Carbon Input and Crop Residue Placement Determine the Carbon Sequestration Efficiency of Soil Management Techniques

This paper aimed to study soil organic carbon (SOC) sequestration under no- tillage (NT) and full inversion tillage (FIT) soil management systems as influenced by crop residue placement. A five-year piece of research was carried out in western Sicily, Italy, on an Opuntia ficus-indica orchard (C-CAM soil) located in a semi-arid Mediterranean climate. Barley was sown annually in the orchard inter-rows at 180 kg ha−1. FIT and NT were compared in interaction with two barley residue managements: (i) removed (rem) and (ii) retained in the field (ret), laid in a split-plot design, with soil management as the main plot and residue management as the sub-plot. The main plot was arranged on two inter-rows, 108 m long and 5 m wide each, replicated three times. SOC (%) and carbon natural abundance (δ13C‰) were determined by using an EA-IRMS. The highest biomass turnover was achieved by FITret (0.85%) vs. NTret (0.46%). The distribution of SOC showed higher values for NT in the top 10 cm soil layer (6.3 g kg−1 in NTret vs. 5.0 g kg−1 in FITret) but lower carbon content in deeper layers. At a depth of 30 cm, FITret maintained 4.4 g kg−1 of SOC, while NTret reached only 3.7 g kg−1, confirming that tillage facilitates the transport and stabilization of carbon in deeper layers. Our results also suggested that when crop residues are left on the soil surface instead of being incorporated into the soil, this may limit the effectiveness of carbon sequestration. Under the experimental tested conditions, which include low susceptibility to erosion processes, the FIT system proved to be an optimal strategy to enhance SOC sequestration and improve the sustainability of agricultural systems in a semi-arid Mediterranean environment.