Beneficial effects of reduced tillage and green manure on soil aggregation and stabilization of organic carbon in a Mediterranean agroecosystem

• Organic C fractionation is a useful tool to understand SOC dynamics affected by agricultural practices.
• Physical-chemical mechanism of SOC stabilization is enhanced with green manure incorporation.
• Combination of green manure and reduced tillage is a good option for SOC sequestration.
• No-tillage did not significantly increase the total SOC in the short-term.

Semiarid Mediterranean agroecosystems need the implementation of sustainable land management (SLM) practices in order to maintain acceptable levels of soil organic matter (SOM). The application of SLM practices helps to maintain soil structure and physical-chemical protection of soil organic carbon (SOC), hence improving soil carbon sequestration and mitigating CO2 emissions to the atmosphere. In an organic, rain-fed almond (Prunus dulcis Mill., var. Ferragnes) orchard under reduced tillage (RT), as the habitual management practice during the 14 years immediately preceding the experiment, we studied the effect of two agricultural management practices on soil aggregate distribution and SOC stabilization after four years of implementation. The implemented practices were (1) reduced tillage with a mix of Vicia sativa L. and Avena sativa L. as green manure (RTG) and (2) no-tillage (NT). Four aggregate size classes were differentiated by wet sieving (large and small macroaggregates, microaggregates, and the silt plus clay fraction), and the microaggregates occluded within small macroaggregates (SMm) were isolated. In addition, three organic C fractions were separated within the small macroaggregates and microaggregates, using a density fractionation method: free light fraction (free LF-C), intra-aggregate particulate OM (iPOM-C), and organic C associated with the mineral fraction (mineral-C). The results show that the combination of reduced tillage plus green manure (RTG) was the most-efficient SLM practice for SOC sequestration. The total SOC increased by about 14% in the surface layer (0–5 cm depth) when compared to RT. Furthermore, green manure counteracted the effect of tillage on soil aggregate rupture. The plant residue inputs from green manure and their incorporation into the soil by reduced tillage promoted the formation of new aggregates and activated the subsequent physical-chemical protection of OC. The latter mechanism occurred mainly in the fine iPOM-C occluded within microaggregates and mineral-C occluded within small macroaggregates fractions, which together contributed to an increase of up to 30% in the OC concentration in the bulk soil. No-tillage favored the OC accumulation in the mineral-C within the small macroaggregates and in the fine iPOM-C occluded within microaggregates in the surface layer, and in the mineral-C occluded within the small macroaggregates and microaggregates at 5–15 cm depth, but four years of cessation of tillage were not enough to significantly increase the total OC in the bulk soil.