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.