Cover crop root-derived organic carbon influences aggregate stability through soil internal forces in a clayey red soil

Abstract

Soil aggregate stability is influenced by soil organic carbon (SOC). However, the influence of SOC from different fall/winter cover crop roots (annual and perennial crops) on aggregate stability through soil internal forces (SIFs) (electrostatic repulsive force (Perf), van der Waals attractive force (Pvdw), and surface hydration repulsive force (Ph)) remains unclear. The objectives of this study were compare three fall/winter cover crops (lucerne, rapeseed, six-year-vetiver (Vet_6Y)) and two control (fallow and fallow with SOC removal (fallow-SOC-R)) to determine SOC effects red soil aggregation through SIFs. Plant root morphological and chemical traits, SOC properties, soil surface charge properties, soil internal forces and aggregate breakdown were all determined. The perennial crop (Vet_6Y) showed the highest root number density (0.21 counts cm−2), second highest root mass density (1.80 mg cm−3), and the lowest root lignin/cellulose ratio (0.69), and thus contributed to higher SOC content and SOC functional groups (mainly polysaccharide-C) than annual crops (rapeseed and lucerne). Such SOC traits in perennial vetiver resulted in a higher specific surface area (SSA), lower surface charge density and surface charge number (Qs) compared to annual crops (rapeseed and lucerne) and controls, and the relationship between the root, SOC and soil surface properties (i.e., SSA) was further confirmed by significant correlation (r ranging from 0.76 to 0.97). Ultimately, these soil surface charge properties after cover crops increased Pvdw, decreased Perf, and thus decreased repulsive net force (Pnet) (the sum of Pvdw, Perf, and Ph) and the average repulsive net force displayed in order of fallow-SOC-R (2.77 MPa) > fallow (2.11 MPa) > lucerne (2.02 MPa) > rapeseed (1.78 MPa) > Vet_6Y (1.45 MPa) at a distance of 1 nm. This resulted in the mass percentage of released small particles (<20, <10, and <2 µm) w (<d) % from aggregate being reduced after cover crops compared to controls. Generally, clayey red soil aggregate stability can be improved by decreasing the repulsive soil Pnet through cover crop root contribution to SOC and SOC-functional groups.