Physical and chemical stabilization of soil organic matter in cropland ecosystems under rice–wheat, maize–wheat and cotton–wheat cropping systems in northwestern India

Abstract

This article examines the impact of rice–wheat, maize–wheat and cotton–wheat cropping systems on total (TOC), labile and stable organic carbon (C) pools, aggregate stability and distribution of organic C in different size aggregates and the mineral fraction (silt + clay) of soil. The rice–wheat soils had significantly (p < .05) lower TOC concentration (4.46 ± 0.15 g C kg−1) compared with the other two cropping systems. Similarly, water-extractable organic C (WEOC, ∼0.48% of TOC), hot-water-soluble C (HWC, ∼5.5% of TOC) and microbial biomass C (MBC, ∼4.2% of TOC) were significantly lower in soils under rice–wheat compared to the other systems. The formation of water-stable aggregates (WSA) was related to the silt + clay (<0.053 mm) fraction of soils. As the silt + clay fraction increased, the proportion of macro-aggregates (>0.25 mm) increased with a concomitant decrease in the proportion of micro-aggregates (<0.25 mm). The carbon preservation capacity (CPC) of macro-aggregates (CPCMacA), micro-aggregates (CPCMicA) and total water-stable aggregates (CPCWSA) was significantly lower in rice–wheat soils compared with the other systems. The CPCMacA, mineral (silt + clay)-associated C (MinAC) and sodium hypochlorite oxidizable C (NaOCl-C) increased with an increase in the proportion of macro-aggregates and the mineral fraction of soils. A linear increase in the stable C pool ((silt + clay)-C and NaOCl-C) and soils’ silt + clay content implies that soils’ fine fraction was the principal mechanism of textural control on soil organic matter (SOM) stabilization.