Soil structure changes under reduced tillage and cover cropping enhance carbon mineralization in Mediterranean croplands

Abstract Conservation agriculture practices like no‐tillage and cover cropping are promoted for rebuilding soil health and sequestering carbon, but their effectiveness in irrigated systems remains questioned. We hypothesized that improved soil structure under conservation practices could inadvertently stimulate microbial respiration upon wetting, potentially offsetting carbon gains. After 19 years of reduced tillage with cover crops (conservation tillage with cover crops [CTCC]) versus standard tillage with no cover crops (STNC), we found CTCC increased surface (0–5 cm) soil carbon by 50%, nitrogen by 83%, and air‐filled porosity by up to 2.5‐fold (580%) at high water potentials. However, CTCC respiration rates increased over 100% under wet conditions (‐0.25 to ‐2 kPa) in 0‐ to 5‐cm depth, likely due to the formation of macropores that become microbial hotspots upon rewetting, accelerating carbon cycling. Normalized respiration showed that each unit of soil carbon under CTCC was mineralized at nearly triple the rate observed under STNC when soils were wet, with this effect decreasing substantially with declining soil moisture and at 5–10 cm. Future work should focus on irrigation intensity and the possibility of amplifying this effect, constraining net carbon accrual despite the overall benefits of conservation practices. Our findings highlight that soil structure–moisture–microbe interactions represent a critical frontier for optimizing conservation agriculture for carbon sequestration, requiring a balanced approach to tillage, organic inputs, and irrigation management to minimize rapid carbon losses.