Enhancing soil organic carbon and structure in Mediterranean rice systems through cover cropping

Cover crops (CC) are recognized for their role in preventing and remediating soil degradation, yet their effects in Mediterranean rice (Oryza sativa L.) systems remain uncertain. This study evaluated the impact of ryegrass (Lolium multiflorum Lam.; RG) and hairy vetch (Vicia villosa Roth; HV) as winter CC on soil organic carbon (SOC) dynamics and physical properties of a Calcaric Fluvisol over three years. The effects were compared to a dry bare fallow (BF), with and without nitrogen (N) fertilizer application to rice. Cover cropping increased topsoil SOC levels (p = 0.015), with particulate organic matter as the primary driver (p = 0.039). Longer periods of cover cropping may be needed for a more prominent effect on mineral-associated organic matter dynamics and C sequestration. While both CC increased aggregation after incorporating rice residues by 17 % (p = 0.005), ryegrass demonstrated a more persistent improvement by also enhancing aggregation at CC termination (p = 0.013). However, hairy vetch may offer greater long-term benefits to soil quality potentially by supporting higher CC and rice residue inputs. Soil penetration data suggests constraints to CC root growth in conventional rice systems and highlights the need to integrate CC with other sustainable management practices to maximize their benefits. This study underscores how winter CC can enhance soil organic matter and restore soil quality of Mediterranean rice systems, despite the limiting conditions for growth.