Soil texture, plant residue quality and aggregate protection determine carbon and nitrogen stabilization under reduced tillage in semiarid rainfed almond orchards

Abstract

Reduced tillage combined with native/spontaneous cover crops are increasingly promoted as climate-smart practices to enhance soil organic carbon (SOC) storage in dryland agriculture, yet the mechanisms controlling carbon and nitrogen (N) stabilization under semiarid Mediterranean conditions remain insufficiently understood. In particular, the effectiveness of these practices may depend not only on plant biomass inputs, but also on plant residue quality, soil texture, microbial activity, and aggregate-mediated protection. In this long-term study (6–20 years), the effects of reduced tillage (twice a year) combined with native cover crops (RT + NCC), relative to traditional tillage (TT; 3–5 times per year to maintain bare soil), were assessed in the top 15 cm of soil (corresponding to the tillage depth) across three rainfed semiarid almond ( Prunus dulcis Mill.) orchards with contrasting soil types (Calcisol and Regosol). Their influence on (i) soil aggregate distribution and stability, (ii) the quantity and quality of OC and N pools across soil aggregate fractions, and (iii) microbial activity and carbon-nitrogen dynamics was evaluated. Soil response to management was strongly site-dependent. The clearest effect was observed in the silty loam soil, where reduced tillage combined with native cover crops increased the free macro and occluded aggregates percentage (about 120%), stimulated microbial activity (enzymatic activities, microbial biomass and mineralized OC), and promoted the redistribution of OC and N into more physically protected pools (increased of about 50% in OC and 35% in N concentration within the occluded aggregates). The soils with higher sand content strongly constrained management outcomes. Across sites, plant residue quantity alone did not explain the observed responses, which were also shaped by residue biochemical quality and local edaphic conditions. Importantly, management practices effects were more evident at the aggregate-fraction level than in bulk soil, where total SOC and N often showed limited changes despite clear increases in protected fractions. Overall, our results demonstrate that the stabilization of OC and N in semiarid Mediterranean almond orchards is governed by the interaction among soil texture, plant residue quality, microbial activity, and physical protection within aggregates. These findings highlight the need for site-specific strategies that maximize the transfer of fresh organic inputs into persistent and protected soil OC fractions.