Cover crops improve the long-term stabilization of soil organic carbon and total nitrogen through physico-chemical protection in rainfed semiarid Mediterranean woody crop systems

Bare soil and inadequate soil tillage are emerging problems worldwide that result in soil quality impoverishment and contribute to global warming. To reverse this situation and to improve soil structure and carbon and nitrogen storage capacity, implementing sustainable land management (SLM) practices is needed. This study evaluates the combined effect of different SLM practices (tillage with/without compost addition) and cover crop types (native/seeded cover crops) and their management (mowing or grazing) on soil organic carbon (SOC) and total nitrogen (tN) stabilization in rainfed semiarid Mediterranean almond orchards under a range of edapho-climatic conditions. To do so, the long-term effects of different SLM practices, such as reduced tillage plus seeded cover crops (RT + SCC), reduced tillage plus native cover crops (RT + NCC) and no tillage plus native cover crops (NT + NCC) on soil water-stable aggregates distribution and different organic carbon and nitrogen pools were estimated at a depth of 0-20 cm (plough layer) at five experimental sites. SLM practices, implemented over 5-12 years (depending on the experimental site), were compared with the traditional tillage (TT) carried out by farmers, which consisted of three-five passes per year to maintain soil bare. Our results demonstrate the cross-site potential of combining reduced tillage and cover crops (regardless of seeded or native) for improving SOC and tN sequestration and stabilization. However, the magnitude of the effect of SLM practices on SOC and tN stabilization depended heavily on the local environmental and management conditions. As seeded cover crops (RT + SCC) accelerated SOC and tN stabilization compared with native cover crops (RT + NCC and NT + NCC), its implementation in these semiarid agro-ecosystems is recommended. However, no tillage implemented as an isolated practice is not recommended in soils prone to compaction, but managed in combination with other SLM practices, such as compost addition or mulching, can be effective for SOC and tN sequestration and stabilization over time.