplemented with N fertilizer or N, P and K fertilizer. The 3.0 Mg ha^-1 rate of biochar return was based on an estimated sorghum dry matter yield of 15 Mg ha^-1 and a 20% biochar yield during pyrolysis of sorghum biomass. Simulated rain was applied at 21 and 34 days after planting and mass loss of N, P and K was measured. A mass balance of total N, P and K was performed after 45 days. Returning 3.0 Mg ha^-1 of biochar did not affect soil total or Mehlich-3 extractable nutrients compared to control soil. Yet, biochar contributed to increased concentration of DRP and mass loss of TP in simulated runoff, especially if topdressed. The mass loss of DRP in runoff from soil topdressed with increasing biochar rates was linearly (r² = 0.94) related to the amount of WEP in biochar. It was estimated that up to 20% of topdressed biochar applied TP was lost in surface runoff after two rain events. In addition, poor recovery of nutrients during pyrolysis and excessive runoff loss of nutrients for topdressed biochar, especially K, resulted in negative nutrient balances. Efforts to conserve nutrients during pyrolysis and incorporation of biochar at rates derived from annual biomass yields will be necessary for sustainable energy crop production.