Landuse impacts on SOC fractions and aggregate stability in typic ustochrepts of Northwest India

Abstract

The soil organic carbon (SOC) and its fractions are good indicators of soil quality and environmental stability. Among the factors affecting SOC pool and fluxes in a watershed, land use changes and soil erosion are factors of importance. The differences in SOC and its fractions among different land uses can help understand the process of soil carbon dynamics. A study was conducted in Typic Ustochrepts of Northwest India to understand the impact of forest, grassland, agricultural and eroded lands on aggregate stability and SOC fractions. The undisturbed soil aggregates were sampled from different land uses in a watershed in Shiwaliks of lower Himalayas. The aggregate stability was determined by shaking the pre-saturated aggregates under water and by single simulated raindrop technique. The SOC, labile carbon, hot-water soluble carbon, particulate organic carbon and aggregate associated organic carbon were determined in aggregates of different sizes as well as in the bulk soils. The water stability of aggregates expressed as mean weight diameter (MWD) and stability index (SISRT) was highest in surface soils (0–15 cm) of grasslands followed by forest, agricultural and eroded lands. The WSA > 2 mm (water stable aggregates > 2 mm) were highest (17.3%) in grasslands and lowest (0.85%) in eroded lands. The eroded soils had 2.2, 7.4 and 3.4 times higher amount of micro-aggregates (WSA < 0.25 mm) than agricultural, forest and grassland soils, respectively. The SOC in surface soils significantly decreased by 27% in forest and 45% in agricultural land from that in grassland soils. In subsoil (15–30 cm), the SOC in eroded, agricultural and grasslands was statistically similar. The SOC stock in the subsoil (15–100 cm) was of significance. The grassland soils could store 41 Mg ha-1 SOC stock compared to 31 Mg ha-1 in the subsurface layer. This difference widened in forestland, where subsoil contained 73.4% of total SOC stock in 100 cm soil profile. Among all the SOC fractions studied, labile carbon was mostly affected by erosion and was 91.6% lower in eroded than in grassland soils. The magnitude of aggregate associated organic carbon decreased with aggregate size in all the land uses. Among the SOC fractions, the aggregate stability under simulated raindrop impact could better be explained (R2 = 0.78) by hot water soluble carbon whereas the water stability of aggregate could better be explained (R2 = 0.69) by particulate organic carbon.