Soil microbial community responses to nitrogen application in organic and conventional rice production

With increasing demand for organic rice (Oryza sativa L.), there is a pressing need for more research on the effects of management practices on soil nutrient cycling and health in organic production. Soil microbial community could be used as an indicator of soil nutrient status and health; however, in drill-seeded, delay-flooded organic rice systems, how a soil microbial community responds to changes in soil properties under different nitrogen (N) management practices is not yet well understood. A greenhouse trial with four replications was conducted to study the effects of a cropping system (organic system receiving an organic certified soil amendment, Nature Safe [N–P–K, 13–0–0]; conventional system receiving a urea [46–0–0] fertilizer), N rate (0, 50, 100, 150, 200, and 250 kg N ha⁻¹), and rice cultivation (RiceTec XP753 and no plant [control]) on phospholipid-derived fatty acids profiling of soil microbial community structure at four growth stages of rice. Soil total microbial biomass (TMB) was significantly affected by the interaction of N rate × rice cultivation and the main effect of growth stage. Total microbial biomass was higher in soils with rice cultivation and soils with high N rates but lower at the growth stages after dry–wet alternations. Cropping system did not affect TMB, but its interaction with rice cultivation or growth stage significantly affected microbial community structure. The fungi/bacteria ratio linearly increased with increasing N rate and was significantly lower at heading than the other stages. Variations in microbial community structure strongly correlated to changes in soil pH, electrical conductivity, and total dissolved N induced by different management practices. This work clearly shows the responses of soil parameters, TMB, and community structure to N rate at different growth stages of organic and conventional rice, which is crucial for refining the criteria and making improved management decisions in organic rice production.