Soils play an essential part in maintaining agroecosystem productivity and understanding the impacts of management practices for agricultural sustainability (Kibblewhite et al., 2008; Lavelle et al., 2014). Tillage and residue management practices under intensive tillage and residue burning agricultural practices improved soil structure, associated protection of SOM and biological activities (Sharma et al., 2019; Saikia et al., 2019a, b), which eventually improves organic matter, soil aggregation, and nutrient cycling in agricultural systems (Lal 1993; Montgomery 2007; Barrios 2007). Phosphorus (P), being the second most imperative nutrient, less available to plants due to its adsorption and precipitation with iron, aluminum and calcium in soils, thereby resulting in the rapid formation of non-labile P forms (Priyadarshi et al., 2018; Ahmed et al., 2019). Soil P transformations vary depending upon the soil type and management practices (Sharma et al., 2014). Moreover, the continuous application of phosphate fertilizers has led to the serious environmental threats including acidification, hardening and P leaching from the soil (Rigo et al., 2019). The sustainability of conventional RWS based on intensive tillage is threatened by scarcity of water, energy and labour, higher production cost and environmental pollution due to burning of crop residues and deteriorating soil health (Montgomery 2007; Srinivasan et al., 2012; Chauhan et al., 2012) and emerging challenges associate with climate change (Jat et al., 2016; Grant and Flaten 2019). To address the aforementioned issues, conservation agriculture (CA)-based practices (minimum tillage, residue retention and crop diversification) are being developed and promoted for rice and wheat production (Gathala et al., 2013). The alternative systems of tillage, green manure (GM) and residue management practices under CA-based practices can potentially lead to significant changes in the availability of nutrients in RWS (Bera et al., 2017; Saikia et al., 2019a). The CA-based practices holds the potential to enhance P availability by altering the soil microbial diversity and enzyme activity, which in turn affects the availability of soil P (Wang et al., 2011; Bhan and Behera 2014; Bezerra et al., 2015). These practices can increase the soil organic matter (SOM), carbon (C) sequestration, soil aggregation (Sithole et al., 2019) and contribute to higher crop yields (Xu et al., 2019).
Soil aggregate size, distribution and stability performs a important role in enhancing the physico-chemical and biological processes in soil (Jiao et al., 2006), and also affects the P forms and availability (Castro-Filho et al., 2002). The organic phosphorus (Po) is usually found in chemically or physically protected forms, which are mineralized slowly into available forms for plant uptake, mostly as a byproduct of SOM decomposition or through the action of specific enzymes. The constant loss of P reservoir in the soil owing to crop harvesting, runoff and leaching can also consume Po and Pi forms rapidly, and thus resulting in the deficiency of P to plant. This P loss is directly associated with the stability of soil aggregates and nutrients distribution within aggregates (Liu et al., 2010). The P distribution among various fractions provides an indication of the potential stability of P in soil which may vary with different management practices. In addition, stable aggregates reduce soil erosion and degradation, surface runoff and crusting (Sithole et al., 2019). Additionally, these CA-based practices comprising ZT with crop residues retention enhances soil aggregation, mean weight diameter of water-stable aggregates and also increases the resistance of aggregates to slaking (Sharma et al., 2019). We hypothesized that tillage, GM and residue management practices may lead to significant changes in P-fractions in soil aggregates. Furthermore, the focus was to elucidate the distribution of different fractions of P (Pi and Po) in the aggregates and how these fractions affected the crop yield under CA-based RWS.