Soil aggregation, glomalin and enzyme activities under conservation tilled rice-wheat system in the Indo-Gangetic Plains

Although many farmers in the Indo-Gangetic Plains (IGP) are currently adopting zero tilled wheat (ZTW), limited information is available on the impacts of conservation tillage and conservation agriculture (CA) practices on soil aggregation, glomalin content, soil enzymatic activities and their inter-relationships. Hence, this study investigated the effects of direct seeded rice (DSR)-ZTW, DSR-ZTW with brown manuring (BM) or mungbean (Vigna radiata) residue retention (MBR) with rice residue retention on soil aggregation, glomalin content and soil enzymatic activities after four years under an irrigated rice (Oryza sativa L.)-wheat (Triticum aestivum L.) system. Eight treatments, involving transplanted rice (TPR)-conventional tilled wheat (CTW), TPR-ZTW, DSR-ZTW and combinations of residue management in DSR-ZTW were evaluated for soil aggregation and microbial activities in the 0-5 and 5-15 cm layers. Plots under mungbean residue + DSR - ZTW + rice residue retention (RR) - ZT summer mungbean + wheat residue retention (DSR + MBR-ZTW + RR-ZTMB+WR) had better microbial indices (except soil ergosterol content) than TPR-CTW in the topsoil and higher MBC and dehydrogenase activity in the 5-15 cm layer than TPR-CTW plots. Plots under DSR + brown manuring (BM)-ZTW + RR (CA module 1) had similar to 53%, 26% and 32% higher ergosterol, glomalin and MBC contents, respectively, in the topsoil (0-5 cm layer) than DSR + BM-ZTW plots. Similarly, plots under CA module 2 had similar to 23% and 29% higher ergosterol and glomalin contents, respectively, than DSR + MBR-ZTW plots. Plots under CA module 2 also had similar to 47% more alkaline phosphatase activity than farmers' practice in the topsoil. These plots also had similar to 247% and 100% more large macroaggregates in the 0-5 and 5-15 cm soil layers than TPR-CTW. However, there were less small macroaggregates in plots under CA module 2 than TPR-CTW in the topsoil. There were significant positive relationships between glomalin, carboxy-methyl cellulase activity and MBC with large macroaggregates in the topsoil and between glomalin and MBC in the 5-15 cm soil layer, indicating the role of glomalin in soil aggregation. Thus, DSR + MBR-ZTW + RR-ZTMB treatment resulted in an improved soil microbial environment after four years of rice-wheat cropping in the IGP, and this practice may be adopted. The enhanced soil properties was mainly due to residue retention of crop residues, zero tillage (ZT) in two crops (in the first three years) and triple ZT (in the fourth year), and growing of a legume crop in the conventional rice-wheat system in the IGP.