Effect of tillage and crop (cereal versus legume) on greenhouse gas emissions and Global Warming Potential in a non-irrigated Mediterranean field

Crop rotation with legumes and conservation tillage systems are advisable practices in rainfed Mediterranean agro-ecosystems, since, in these areas, soils are broadly characterized by low organic matter contents and low fertility. These management practices can substantially modify the dynamics of soil greenhouse gas (GHG) emissions, carbon sequestration and carbon dioxide (CO2) emissions derived from system inputs and farm operations. In this context, a field experiment was conducted under Mediterranean conditions to evaluate the effect of three long-term tillage systems (no tillage (NT), minimum tillage (MT) and conventional tillage (CT)) and two crops (vetch (V, Vicia sativa L.) versus barley (B, Hordeum vulgare L.)) on nitrous oxide (N2O), methane (CH4) and CO2 emissions during one year. Crop yields, soil mineral nitrogen concentrations, dissolved organic carbon and GHG fluxes were measured during the growing season. Soil organic carbon was measured in spring 2012 (18 years after the beginning of a long-term tillage experiment) and together with input and fuel consumption by farm machinery was used to compare the Global Warming Potential (GWP) of the different crop and tillage treatments. Cumulative fluxes of N2O ranged between 0.16 (B-MT) and 0.29 (V-MT) kg N2O-N ha−1 yr−1, resulting in a lower impact on Net GWP than in previous studies. A significant 'tillage × crop' interaction was observed in cumulative N2O emissions (V resulted in higher N2O losses than barley in CT and MT, whereas similar fluxes were observed under NT), which was influenced by soil water-filled pore space, dissolved organic carbon content and denitrification losses, in spite of the presumable predominance of nitrification. Neither tillage nor crop influenced CH4 or CO2 emissions. Yield-scaled N2O emissions (YSNE) were low (<4 g N2O-N kg aboveground N uptake−1) and significantly higher in B than in V. The most sustainable crop and tillage treatments in terms of GWP were non-fertilized-V and NT, due to higher carbon sequestration and lower fuel consumption under NT and the absence of mineral N fertilizers in V. These crop and tillage treatments could be considered good management strategies in rainfed Mediterranean agro-ecosystems since they provide the best balance between soil emissions, YSNE and Net GWP.