Effects of conservation tillage, controlled traffic and regulated deficit irrigation on soil CO2 emissions in a maize-based system in Mediterranean conditions

Conservation tillage is promoted as a potential agriculture practice to reduce carbon dioxide (CO2) emissions but little is known on its impact in irrigated Mediterranean conditions, and particularly, when combined with controlled traffic, adopted to avoid soil compaction effects on the crops, and with regulated deficit irrigation (RDI), adopted to conserve water. CO2 effluxes were measured during the 2016/2017 and 2017/2018 irrigated maize-cropping and fallow periods on a long-term tillage experiment established in Cordoba (Spain) in which two tillage systems, conventional with residues incorporated (CTR) and zero tillage with surface residues (ZTR), are compared, both combined with controlled traffic. Additionally, two irrigation treatments were introduced: full irrigation (FI) and RDI. We hypothesized that ZTR paired with RDI would make this irrigation strategy more effective for reducing CO2 emissions. Although tillage and traffic affected CO2 effluxes, RDI did not in spite of saving 100 mm of water. Frequent irrigations maintained similar superficial soil conditions in FI and RDI. In the short term, soil CO2 effluxes were higher in CTR than in ZTR after soil preparation and during crop growth, although only significantly in the first case. However, accumulated CO2 emission during the cropping period (163 days) was 1.8 times higher for CTR than ZTR (2126 and 1177 g m-2, respectively). The accumulated emission during the fallow period (202 days) was less relevant and similar for both systems (628 g m-2). Spatially, crop lines emitted the double CO2 than furrows during the cropping period in both tillage systems, and in ZTR during the fallow, showing the relevance of the measuring point locations. Three diurnal soil CO2 efflux curves supported the results. In irrigated Mediterranean maize crops, ZTR combined with controlled traffic can be an efficient soil management system to reduce CO2 emissions, and can be paired with RDI for water saving.