Studies On Effects And Mechanisms Of Water Conditions And Nitrogen Fertilizer Levels On Nitrogen Use Efficiency In A Rice Cropping System

Soil water conditions and nitrogen (N) nutrient are two major controllable factors to obtain higher rice yield and to achieve better economic return in a rice paddy ecosystem. Proper water management pattern and fertilization are considered as the effective ways for improving N use efficiency (NUE), increasing rice yield and saving water resource. In this study, combining with15N tracing technique, ion-exchange resin (IER) method and phospholipid fatty acid (PLFA) analysis technique, the pot experiment and incubation experiment were conducted to investigate the effects of water management patterns and N fertilizer levels on N uptake and utilization efficiency in rice, as well as the effects of water conditions and N levels on soil N-supplying capacity and microbial community structure. The objective was to clarify the scientific basis for establishing appropriate water-N fertilizer management strategy for rice production. The main results were as follows:(1) The rice pot experiment was designed with four N levels of0(NO),126(N1),157.5(N2) and210(N3) kg N ha-1under two water management patterns of continuous water-logging irrigation (WLI) and water-controlled irrigation (WCI) to explore the effects of water management patterns and N fertilizer levels on the rice yield and NUE by15N tracing technique. The results indicated that the rice yield significantly increased by7.4%under WCI compared with that under WLI. As N fertilizer enhanced, the rice yield and NUE were increased at lower N fertilizer levels and then decreased at higher N fertilizer levels. The highest rice yield and NUE, as well as the lowest loss of N fertilizer were achieved under N2treatment.(2) A soil incubation experiment was designed with two N levels of0(NO) and250(N250) mg N kg-1DW) under two water conditions of Moist (keeping the soil water content at70%of the field capacity) and Flooded (about3cm water layer above the soil surface). Two paddy soils of Alluvial soil and Purplish clayey soil were used in this incubation experiment. The KCl extracting method and IER method were used to detect soil mineral N contents during incubating periods. The results illustrated that the variation trends of soil NH4+-N or NO3--N content detected by two extracting methods were coincident. Specifically, NO3--N was the main mineral N in Alluvial soil, while it was NH4+-N in Purplish clayey soil; the Moist treatment significantly increased the soil NO3--N content, while the soil NH4+-N content was significantly increased by the Flooded treatment; the N250treatment significantly increased soil mineral N contents. By the correlation analysis, the significant positive correlations were found between the soil mineral N contents detected by the KCl extracting method and IER method. It is clear that the IER method can be used in situ to monitor the soil N-supplying capacity in paddy soil.(3) By the IER method, the dynamic trends of soil N-supplying capacity during the rice growth stages, as well as the relationship between the soil N-supplying capacity and the accumulation of N in rice plant were studied. The results indicated that the content of soil RAQ-NO3--N under WCI was significantly higher than that under WLI both at the tillering (10-40d after transplanting) and the grain-filling stages (60-90d after transplanting); however, there was no significant effect between WCI and WLI on soil RAQ-NH4+-N during the rice growing period. The contents of soil RAQ-NH4+-N and RAQ-NO3--N increased as N level enhanced, which mainly manifested at the early rice growing stage. The application of excessive N fertilizer (N3) increased the risk of N loss, which was detrimental to the improvement of NUE. The main form of resin exchangeable N in soil was NO3--N under NO treatment. As the passing of rice growing period, the ratios of RAQ-NO3--N in soil mineral N were gradually increased in N1, N2and N3treatments, especially under the WCI condition. The WCI regime and proper N fertilizer level (N2) were beneficial for promoting soil N-supplying capacity. According to the correlation analysis, the significant positive correlations were revealed between the soil mineral N accumulated by the ion-exchange resin capsule and the N content, total N uptake by rice plant as well as grain yield. It is obvious that the soil mineral N extracted by resin capsules can properly characterize the soil N-supplying capacity. Further-more, it is implied that the effects of water management pattern and N fertilization level on N uptake and utilization efficiency may be caused by their direct effects on soil N-supplying capacity.(4) The effects of water management patterns and N fertilizer levels on the soil microbial community structure were investigated in a pot experiment. Phospholipid fatty acid (PLFA) analysis was conducted to track the dynamics of soil microbial communities at the tillering, grain-filling, and maturity stages. The results showed that, soil microbial biomass (measured as total PLFAs), bacterial and fungal PLFAs were observed as an increase from the tillering to the grain-filling stage and a decrease at the maturity stage except for actinomycetic PLFAs, which decreased continuously from the tillering to the maturity stage. Soil microbial biomass was significantly higher under WCI than that under WLI mainly at the grain-filling stage, whereas the fungal PLFAs detected under WCI were significantly higher than those under WLI at the tillering, grain-filling, and maturity stages. The application of N fertilizer also significantly increased soil microbial biomass and the main microbial groups under WLI and WCI conditions. However, there were no significant differences among the three N levels, particularly between N2and N3(75%of N3) treatments, which indicated that there were no dose-dependent effects at the N levels. There was no evident effect if excessive N fertilizer was applied for the increase of soil microbial biomass.In conclusion, the N2(157.5kg N ha-1) treatment in combination with WCI (water-controlled irrigation) is the optimal irrigation and nitrogen application regime, considering getting higher rice yield and NUE, reducing the N loss, as well as economical water and N use.