Soil nitrogen availability alters rhizodeposition carbon flux into the soil microbial community

Purpose Soil microorganisms are important in the cycling of plant nutrients. Soil microbial biomass, community structure, and activity are mainly affected by carbon substrate and nutrient availability. The objective was to test if both the overall soil microbial community structure and the community-utilizing plant-derived carbon entering the soil as rhizodeposition were affected by soil carbon (C) and nitrogen (N) availability. Materials and methods A C-13-CO2 steady-state labeling experiment was conducted in a ryegrass system. Four soil treatments were established: control, amendment with carboxymethyl cellulose (CMC), amendment with ammonium nitrate (NF), combined CMC and NF. Soil phospholipid fatty acid (PLFA) and C-13 labeling PLFA were extracted and detected by isotope ratio mass spectrometer. Results and discussion The combinedCMCandNF treatment with appropriate C/N ratio (20) significantly enhanced soil microbial biomass C and N, but resulted in lower soil inorganic N concentrations. There was no significant difference in soil PLFA profile pattern between different treatments. In contrast, most of the C-13 was distributed into PLFAs 18:2 omega 6,9c, 18:1 omega 7c, and 18:1 omega 9c, indicative of fungi and gram-negative bacteria. The inorganic-only treatment was distinct in C-13 PLFA pattern from the other treatments in the first period of labeling. Factor loadings of individual PLFAs confirmed that gram-positive bacteria had relatively greater plantderived C contents in the inorganic-only treatment, but fungi were more enriched in the other treatments. Conclusions Amendments with CMC can improve N transformation processes, and the ryegrass rhizodeposition carbon flux into the soil microbial community is strongly modified by soil N availability.