Carbon and nitrogen availability in paddy soil affects rice photosynthate allocation, microbial community composition, and priming: combining continuous .sup.13C labeling with PLFA analysis

Background and aims Carbon (C) and nitrogen (N) availability in soil change microbial community composition and activity and so, might affect soil organic matter (SOM) decomposition as well as allocation of plant assimilates. The study was focused on interactions between C and N availability and consequences for rhizodeposition and microbial community structure in paddy soil. Methods Rice continuously labeled in a .sup.13CO.sub.2 atmosphere was fertilized with either carboxymethyl cellulose (CMC) (+C), ammonium sulfate (+N), or their combination (+CN), and unfertilized soil was used as a control. .sup.13C was traced in aboveground and belowground plant biomass, soil organic matter, and microbial biomass. Microbial community composition was analyzed by phospholipid fatty acids (PLFAs). Results +CN application led to a higher yield and lower root C and N content: .sup.13C assimilated in shoots increased by 1.39-fold and that in roots decreased by 0.75-fold. Correspondingly, after +CN addition, .sup.13C from rhizodeposits incorporated into SOM and microorganisms decreased by 0.68-fold and 0.53-fold, respectively, as compared with that in the unfertilized soil. The application of +C or + N alone resulted in smaller changes. CMC led to a 3% of total N mobilized from SOM and resulted in a positive priming effect. Both fertilizations (+C, +N, or + CN) and plant growth stages affected soil microbial community composition. With decreasing microbial biomass C and N, and PLFA content under +CN amendment, +CN fertilization decreased Gram-positive (G+)/ Gram-negative (G-) ratios, and resulted in lower G+ bacteria and fungi abundance, whereas G- and actinomycetes were stimulated by N fertilization. Conclusions Organic C fertilization led to a positive N priming effect. Organic C and mineral N application decreased C input by rhizodeposition associated with lower .sup.13C recovery in SOM and microbial incorporation. C and N addition also altered microbial community composition, as +CN decreased content of microbial groups, such as G+ bacteria and fungi, but +N stimulated G- bacteria and actinomycetes.