Accumulation of organic carbon in various soil aggregate sizes under different land use systems in a semi-arid environment

Aggregate-protected soil organic matter remains an important platform for organic carbon stabilisation and a sink for atmospheric CO2 emission, which play a crucial role in sustainable functioning of terrestrial ecosystems. This study was conducted to determine water stable soil aggregates, dry stable aggregates, and soil organic carbon (SOC) associated with aggregate sizes in surface (0-5 cm) and subsurface (5-15 cm and 15-30 cm) soil layers under four land use systems (forests, exclosures, grazing lands and cultivated lands) in the semi-arid northern Ethiopia. Dry- and wet-sieving analysis were applied to fractionate the soil into macroaggregates (> 0.25 mm) and microaggregates (< 0.25 mm). Particle size distribution and SOC content in macroaggregates and microaggregates (from which soil stability indices were developed) were determined using routine laboratory procedures. Result shows that land use systems had impact on soil aggregate distribution: microaggregates were dominant (more than 60 %) under cultivated land while macroaggregates were preponderant in grazing land (72-95 %). Aggregate stability index (ASI) followed the increasing order of grazing land > forest land/exclosure > cultivated land, in the surface layers, while structural stability index (SSI) was in the increasing order of forest > grazing land > exclosure > cultivated land for the top soils. SOC had strong and weak positive correlation with SSI and ASI respectively. This implies that SOC has more limited role in improving aggregation than other unexplained variables (not investigated in this study) like clay mineralogy, wettability characteristics, microbial activity and composition, and biotic factors in the study area. Correlation and regression analyses implied that dry-sieving of soil is as good as wet sieving in determining aggregate size distribution and soil aggregation in the study area. Therefore dry-sieving could be described as a "climate-smart" and quick sustainable alternative to the more time-consuming and tedious wet sieving method in a semi-arid dryland. Vegetation restoration improved soil organic carbon sequestration associated with aggregate sizes in the exclosures. Land use systems and management practices for reclamation of degraded land, in addition to changes in land use exerts positive and negative impacts on the organic carbon stock in soil aggregate fractions.