Long-term impact of diversified crop rotations and nutrient management practices on soil microbial functions and soil enzymes activity

The long-term effect of pulse crops such as pigeonpea (Cajanus cajan L.), chickpea (Cicer arietinum L.) and mungbean (Vigna radiata L.) in crop rotation and nutrient management on soil microbial functions and enzymes activity was studied. We presumed that pulse-inclusive crop rotations and integrated nutrient management would have higher soil enzymes activity and microbial biomass carbon compared with continuous maize (Zea mays L.) - wheat (Triticum aestivum L.) rotation. We used a thirteen years old experiment in Typic Ustochrept soil of Kanpur, India. The treatments consisted of four crop rotations such as maize-wheat (MW), maize-wheat-mungbean (MWMb), maize-wheat-maize-chickpea (MWMC, two years rotation), and pigeonpea-wheat (PW) each with three levels of nutrient management treatments such as without fertilizers (control: CT), integrated nutrient management (INM), and recommended inorganic fertilizers (RDF). Crop rotations with legume in each year (PW and MWMb rotations) increased soil organic carbon (SOC), soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) compared with those of MWMC and MW (p ≤ 0.05). The PW, MWMb, and MWMC rotations increased the activity of alkaline phosphatase (20–80%), arylsulfatase (16–35%), β-glucosidase (9–16%), dehydrogenase (52–79%) and protease (5–51%) than that of MW rotation. In general, nutrient management treatments followed the order of INM > RDF ≥ CT (p ≤ 0.05) for most of the studied soil parameters. Notably, the activity of acid phosphatase, arylsulfatase and dehydrogenase enzymes was comparable in CT and RDF after thirteen years. It indicated that long-term use of chemical fertilizers could not increase these enzymes activity and warranted the need for organic amendments addition. The increased SOC and enzymes activity (particularly acid/alkaline phosphatase, β-glucosidase and arylsulfatase) in pulse-inclusive rotations were directly reflected in base-crop (wheat) productivity. The effect of crop rotations and nutrient management was mostly additive and thus an oversized increase in soil enzymes activity, SMBC and SOC was noticed in PW and MWMb with INM over the remaining crop rotations × nutrient management combinations. Thus, inclusion of pulses in crop rotation and addition of organic amendments (farmyard manure/crop residue) could be the long-term sustainable strategy to ensure optimum biological functioning of the tropical soils. The study further suggested that continuous addition of inorganic fertilizers could be detrimental to the soil enzymes activity and soil biological health in the long-run.