Alternatives to maize monocropping in Mediterranean irrigated conditions to reduce greenhouse gas emissions.

Winter legume cover crops or double-cropping in high N-fertilizer maize-based sprinkler-irrigated systems enhance agroecosystem diversity and potentially increase yields. However, the effects on direct N2O emissions and global warming potential (GWP) have not been fully established. For two years, in the Ebro Valley (Spain), four maize-based systems consisted of: long-season maize (Zea mays) with winter fallow period (F-LSM) the reference system; or after a leguminous cover crop (common vetch, Vicia sativa) (CC-LSM); and short-season maize after a cereal crop (barley, Hordeum vulgare) (B-SSM) or after a leguminous crop (pea, Pisum sativum) (P-SSM). They were assessed in terms of productivity, direct greenhouse gasses emissions (GHG: N2O, CH4, CO2), and global warming potential (GWP). Direct GHG emissions were measured using the static chamber technique, while soil parameters were monitored. Crop yields and nitrogen uptake were also quantified. GHG emissions linked to management and inputs were calculated to obtain GWP and greenhouse gas intensity (GHGI). The most productive system (B-SSM) obtained the highest direct (79 %, 35 %, and 30 % higher than the F-LSM, P-SSM, and CC-SSM, respectively) and scaled N2O emissions. The P-SSM system had similar N-uptake-scaled emissions to the monocropping (MC) systems. Irrigation, fertilizer, and farm operations accounted for the 26 %, 31 %, and 27 % of the total indirect emissions, respectively. Fertilizer production-related emissions in B-SSM and F-LSM systems were 172 % and 45 % higher than the average emissions in the systems with legumes (461 kg CO2eq. ha-1). Diversified systems lead to slightly higher GHGI values than the reference system (F-LSM). However, no differences were found between the F-LSM and P-SSM systems in GWP (4521 and 5512 kg CO2-eq. ha-1, respectively) or GHGI (144 and 158 kg CO2-eq. ha-1, respectively). The P-SSM system may be a potential alternative for increasing the diversification of maize-based irrigated agrosystems without increasing GHG emissions.

Management of agricultural soils for greenhouse gas mitigation: Learning from a case study in NE Spain.

A portfolio of agricultural practices is now available that can contribute to reaching European mitigation targets. Among them, the management of agricultural soils has a large potential for reducing GHG emissions or sequestering carbon. Many of the practices are based on well tested agronomic and technical know-how, with proven benefits for farmers and the environment. A suite of practices has to be used since none of the practices can provide a unique solution. However, there are limitations in the process of policy development: (a) agricultural activities are based on biological processes and thus, these practices are location specific and climate, soils and crops determine their agronomic potential; (b) since agriculture sustains rural communities, the costs and potential for implementation have also to be regionally evaluated and (c) the aggregated regional potential of the combination of practices has to be defined in order to inform abatement targets. We believe that, when implementing mitigation practices, three questions are important: Are they cost-effective for farmers? Do they reduce GHG emissions? What policies favour their implementation? This study addressed these questions in three sequential steps. First, mapping the use of representative soil management practices in the European regions to provide a spatial context to upscale the local results. Second, using a Marginal Abatement Cost Curve (MACC) in a Mediterranean case study (NE Spain) for ranking soil management practices in terms of their cost-effectiveness. Finally, using a wedge approach of the practices as a complementary tool to link science to mitigation policy. A set of soil management practices was found to be financially attractive for Mediterranean farmers, which in turn could achieve significant abatements (e.g., 1.34 MtCO2e in the case study region). The quantitative analysis was completed by a discussion of potential farming and policy choices to shape realistic mitigation policy at European regional level.