Microbial strategies for phosphorus acquisition in rice paddies under contrasting water regimes: Multiple source...

Microbial acquisition and utilization of organic and mineral phosphorus (P) sources in paddy soils are strongly dependent on redox environment and remain the key to understand P turnover and allocation for cell compound synthesis. Using double ^32/33P labeling, we traced the P from three sources in a P-limited paddy soil: ferric iron-bound phosphate (Fe-P), wheat straw P (Straw-P), and soil P (Soil-P) in microbial biomass P (MBP) and phospholipids (Phospholipid-P) of individual microbial groups depending on water regimes: (i) continuous flooding or (ii) alternate wetting and drying. ^32/33P labeling combined with phospholipid fatty acid analysis allowed to trace P utilization by functional microbial groups. Microbial P nutrition was mainly covered by Soil-P, whereas microorganisms preferred to take up P from mineralized Straw-P than from Fe-P dissolution. The main Straw-P mobilizing agents were Actinobacteria under alternating wetting and drying and other Gram-positive bacteria under continuous flooding. Actinobacteria and arbuscular mycorrhiza increased P incorporation into cell membranes by 1.4-5.8 times under alternate wetting and drying compared to continuous flooding. The Fe-P contribution to MBP was 4-5 times larger in bulk than in rooted soil because (i) rice roots outcompeted microorganisms for P uptake from Fe-P and (ii) rhizodeposits stimulated microbial activity, e.g. phosphomonoesterase production and Straw-P mineralization. Higher phosphomonoesterase activities during slow soil drying compensated for the decreased reductive dissolution of Fe-P. Concluding, microbial P acquisition strategies depend on (i) Soil-P, especially organic P, availability, (ii) the activity of phosphomonoesterases produced by microorganisms and roots, and (iii) P sources - all of which depend on the redox conditions. Maximizing legacy P utilization in the soil as a function of the water regime is one potential way to reduce competition between roots and microbes for P in rice cultivation.