Land conversion impacts on soil macroaggregation, carbon sequestration and preservation in tree orchards located in Mediterranean environment (Spain)

Agricultural soils degradation is a global challenge that affects food security, climate change, environmental risks, and the loss of biodiversity and ecosystem services. In this on-farm study conducted in the Mediterranean mountain area of southern Spain, we used a natural area (NAT) as reference and compared the effect of three tree cropping systems – conventional olive (COL), organic olive (OLO) and organic almond (OAL)– on soil aggregation, aggregates-associated soil organic carbon and carbon preservation capacity. This was conducted to evaluate the hypothesis that the organic management system affects soil stability, soil organic carbon sequestration and soil organic carbon distribution among aggregate fractions. Soil samples were collected at five different depths (0–10, 10–20, 20–30, 30–40 and 40–50 cm depth) to analyse the management impacts on deeper soil layers. Aggregation indices showed an increase in the topsoil layers (0–10 and 0–20 cm depth) under organic managements with higher large macroaggregates (> 2000 µm) percentages in OLO (+10.9 % and 21.8 %) and OAL (+15.25 % and 28.5 %) respectively compared to COL. A significant decarbonization process was detected and the total carbon preservation capacity (0–50 cm depth) decreased from NAT (159.24 g kg—1) to cultivated plots, that stored a − 53.8 % OLO (73.45 g kg—1), − 66.3 % OAL (53.69 g kg—1) and − 70.4 % COL (47.22 g kg—1). In this regard, the management change from conventional to organic increased total carbon preservation capacity and caused a change in the distribution of soil organic carbon pool observed in a higher carbon content in large macroaggregates (+47.3 %). These findings indicate that organic farming affects not only to the soil organic carbon sequestration but also to the extent of soil organic carbon protection.