Rice is the primary staple grain crop grown in India, where rice fields are flooded throughout the entire crop growth period (Dash et al., 2015). As the demand for water increases in domestic and industrial sectors, the agricultural sector must allocate some of its water supply to meet the needs of other sectors. Water requirements in paddy can be reduced while increasing water productivity by replacing the continuous flooding (CF) water management system with alternate wetting and drying (AWD) without significantly sacrificing yield. The success and adoption of the AWD system will depend on the following factors: (1) the amount of water saved, (2) the availability of soil nutrients to the crop, and (3) the grain yield produced, compared to the CF system. This study focuses specifically on the second factor, which is nutrient transformation and availability, particularly phosphorus (P) availability and transformation under CF and AWD water management systems.
Phosphorus is an essential macronutrient for rice, but its availability in the soil has always been a problem due to slow diffusion and high fixation rates (Yigezu et al., 2023). P availability in soil is influenced by three main mechanisms: (1) dissolution and precipitation, (2) absorption and desorption, and (3) biotransformation (Adediran et al., 2021). These mechanisms are affected by the state and condition of the soil, including water flux and oxidation-reduction state. The condition of the soil rhizosphere remains stable in dryland situations, whereas in the water environment of paddy fields, the changing condition of the rhizosphere significantly impacts P availability (Turner and Gilliam, 1976; Bagheri et al., 2021). Therefore, water management in paddy fields directly affects the availability of P for the crop.
Water management and nutrient balance both play crucial roles in determining nutrient availability. Specifically, the application rate of nitrogen (N) under different water regimes can impact the rhizosphere environment and influence phosphorus (P) availability in soil. Studies have demonstrated that the continuous use of balanced chemical fertilizers improves soil P availability, rice P absorption, and yield (Bhattacharyya et al., 2015). Changes in soil pH and redox potentials (Eh) in paddy fields under CF and AWD can also affect P availability, as well as P adsorption and desorption properties (Seng et al., 1999). Even in acidic soils, calcium plays a significant role in modulating P dynamics under CF (Scalenghe et al., 2014). However, the excessive use of chemical fertilizers can lead to P build-up in the soil, limiting P availability (Ramaekers et al., 2010). To mitigate these negative effects and improve soil fertility in rice production systems, various agronomic practices, such as straw incorporation and AWD water management, have been implemented (Jiang et al., 2021). However, little is known about the effect of N fertilizer on soil P availability under different water management systems.
Under the CF water management system, the rhizosphere remains anaerobic throughout the rice growing period. This anaerobic condition lowers the redox potential, converts Fe3+ to Fe2+, and releases P from insoluble iron phosphate, potentially increasing P availability (Rakotoson et al., 2014). Conversely, reduced conditions can facilitate the absorption of more P by amorphous iron oxides with additional P binding sites, reducing P availability (Zhang et al., 2003). However, since the reduced soil is exposed to aerobic conditions during sampling and experimentation, the increased P availability can be considered an experimental artifact. Estimating P in the laboratory using air-dried soil may lead to an overestimation of P due to increased P precipitation (Brand-Klibanski et al., 2007).
The AWD water management system consists of two phases: drying and wetting. Most studies on soil P availability have focused on the wetting phase, with limited emphasis on P availability during the drying phase. Many researchers have used air-dried soils collected after paddy harvest to assess the impact of water management on soil P availability (Xu et al., 2020), while a few have compared changes in available P and P adsorption in flooded soils and their air-dried samples (Maftoun et al., 2006). However, this may not accurately represent the soil's P response to different water management regimes throughout the entire paddy growth period. In actual paddy field conditions during the AWD drying cycle, the soil retains a high level of water content. Therefore, the extent of soil dryness and wetness in samples collected for the study becomes crucial in determining soil P availability. During the drying phase, both the forms of iron and P in the soil undergo changes (Xu et al., 2020). As a result, fresh soil samples collected under different water management systems may provide a more accurate representation of the actual P condition in paddy soils.
Microbial biomass phosphorus (MBP) constitutes a significant portion of the soil organic P pool, accounting for 2–5% of the total organic P content in arable soil (Hedley and Stewart, 1982). MBP is highly active and plays a key role in nutrient cycling. Due to its fast turnover rate, MBP can swiftly enhance P availability. Throughout the life cycle of soil microbes, P availability decreases as a result of immobilization, while P levels in the soil rapidly rise after microbe’s death due to rapid mineralization. In anaerobic (flooded) soils, the abundance of Fe3+ reducing bacteria increases (Wang et al., 2019), facilitating the release of P anions through microbiological dissolution of ferric oxides (Maranguit et al., 2017). Conversely, soils under AWD water management exhibit a more diverse and enriched microbial population (Majumder et al., 2021). Previous research has primarily focused on P availability in soil based on absorption-desorption phenomena under different water management systems (Bai et al., 2017), but the relationship between MBP and available P under various water management systems remains unclear.
The present study investigates the influence of water management and nitrogen (N) addition on P availability in paddy soil. Its other objective is to examine the relationships between soil available P, redox state, pH, and MBP under various water management regimes. The hypothesis posits that AWD irrigation, with or without N addition, will decrease P availability in paddy soil compared to CF irrigation. Accordingly, the study aims to achieve the following objectives: (1) To quantify the impact of water management (CF and AWD) on soil P availability, and (2) To determine the potential effect of different N fertilizer application rates on P availability in soil under CF and AWD conditions.