Nitrogen Cycling In Rice-Wheat Rotation System Under Optimized Nitrogen Management

Excessive nitrogen fertilization generally exists in rice-wheat rotation system in middle and lower reaches of Yangtze river, which cause not only the decrease of nitrogen use efficiency but also the increasing of the risk of environmental pollution. Therefore, identifying the N cycling processes in field scale as well as in regional scale has become an essential need for optimizing N application and improving the N use efficiency in agricultural soils in middle and lower reaches of Yangtze river. To gain better insight into the nitrogen cycling in rice-wheat rotation system under optimized nitrogen management in middle and lower reaches of Yangtze river, field plot trials combined with micro-plot experiment with 15N tracing were established in Hubei province from 2007 to 2008 to monitor ammonia volatilization (AV), denitrification and N₂O emission, NO₃--N accumulation and 15N fate under different nitrogen application strategies. For both field trail and micro-plot experiment with 15N tracing, four N treatments included: (1) conventional N application (225 and 210kgN·ha^-1 for winter wheat and rice, respectively), (2) reduced N application (157.5 and 147kgN·ha-1 for winter wheat and rice, respectively), (3) optimized N application (increase ratios of dress N to base N fertilizer and split application based on crop N uptake at different growth stage, same to N rate in reduced N application), and(4) control with no nitrogen applied. For the treatments of conventional and reduced N application, 1/2 of nitrogen fertilizer applied as basal and 1/2 as topdressing at elongating stage for winter wheat, and 1/2 as basal and 1/2 as topdressing at tillering stage for rice. For optimized N application, 1/3 nitrogen fertilizer applied as basal, and 1/3 as topdressing at elongating stage and 1/3 at ear bearing stage for winter wheat, and 1/3 as basal, and 1/3 as topdressing at tillering stage and 1/3 at ear bearing stage for rice. A continuous airflow enclosure method was adopted to determine the ammonia volatilization (AV) loss, N₂O emission was analyzed using the static chamber-gas chromatograph method, and soil NO₃^--N and NH₄⁺-N was determined by flow injection analysis. Based on above observation, the reliability and sensitivity of Denitrification-Decomposition simulation model (DNDC) for forecasting nitrogen cycling and balance were tested in field scale, and influencing factors of various N cycling pathways were identified. Meanwhile, Qianjiang city of Hubei province was taken as the study area, DNDC coupled with geographic information system (GIS) was used to estimate the nitrogen cycling and balance in rice-wheat rotation system in regional scale. The main findings obtained are summed up as follows:1. Field plot trials showed that grain yield and N uptake for wheat and rice in the treatment of optimized N application were slightly higher than those under conventional N application, there is no significant difference within three N fertilizer application treatments, except no N fertilizer application. Compared to conventional N fertilization, the apparent N recovery under optimized N fertilization improved by 12.46% and 7.96% for wheat and rice, respectively. During whole growing season of wheat and rice, the apparent loss of N in the treatments of conventional, reduced and optimized N application were 179.7, 115.2 and 80.8kgN/ha respectively, accounting for 41.3%, 37.8% and 26.5% of the total N fertilizer input.2. The AV loss generally increased with the urea application rate, and affected by the temperature and soil water condition. In wheat growing season, the AV from fertilizer N lasted for 7-10 days after fertilization; while lasted for 5-7 days in rice growing season. Majority of the AV loss occurred in the rice season, occupied by 74.08%-78.65% of total AV loss in the entire rotation system. Compared to conventional N fertilization, the AV loss reduced N 26.28kg/ha, the loss rate reduced 32.90% under optimizing N application, showing that optimized N application is a rational and practicable N fertilization mode for reducing AV in rice-wheat rotation system.3. There were significant seasonal variations of N₂O emission flux during rice-wheat growing season. Total N₂O emission generally increased with the N application rate, and majority of N₂O emission occurred during the wheat season. In wheat growing season, N₂O emission varied from 2.43 to 4.84 kgN₂O/ha which accounted for 0.54% to 0.74% of applied N; While in rice growing season, N₂O emission varied from 0.89 to 2.45 kgN₂O/ha which ranged from 0.39% to 0.47% of applied N. Compared to conventional N application, N₂O emission loss rate reduced 23.87%, under the optimizing N application, indicating that optimizing N application associated with increasing ratios of dress N to base N fertilizer and times of split application based on plant N requirement at different growth stage could obviously reduce the N₂O emission under the rice-wheat rotation system.4. Micro-plot experiment with 15N tracing showed that N recovery for wheat and rice under optimizing N application was improved by 6.19% and 3.07%, respectively, the residual rate in soil increased by 4.76% and 2.49% in wheat and rice season respectively, the loss rate of fertilizer 15N decreased by 10.95% and 5.57%, respectively, in comparison with conventional N application. Cumulative AV amounts from fertilizer 15N under optimizing N application reduced by 41.27% and 54.04%, and denitrification losses reduced by 35.18% and 35.29% during wheat and rice growing seasons, respectively, compared to conventional N application.5. In field scale, a higher reliability was found between soil ammonia volatilization and N₂O emission simulated by DNDC model and field measured results, suggesting that the DNDC model could be used for describing N loss in soils under different nitrogen fertilizer management measures. According to sensitivity test on the DNDC model, both grain yield and crop N uptake were significantly affected by temperature and N application rate, AV was significantly influenced by fertilizer N type and the nitrogen application rate, N₂O emission significantly affected by temperature, soil pH and soil organic carbon.6. In regional scale, the regional N surpluses were 896.84 and 620.08t under the conventional N application and optimizing N application respectively. Compared to the conventional N application, AV, the nitrate leaching, and nitrification and denitrification loss reduced by 80.7%, 38.80% and 62.51% under optimizing N application respectively. The spatial heterogeneity in soil nutrients and environmental parameters result in the spatial variance of regional N outputs.