Does Increasing Soil Carbon Increase Soil Nitrous Oxide Emissions from a Cropping Soil in a Semiarid Region?.

Increasing soil organic carbon (SOC) is promoted as a strategy for sequestering carbon dioxide (CO₂) and mitigating anthropogenic greenhouse gas (GHG) emissions. Although increasing SOC can benefit crop productivity, it has enhanced nitrous oxide (N₂O) emissions in temperate environments. Our field-based study investigated the extent to which increasing SOC altered N₂O and methane (CH₄) emissions from a rainfed, cropped soil in a semiarid region. The experimental design included two organic matter (OM) additions (no OM, plus OM) by two N fertiliser rates (0, 100 kg ha^-1 yr^-1) by three replicate plots. Organic matter (chaff) had been applied to a free-draining sandy soil every three years (80 t ha^-1 applied to date) since 2003, and tillage had occurred annually. Soil organic carbon (0–100 mm) averaged 1.20 % in the plus OM treatment versus 0.64% in the no OM treatment during the present study. Nitrous oxide and CH₄ fluxes were measured for two years on a sub-daily basis using soil chambers connected to a fully automated system that analysed trace gases by gas chromatography. Increasing SOC increased soil N₂O emissions (and inhibited CH₄ uptake), but also improved grain yield and quality. Nitrous oxide emissions were low by international standards and represented up to 0.1% of the N fertiliser applied. Total N₂O emissions after two years were ranked: plus OM, plus N (409 g N₂O-N ha^-1) > plus OM, no N (202 g N₂O-N ha^-1) > no OM, plus N (41 g N₂O-N ha^-1) = no OM, no N (14 g N₂O-N ha^-1). Increasing SOC has also decreased CH₄ uptake by 30%. Land management practises that increase SOC in sandy-textured rainfed, cropping soils in semiarid regions should be encouraged as it can improve grain yield without substantial increases in soil N₂O emissions.