Microbial inoculant has little effect on greenhouse gas emissions following cover crop incorporation

A net negative emissions technology is the transformation of CO₂ and its storage in agricultural soils in form of soil organic carbon (SOC). One possibility to increase SOC stocks in agriculture is to grow and incorporate cover crops in the upper soil layer. However, incorporation of fresh plant material can also increase N₂O emissions and thereby reduce the overall greenhouse gas mitigation effect. While the effect of removing plant material is relatively well understood, the effect of different incorporation methods and the inoculation of cover crops with microbial inoculant (MI) is still poorly known. To investigate these effects, we conducted an incubation study and a field trial where a grass-clover cover crop was followed by maize. We measured greenhouse gas emissions (N₂O, CO₂), soil parameters (Nmin, DOC, soil moisture and temperature), soil organic carbon (SOC) stocks and maize yield. In the four weeks following cover crop incorporation, shallow rotary tillage induced 30 % higher CO₂ emissions than ploughing and removal of cover crop biomass resulted in significantly lower N₂O and CO₂ emissions as if it was mulched and inoculated with MI. Regarding the whole season, removal of aboveground cover crop biomass reduced N₂O field emissions in tendency by 21 %, whereby the trend in N₂O reduction by adding MI in the field was less pronounced. Total N₂O emissions did not differ between tillage implements used for incorporation. SOC stocks did not change within 0–20 cm within a year. Maize yield was 23 % higher with ploughing than rotary tillage. Overall, the addition of MI during cover crop incorporation might improve the greenhouse gas balance, but potential effects are superimposed too strongly by other management and meteorological factors. Therefore, claims that MI are an option to mitigate greenhouse gas emissions from agriculture remain weakly substantiated.