Effect of six engineered biochars on GHG emissions from two agricultural soils: A short-term incubation study

Abstract

Biochar production for soil amendment was recently proposed as a tool to mitigate climate change, reducing soil greenhouse gas (GHG) emissions and sequestrating carbon (C) in soil. The aim of this research project was to test the hypothesis that only biochars with specific requirements (low H/Corg and O/Corg ratios, high C/N ratio) can reduce soil N2O emissions without increasing CO2 emissions in the short term. A 45-days incubation study was carried out, in which six engineered biochars made from the pyrolysis of wood, switchgrass and the solid fraction of pig manure (SFPM), were amended to two agricultural soils (loamy sand and silt loam) at a dose of 2% (w/w) in 1-liter jars. Soil moisture content was adjusted at 80% of water-filled pore space with a solution of ammonium nitrate that corresponds to 170 kg of nitrogen per hectare. N2O and CO2 emissions were analysed on days 2, 3, and then weekly. Soil chemical properties and bacterial richness, composition and taxonomy were analysed after the incubation period. When compared to the control soils without biochar, N2O emissions were decreased by 42 to 90%, but only in the silt loam amended with biochars made from wood and switchgrass, these biochars having a high C/N ratio (>30). Lower N-NH4+ and N-NO3− concentrations were observed in these biochar treatments than in control soil. Moreover, two bacterial classes (Deltaproteobacteria and Thermoleophilia) were correlated with a decrease in N2O emissions. For each type of biochar, those produced at the highest temperature with low O/Corg and H/Corg ratios resulted in the lowest increase in CO2 emissions, which could indicate a higher biochar carbon stability. Overall, results of this study demonstrated that biochar can either increase or decrease soil GHG emissions depending on its properties, and that the effect can differ according to soil properties. Future long-term studies in the field in the presence of crop should be carried out in order to validate the conclusions of this study.