Double-cropping, tillage and nitrogen fertilization effects on soil CO2 and CH4 emissions

Double-cropping is increasingly embraced for its economic advantages in irrigated crops within the Ebro Valley region. However, the implications of this practice for soil carbon dioxide (CO₂) and methane (CH₄) emissions remain poorly understood. This study aimed to assess the impact of legume-maize double-cropping on soil CO₂ and CH₄ emissions and to identify optimal tillage systems and nitrogen fertilization rates for enhancing soil carbon budget. Conducted in Agramunt (NE Spain), the research transformed a long-term tillage and nitrogen fertilization field experiment initiated in 1996 under rainfed conditions. It transitioned into a maize monocrop system in 2015 and a diversification experiment in 2018. Maize monocrop was compared to a legume-maize double-cropping with two tillage systems (conventional tillage and no-tillage) and three mineral nitrogen fertilization rates (zero, medium and high). The legumes used were pea for grain (2019), vetch for green manure (2020) and vetch for forage (2021). The use of legumes in the double-cropping system had a significant impact on CO₂ fluxes. CO₂ emissions in the double-cropping ranged from 250 to 8070 mg CO₂-C m⁻² day⁻¹, while in the monocropping, the fluxes were in the range of 150–5790 mg CO₂-C m⁻² day⁻¹. Due to the higher biomass production and extended crop cycle in the double-cropping, along with increased residue decomposition due to the higher amount of residue provided, the cumulative CO₂ emissions were higher in the double-cropping. Within the double-cropping, the response of cumulative soil CO₂ emissions to tillage depended on the legume used and residue management. For legumes with a low nitrogen content, conventional tillage resulted in lower cumulative CO₂ emissions, while for legumes with a high nitrogen content, the lowest emissions were observed in the no-tillage system. The increased incorporation of crop residues in the combined approach of double-cropping and no-tillage led to a greater net ecosystem carbon budget, reaching 1960 kg C ha⁻¹. However, the use of high or medium nitrogen fertilization rates did not have a significant impact on CO₂ emissions or the net ecosystem carbon budget. Cumulative soil CH₄ emissions were only affected by soil tillage in 2019 and 2021, with greater net CH₄ uptake under no-tillage compared with conventional tillage. This work underlines the importance of using legume-maize double-cropping together with no-tillage and N fertilization reduction to control the soil CO₂ and CH₄ emission yields, while maintaining crop yields under Mediterranean conditions.