Carbon storage in a rainfed Mediterranean vertisol: Effects of tillage and crop rotation in a long‐term experiment

The storage of carbon (C) in cultivated soils can be increased with the adoption of different practices. The objective of this study was to determine soil organic carbon (SOC) storage in the 0‐ to 90‐cm depth profile, in four different soil layers (0–15, 15–30, 30–60 and 60–90 cm) in a long‐term (29 years) experiment established in 1986 on a rainfed Mediterranean Vertisol in southern Spain. The treatments studied were: conventional tillage (CT) versus no‐tillage (NT); five 2‐year crop rotations (wheat–chickpea, wheat–sunflower, wheat–bare fallow, wheat–faba bean, and continuous wheat; and nitrogen (N) fertilizer applied to wheat at four rates (0, 50, 100 and 150 kg N ha−1). The SOC accumulation was higher in the 30–60‐cm layer (9.2 Mg ha−1) due to the size of the characteristic cracks of Vertisols under semiarid conditions. Over the 29‐year study period, the SOC in the 0–90‐cm layer increased by 23.6 Mg ha−1 due to the change in residue management. The NT treatment exhibited a higher mean annual rate of organic C accumulation compared with the CT treatment (1.0 and 0.66 Mg ha−1 year−1, respectively) due to the retention of the mulched residue. Additionally, crop rotation influenced the rate of organic C accumulation, with wheat‐faba bean, wheat‐sunflower and continuous wheat exhibiting the highest levels of C storage in comparison to the other treatments. In rainfed Mediterranean agriculture, the selection of no‐tillage along with a rotation with legumes is key to improving soil fertility and increasing C reserves and the rate of C accumulation by soil. Highlights The SOC increase over the 29‐year study period was 23.6 Mg ha−1 in the 0–90‐cm soil profile. The characteristic cracks of Vertisol increase SOC stocks in deeper soil layers. No tillage accumulated 66.2% more SOC than conventional tillage in the 0–90‐cm profile. The wheat–sunflower rotation had the highest annual rate of C sequestration (1 Mg ha−1 year−1) and wheat–chickpea the lowest (0.6 Mg ha−1 year−1) over 29 years.