Soil Aggregation and Organic Carbon Protection in a No-Tillage Chronosequence Under Dryland Mediterranean Conditions.

No-tillage is widely accepted as a soil management practice that has the potential to increase the soil organic carbon (SOC) pool contributing to mitigate CO₂ increase in the atmosphere. However, few studies have attempted to quantify the role of NT maintenance over time on SOC protection within aggregates. The objective of this work was to study the effects of a NT chronosequence on soil aggregation and SOC protection in a dryland Mediterranean agroecosystem located in NE Spain. A NT chronosequence was established with five phases: (i) conventional tillage (CT); (ii) NT for 1 year (NT-1); (iii) NT for 4 years (NT-4); (iv) NT for 11 years (NT- 11) and (v) NT for 20 years (NT-20). Water-stable aggregates and their C concentration and total SOC were measured for every chronosequence phase from four soil depths (i.e., 0-5, 5-10, 10- 20, 20-30 cm). Water-stable large macroaggregates (2-8 mm) were 0.02, 0.12, 0.32 and 0.31 g g^-1 dry soil for the NT-1, NT-4, NT-11 and NT-20 phases, respectively. Following a similar trend, SOC level reached its maximum after 11 years under NT in soil surface. Significant differences were also found on the C concentration within microaggregates with 10.2, 10.1, 16.4, 18.1 and 19.1 g C kg^-1 for the CT, NT-1, NT-4, NT-11 and NT-20, respectively. In Mediterranean semiarid agroecosystems, SOC protection is maximized when NT is maintained over time due to the increase in the proportion of water-stable macroaggregates and microaggregate-C concentration.