Effects of rice-straw returning on gaseous nitrogen loss and microorganisms in tobacco field after topdressing

Abstract: Topdressing is widely used to reduce the emission of nitrous oxide (N2O) and ammonia (NH3), thereby to alleviate greenhouse effect and haze under various agricultural measures in management of a tobacco field. In this study, a rice/tobacco location experiment was performed to explore the comprehensive effects of rice-straw returning on the loss of nitrogen and related microbial communities in 2017. Three treatments were selected, including the single application of chemical fertilizer (NPK), chemical fertilizer + rice straw returning to field (NPKS), and chemical fertilizer + rice straw returning + cake fertilizer (NPKSB). Ammonia volatilization was determined by continuous air extraction in a closed chamber. The emission of N2O was measured by a static chamber and gas chromatography. A high-throughput sequencing method was used to evaluate soil microorganisms. The results showed that the rate of NH3 volatilization and N2O emission first rose after topdressing, and then declined when reaching the maximum in 2-3 days. In the NPK treatment, the amount of NH3-N emission was (1.45±0.04) kg/hm2, and the amount of N2O-N emission was (2.49±0.23) kg/hm2 in topdressing. The rate of nitrogen loss was 6.97% ± 1.07%, and that of N2O emission was 4.81%±1.13%, which accounted for 62.98% of gaseous nitrogen, indicating that the N2O emission was the main gas loss. It infers that the long-term straw returning increased soil water content (8.4%-10.1%), and changed the microbial community structure of ammonia oxidizing bacteria (AOB, Ammonia Oxidizing Bacteria) and N2O reductase gene (nosZ), where the relative abundance of rhizobia was significantly lower, whereas, that of rhodospira was dramatically higher than that of chemical fertilizer alone. When the rice straw was returned to the field, the emission of N2O increased by 55.35% and NH3 volatilization decreased by 11.43%, as well as the loss rate of gaseous nitrogen was 10.34%±1.40%. The NPKSB treatment increased the soil water content, while, changed the microbial community structure of AOB and nosZ genes, indicating a significant increase in the relative abundance of Burkholderia, compared with the NPK treatment. In rice straw returning with cake fertilizer, the emission of NH3 volatile nitrogen decreased by 8.91%, whereas, the emission of N2O-N was similar to that of NPK treatment. The N2O emission decreased by 27.82% for the straw returning combined with cake fertilizer, compared with the NPKS treatment. There was no significant difference in NH3 volatilization between the treatments of NPKS and NPKSB. In the correlation analysis, the loss of NH3 volatilization was negatively correlated with the organic matter and moisture content in soil, whereas, there was no significant correlation with the structure of microbial community, particularly on nosZ gene and AOB gene. The N2O emission was negatively correlated with rhizobia, while positively correlated with rhodospiridae. The straw returning combined with cake fertilizer can be used to reduce the NH3 volatilization, whereas, to inhibit the N2O emission caused by straw in a tobacco field soil.