Relationship between greenhouse gas emissions and changes in soil gas diffusivity in a field experiment with biochar and lime.

Reducing greenhouse gas emissions from arable soil while maintaining productivity is a major challenge for agriculture. Biochar is known to reduce N2O emissions from soil, but the underlying mechanisms are unclear. This study examined the impact of green waste biochar (20 Mg ha−1) and lime (CaCO3; 2 Mg ha−1) application on soil gas transport properties and related changes in these to soil N2O and CO2 emissions measured using automated chambers in a field experiment cropped with maize. In situ soil water content monitoring was combined with laboratory measurements of relative soil gas diffusion coefficient (Dp/D0) at different matric potentials, to determine changes in Dp/D0 over time. Cumulative N2O emissions were similar in the control and lime treatment, but much lower in the biochar treatment. Cumulative CO2 emissions decreased in the order: lime treatment > biochar treatment > control soil. When N2O emissions were not driven by excess N supply shortly after fertilisation, they were associated with Dp/D0 changes, whereby decreases in Dp/D0 corresponded to N2O emissions peaks. No distinct pattern was observed between CO2 emissions and Dp/D0. Cumulative N2O emissions were positively related to number of days with Dp/D0 < 0.02, a critical limit for soil aeration. These results indicate that improved soil gas diffusivity, and hence improved soil aeration, may explain the effect of biochar in reducing N2O emissions. They also suggest that knowledge of Dp/D0 changes may be key to explaining N2O emissions. [ABSTRACT FROM AUTHOR]