Conservation Practices Boost Soil-Protected Organic Carbon Stocks in Olive Orchards

Carbon farming practices are pivotal for enhancing soil organic carbon (SOC) storage in agricultural systems. This study focuses on evaluating the effects of spontaneous cover crops as a conservation strategy compared to conventional management practices on total, non-protected, and protected SOC fractions, as well as carbon saturation, in olive groves across 13 paired sites (26 sites in total) in Andalucía, Spain. The research evaluates organic carbon concentrations in different soil fractions: non-protected (250–2000 µm), physically protected (53–250 µm), and chemically protected (<53 µm). The results reveal that olive groves managed with temporary spontaneous cover crops (CC) over the last 8–12 years generally exhibit higher SOC concentrations compared to those managed conventionally (BS), with significant differences observed across multiple sites. CC sites exhibited higher carbon stocks, with protected carbon averaging 42.6 Mg C ha−1 compared to 29.7 Mg C ha−1 in BS, and non-protected carbon at 10.3 Mg C ha−1 versus 4.8 Mg C ha−1. A direct relationship was identified between total SOC and both protected and non-protected carbon fractions, indicating that the soil of the studies olive orchards is far from being saturated in protected SOC. Moreover, the soil of the CC olive farms had a lower carbon saturation deficit (45.3%) compared to BS (67.2%). The findings show that maintaining the cover crops in olive orchards significantly contributed to carbon sequestration and reduced carbon saturation deficits by increasing the stocks of protected SOC.