Long-term nutrient management reshapes soil carbon fractions and biological fertility: Evidence from a 43-year semi-arid Vertisol experiment

Improving soil carbon quality and biological fertility through long-term nutrient management is essential for sustaining productivity, reducing input dependence and enhancing climate resilience in rainfed Vertisols. This study evaluated the effects of nine treatments including control, inorganic fertilisation (50 % RDF, 100 % RDF, RDF + Zn), organic amendments [farmyard manure (FYM) (50 % NFYM, sole FYM), crop residues (CR) (50 % NCR)] and integrated nutrient management (INMFYM, INMCR) on soil carbon pools, oxidisable carbon fractions, carbon management indices (CMI) and enzyme activities in a permanent manurial experiment. Soil organic carbon (SOC) ranged from 5.66 (control) to 11.19 g kg-1 (INMFYM), both in the cotton phase. Inorganic treatments enhanced soil inorganic carbon (SIC), suggesting an inverse relationship with organic inputs. Sole FYM enriched the very labile fraction, whereas INM enhanced the labile and less labile fractions and non-labile carbon was highest under RDF + Zn. CMI ranged from 136.7–307.1 and was highest under sole FYM. The 50 % NFYM recorded higher permanganate oxidisable carbon (POXC) and microbial biomass carbon (MBC) than 100 % RDF despite lower SOC, highlighting the importance of substrate quality. Enzyme activities were consistently lower in the cotton phase. Structural equation modelling indicated that SOC was associated with dehydrogenase activity indirectly through MBC, which mediated 86.5 % of this effect, whereas POXC was the primary driver of β-glucosidase (BGA) and cellulase activities (CA). These findings demonstrate that biological fertility in dryland Vertisols is governed by active, biologically accessible carbon pools rather than total SOC and that sustained organic and integrated inputs are essential for enhancing carbon quality and microbial enzyme function.