Alternate wetting-drying had no preferences for rice P uptake but increased microbial P allocation to phospholipids: Evidence from dual 32P and 33P labeling

Alternate wetting-drying (AWD) in rice cultivation controls soil redox conditions and consequently nutrient solubility. Under low redox potential, ferric iron reduction leads to bound phosphate (Fe(III)–P) dissolution, but lack of oxygen retards organic phosphorus (Porg) mineralization. Microorganisms accelerate Porg mineralization as the redox potential increases during drying, but it is not known which P source will be preferentially taken up by microorganisms and plants. Using double 32/33P labeling, we traced for the first time the P released from inorganic (32P) and organic (33P) sources into microbial biomass P (MBP), phospholipids, and plants under continuous flooding (CF) and AWD. The CF rapidly induced reducing conditions that increased Fe–P dissolution and thus P availability. The AWD had no preference for 32P and 33P incorporation into plants. However, the ratios of 32P in roots to 32P in MBP or to 32P in phospholipids in rooted soil were 4–9 times higher than the respective ratios of 33P. Moreover, the ratios of 33P in roots to 33P in MBP or to 33P in phospholipids were always <0.6. Thus, plants and microorganisms were more competitive for P from Fe–P and from Porg, respectively. The AWD increased 32P and 33P incorporation into phospholipids in rooted soil by 53–56% as compared to CF. Also, the ratio of 32P in phospholipids to 32P in MBP was 1.4–2.2 times higher under AWD than under CF. Thus, AWD stimulated microorganisms to allocate more P for cell membrane synthesis as compared to CF. Plants were similarly effective to take up P from inorganic and organic sources. Increased root biomass stimulated arbuscular mycorrhizal symbiosis (represented by 16:1ω5c) and thus 33P recovery in roots under AWD. In turn, the acceleration of microbial P turnover and the microbe-specific preference for Porg highlight the importance of arbuscular mycorrhiza for plant P uptake under fluctuating water conditions.