Effects of land management change on soil aggregates and organic carbon in Mediterranean olive groves

Abstract

Carbon sequestration in agricultural soils has been defined as a positive strategy in the mitigation of global warming. The selection of the appropriate soil management system to achieve this goal is fundamental. In this study the effect of two tillage treatments, conventional tillage (CT) and no tillage with bare soil (NT) was analyzed in the complete soil profile (by horizons) in a rainfed olive grove under Mediterranean conditions. The objective was to determine the influence of treatments on soil aggregation and organic carbon (OC) distribution within aggregate fractions both in the surface horizon (0–32.7 cm in CT and 0–21 cm in NT) and in depth (32.7–119.7 cm in CT and 21.7–110 cm in NT) (Bw, BC and C horizons). Soil samples from all horizons were separated into four aggregate-size fractions: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm), and silt + clay (<53 μm) by wet-sieving method. Aggregate stability results showed that mean weight diameter of aggregates (MWD) increased in depth in both treatments and the A horizon contained 47.6% (CT) and 55.8 (NT) more percentage of microaggregates. NT obtained greater MWD values (0.76 vs. 0.61 mm) and more proportion of water stable large macroaggregates (15.9% vs. 6.6%) than CT only in the A horizon. Conversely, CT achieved higher MWD values than NT in the depth and more proportion of macroaggregates (large macroaggregates + small macroaggregates) were found in CT in all depths. In addition, CT had larger contribution to SOC stock within aggregate fractions (24.9 Mg ha−1) than NT (22.4 Mg ha−1). This study indicates that the adoption of NT with bare soil did not increase the OC pool in aggregate fractions and therefore it is not always the best option management change to increase SOC in Mediterranean areas.