Soil biological quality and microbial functional diversity under diversified conservation agriculture systems in the Indo-Gangetic Plain

The intensive rice–wheat system leads to deterioration of soil quality, groundwater pollution and land degradation due to excessive use of tillage, fertilizers, pesticides and irrigation water. Conservation agriculture (CA) practices and diversification of cropping systems offer potential pathways to restore soil quality while supporting food and feed demands to cater the human needs. The current study undertaken under a long-term CA experiment during 2022-23 and 2023-24 to (i) evaluate the effects of diversified CA system scenarios on soil organic carbon (SOC), nutrient availability, microbial biomass, enzymatic activity, and microbial functional diversity, and (ii) identify sensitive biological indicators using PCA and correlation analysis to derive SBI. In this study, seven contrasting production scenarios-ranging from conventional to conservation, traditional to futuristic diversified systems, farmers’ practice to improved agronomy, residue removal to retention were practiced and evaluated. Results showed that diversified CA scenarios significantly improved soil quality indicators compared with the conventional rice–wheat system (Sc1). Among the different scenarios, rice–wheat–mungbean (Sc3) and maize–mustard–mungbean (Sc4) managed with ZT and residue retention recorded significantly higher soil organic carbon (by 67.9 and 43.4%), available N (by 25.2 and 20.2%), available P (by 48.7 and 35.5%), and available K (by 37.6 and 30.2%), respectively over Sc1. These systems also recorded significantly increased microbial populations, including nitrogen fixers (75.6 and 55%), Azotobacter (203 and 207%), Actinomycetes (212 and 243%), and Thiobacillus (28.8 and 4.1%) compared to that of Sc1, respectively. Functional diversity measured through average well color development (AWCD), Shannon index, and substrate utilization patterns confirmed higher microbial metabolic capacity under diversified systems being highest with Sc3 and Sc4. Principal component analysis identified nitrogen-fixing bacteria, alkaline phosphatase, AWCD, polymer utilization, Thiobacillus , and amines as key sensitive biological indicators contributing maximum to a composite soil biological index (SBI), which was highest under Sc3 (+246%) and lowest under Sc1. Overall, diversified rice–wheat–mungbean (Sc3) and maize–mustard–mungbean systems following conservation agriculture practices stood out as a potential cropping system for restoring soil fertility and biological quality in intensively managed Indo-Gangetic Plain. • Conventional rice–wheat compared with diversified conservation agriculture systems • CA rice–wheat–mungbean and maize–mustard–mungbean improved SOC and soil biology • Diversification enhanced microbial metabolism and functional diversity indices. • CA diversification increased soil biological index fourfold.