Structural Stability and Organic Matter Stabilization in Soils: Differential Impacts of Soil Salinity and Sodicity

The present study investigated the quality of soil organic matter (SOM) and carbon (C) stability in saline and sodic soils under rice–wheat cropping system in south-western Punjab (India). The main objective of this study was to evaluate the impact of salt-affected landscapes on soil physical, chemical, biological, and microbial attributes which inter alia influence SOM quality and vice versa. The C stability in soils under saline and sodic landscapes was studied vis-à-vis normal soils based on clay dispersion ratio (CDR), clay flocculation index (CFI), distribution of macro- (> 0.25 mm) and micro-aggregates (< 0.25 mm), aggregate ratio (AR), C preservation capacity (CPC), total organic C (TOC) fractions of variable oxidizability, and partitioning of SOM as humic acid (HA), non-humic acid (NH), and fulvic acid (FA). Soil enzymatic activity, C mineralization, and basal soil respiration (BSR) were determined to estimate microbial (qmic) and mineralization quotients (qM) for soils under salt-affected landscapes. Soil-related glomalin protein (TG) was determined to establish relationship between aggregate breakage and eventually the release of C. Sodic landscapes had significantly (p < 0.05) higher CDR (by ~ 17.6 and 12.6%) but lower CFI (by ~ 25.0 and 17.2%) than saline and normal landscapes, respectively. The AR was significantly lower by ~ 28.8% and 50.9% in saline and sodic, compared with the normal landscapes. The salt-affected soils had significantly lower NH, HA, and FA concentration than the normal soils. The stable C pool comprised ~ 69.9% of TOC in saline as compared to ~ 55.1% in sodic and ~ 66.1% in normal landscapes. Macro-aggregate breakage and C release was related to decreased TG content and was discernible as significant reduction in aggregate associated C and CPC in sodic soils. A significantly higher qM for normal soils (by ~ 8.1% of TOC) than saline (by ~ 4.5%) and sodic soils (~ 5.3%) was responsible for higher C mineralization and BSR in soils. These results revealed that loss of TOC pool in sodic soils was ascribed to significantly higher dispersion ratio (DR) because of increased WDS and WDC causing reduction in proportion of water stable aggregates (WSA). Aggregate breakage in sodic soils resulted in loss of stable C pool with concomitant increase the active C pool. The lower qmic for soils under salt-affected landscapes revealed stressed microbial biomass due to excessive salt accumulation. The study highlights great potential for increasing SOM stabilization and structural stability of salt-affected soil with the adoption of appropriate land-use management strategies. These results underpin considerable potential for C sequestration in salt-affected soils through land rehabilitation by reclamation.