Tillage, green manure and residue retention improves aggregate-associated phosphorus fractions under rice–wheat cropping

The sustainability of the rice–wheat system is threatened due to the deterioration of soil health and emergence of new challenges of climate change caused by low nutrient use efficiency and large scale burning of crop residues. The conservation agriculture based on tillage intensity, crop residue retention and raising green manuring (GM) crops during the intervening period between wheat harvest and rice establishment offers opportunities for restoration of phosphorus (P) dynamics and stimulate phosphatase activities within the macro-and micro-aggregates. Phosphorus and phosphatase activities in the soil aggregates affected by different residue management practices remain poorly understood. Thus, soil samples were obtained after a five-year field experiment to identify the effect of tillage, green manure and residue management on aggregate-associated phosphorus fractions. Four main plot treatments in rice included combination of wheat straw and GM were conventional till puddled transplanted rice (PTR) with no wheat straw (PTR(W0)), PTR with 25% wheat stubbles retained (PTR(W25)), PTR without wheat straw and GM (PTR(W0) + GM), and PTR with wheat stubbles and GM (PTR(W25) + GM). Three sub-plots treatments in the successive wheat crop were conventional tillage (CT) with rice straw removed (CTW(R0)), zero tillage (ZT) with rice straw removed (ZTW(R0)) and ZT with rice straw retained as surface mulch (ZTW(R100)). Results of the present study revealed significantly higher phosphorus fractions (HCl-P, NaHCO(3)-P(i) and NaOH-P(o)) in treatment PTRW(25) + GM and ZTW(R100) compared with PTRW(0)/CTW(R0) within both macro- and micro-aggregates. The total phosphorus (P), available P, alkaline phosphatase and phytin-P were significantly higher under ZTW(R100) than CTW(R0). The principal component analysis identified NaOH-P(o), NaHCO(3)-P(i) and HCl-P as the dominant and reliable indicators for evaluating P transformation within aggregates under conservation agriculture-based practices.