Two decades of conservation agriculture enhances soil structure, carbon sequestration, and water retention in Mediterranean soils

Conservation agriculture offers a pathway for enhancing soil health with climate co-benefits in Mediterranean agricultural systems. This study examined long-term impacts of combining reduced tillage with cover cropping over 20 years in California's Central Valley, providing insights into soil system response under sustained conservation management. We assessed soil physical, chemical, and structural properties comparing reduced tillage with cover crops (CTCC) to standard tillage without cover crops (STNC), employing density fractionation and spectroscopic analysis to understand carbon protection mechanisms. After two decades, conservation agriculture produced soil conditions consistent with the approach toward a management-induced steady state described for systems managed for 15–20 years (Caruso et al., 2018; West and Post, 2002), characterized by shifts in carbon stabilization pathways. Water-stable aggregate analysis revealed the most pronounced management effects, with CTCC exhibiting 136 % greater stability than STNC, indicating substantial improvements in soil structural integrity. These structural enhancements corresponded with a reorganization of carbon protection mechanisms: CTCC disproportionately enriched the occluded light fraction (44.1 % vs. 35.4 % of total recovered carbon in STNC), indicating that aggregate-mediated physical protection becomes a dominant carbon stabilization pathway under long-term conservation management. Mineral-associated organic carbon saturation analysis revealed that both management systems remained well below theoretical maximum capacity (11.5 % vs. 7.4 % saturation for CTCC and STNC, respectively), indicating that mineral surface availability is not the primary constraint on carbon storage in this Mediterranean system. Physical property improvements under CTCC included 15 % lower bulk density and 13 % greater water retention at field capacity, though benefits were concentrated in the surface horizon. Our findings demonstrate that two decades of conservation agriculture substantially altered soil functioning through aggregate-mediated physical protection, while creating measurable improvements in soil structural integrity and water retention capacity. These mechanism shifts are consistent with a soil system that maintains enhanced functionality and continued carbon sequestration potential in Mediterranean agricultural systems.