Incorporation of 13C-labelled rice rhizodeposition into soil microbial communities under different fertilizer applications

The application of nitrogen fertilizer is an important agronomic practice for stimulating plant production but is also reported to affect soil microorganisms, either directly or indirectly, via increased plant root exudation. In order to investigate the effect of nitrogen fertilizer on microorganisms utilizing root exudates, we chose three nitrogen application levels (0, 100, 200 mg kg−1 N) in a rice planted system and labelled the rice plants with continuous 13CO2. Phospholipid fatty acids (PLFA) were then extracted and determined by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Results showed that nitrogen application increased rice biomass and enhanced the δ13C value of the rice plant stem, root and soil. PLFA results suggested that nitrogen alone did not alter the microbial biomass and community structure directly, but that rice plant growth enhanced the microbial biomass and altered the microbial community structure greatly. The difference in microbial community structure between planted and unplanted treatments was mainly due to a higher percentage of the PLFAs 18:2ω6,9 and 16:0 and a lower percentage of i16:0 and a15:0 in the planted treatment. For planted treatments, nitrogen application significantly enhanced the total amount of 13C label and altered the microbial community which utilized the root exudates. Our results suggest that the nitrogen effect on the microbial community utilization of labelled materials was mediated by increased root rhizodeposition and provided evidence that fungi and Gram-negative bacteria were the main consumers of rice root exudates. We conclude that the impact of rice rhizodeposition on the soil microbial community utilization of root exudates is modified by nitrogen fertilizer application based on the 13C-PLFA results.