Soil organic carbon sequestration when converting a rainfed cropping system to irrigated corn under different tillage systems and N fertilizer rates

The aim of this study was to evaluate the impact of 21 years of tillage and N fertilization and the conversion from a rainfed to an irrigated cropping system on soil organic C (SOC). The study was carried out in northeastern Spain in a long-term tillage and N rate field experiment established in 1996 under barley rainfed conditions, which in 2015 was converted into irrigation with corn. Three types of tillage (conventional tillage, CT; reduced tillage, RT; no-tillage, NT) and three mineral N fertilization rates (0, 60, and 120 kg N ha(-1) under barley, and 0, 200, and 400 kg N ha(-1) under corn) were compared. Annual C-inputs as aboveground crop residues and annual SOC sequestration rate (SOCrate) (0-40 cm depth) were calculated in three different periods (P1, P2 and P3) under rainfed (-R) and irrigated (-I) conditions (P1-R, from 1996 to 2009; P2-R, from 2009 to 2015; P3-I, from 2015 to 2017). At the end of P3-I, particulate organic C (POC) was measured from the 0-5, 5-10, 10-20, 20-30, and 30-40 cm depths. Averaged over all treatments, SOCrate was 492, 222, and 969 kg C ha(-1) yr(-1) for P1-R, P2-R, and P3-I, respectively. In P1-R and P3-I, C-input explained 70% of the variability of SOCrate. In P1-R, SOCrate followed the order NT > RT > CT, while for N rate, order was high > medium > 0. In P3-I at the highest N rate, SOCrate followed the order NT > RT > CT. In P2-R, SOCrate did not show differences between tillage and/or N rate treatments. The increase in SOC after conversion from a rainfed to an irrigation system was mainly explained by POC, which was increased by 75% compared to the previous rainfed period. The modification of the cropping system through the introduction of irrigation and adequate crop management practices under no-tillage and adjusted N fertilizer rates can contribute to the sequestration of large amounts of atmospheric CO2.