• Energy Research

AbstractCrop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N2O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short‐term (months) effects of the crop residues on N2O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium‐term (years) and long‐term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long‐term effects of residue addition on N2O emissions.

Biochar application has been suggested as a strategy to decrease nitrous oxide emissions from agricultural soils while increasing soil C stocks, especially in tropical regions. Climate change, specifically increasing temperatures, will affect soil environmental conditions and thereby directly influence soil N2O fluxes. Here, we show that Miscanthus giganteus biochar applied at high rates suppresses the typical warming-induced stimulation of N2O emissions. Specifically, in experiments with high biochar addition (25 Mg ha−1), N2O emissions under 40°C were equal to or even lower compared to those observed at 20°C. In this sense, the mitigation potential of biochar for N2O emissions might increase under the auspices of climate change.

Abstract The conversion of agricultural residues into biochar produces a material with agronomic and environmental benefits. Biochars have been found to positively or negatively affect soil greenhouse gas (GHG) emissions, with the variability being explained by differences in biochar properties and environmental conditions. This study evaluated the effect of soil moisture conditions on the emissions of GHGs from soils amended with different types of biochar. For that, we added biochars to a sandy soil and incubated the samples at 23 °C for 90 days under two moisture conditions. While CH4 fluxes remained not significantly different from zero in all treatments, CO2 effluxes were stimulated at higher soil moisture contents, though the stimulating effect varied with biochar type. Soils amended with biochars with a narrow C:N ratio showed a higher CO2 efflux than soils amended with biochars with a wide C:N ratio. There were significant main effects of pyrolysis temperature and soil moisture conditions on the N2O fluxes from the treatments, however total N2O fluxes from biochar amended soils were overall not significantly different from the control due to treatment interactions. Our study indicates that the main effect of biochars on the GHG balances of soils is related to an increase in soil C stocks, with little short-term changes in soil CH4 and N2O fluxes. Nevertheless, short-term increases in GHG emissions due to biochar addition to soils can be easily mitigated by use of biochars with high C:N ratios produced under high temperatures and by avoiding application to moist soils.