Nitrogen Cycling And Balance In Winter Wheat-Summer Corn Rotation System Under Optimized Nitrogen Management

Due to huge food demand, increasing crop yield by fertilization and reducing negative environmental impacts from fertilization is urgent for sustaining agricultural development in China, especially in the north China plain(NCP). Excessive N application in a winter wheat-summer corn rotation system in the NCP has been shown to led to the low N use efficiency and to increase the risk of nitrate-N leaching to groundwater. Up to date, much research work has been done in the field of ammonia volatilization, denitrification, and NO₃^- movements, but generally with only one or a few of components concerned in one experiment with different N application rates, the field measurement of the N cycling processes under optimized N application has become an essential need in the general requirement to improve the N use efficiency in agricultural soils.To gain better insight into the nitrogen cycling and balance under winter wheat-summer corn rotation system in NCP, field plot trials combined with micro-plot experiment with ¹⁵N tracing were established in Hengshui region (Hebei 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 (300 and 240 kg N ha-1 for winter wheat and summer corn, respectively), (2) reduced N application (210 and 168 kg N ha-1 for winter wheat and summer corn, 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 1/3 as basal and 2/3 as topdressing in the bell-mouthed period for summer corn. For optimized N application, 1/3 nitrogen fertilizer applied as basal, and 1/3 as topdressing at elongating stage and 1/3 at booting stage for winter wheat, and 1/5 as basal, and 3/5 as topdressing in the bell-mouthed period and 1/5 at silking stage for summer corn. After each N application, volatile NH3 and N₂O gas were collected and determined immediately, and soil were sampled for NO₃^--N and NH₄⁺-N determination. An enclosed intermittent vent method was adopted to determine AV, denitrification and N₂O emission from soil was measured using acetylene inhibition - intact soil core incubation technique, and soil NO₃^--N and NH₄⁺-N was determined by flow injection analysis. Meanwhile, dry and wet atmosphere deposition of nitrogen in the experimental site was measured as well. Based on above observation, the reliability and sensitivity of DeNitrification-DeComposition simulation model (DNDC) for forecasting nitrogen cycling and balance were tested, and influencing factors of various N cycling pathways were identified. The main findings obtained are summed up as follows:1. Field plot trials showed that grain yield and N uptake for winter wheat and summer corn 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 percentage points14.3 and 15.2 for winter wheat and summer corn, respectively. During whole growing season of winter wheat and summer corn, the apparent loss of N in the treatments of conventional, reduced and optimized N application were 177.6, 102.9 and 58.4kg N/hm² respectively, accounting for 32.8%, 27.2% and 15.4% of the total N fertilizer input.2. AV from soil in both winter wheat and summer corn season occurred mainly within 14 days after fertilization, and the cumulative AV amount from fertilizer N in winter wheat season was found to be less than that in summer corn season. Cumulative AV amounts from soil of winter wheat-summer corn rotation system were observed to be reduced with decreasing of N application rate. Total AV amounts under conventional N fertilization was 2.28 and 2.03 times as high as that under optimized N fertilization, showing that optimized N application is a rational and practicable N fertilization mode for reducing AV in winter wheat-summer corn rotation system.3. There were significant seasonal variations of denitrification rate and N₂O emission flux during winter wheat-summer corn growing seasons, each having a positive correlation with soil water content and inorganic nitrogen concentration (NH₄⁺-N, NO₃^--N). Total denitrification losses and N₂O emission amounts from soil in winter wheat-summer corn rotation system were observed to be reduced with decreasing of N application rate, and which in winter wheat season were found to be less than those in summer corn season. Total denitrification losses and N₂O emission amounts under conventional N fertilization were 1.62 and 1.67 times respectively as high as those under optimized N fertilization during winter wheat growing season, and 2.01 and 2.00 times during summer corn growing season, optimizing N application could reduce denitrification losses and N₂O emission through modifying soil inorganic nitrogen concentration and its temporal distribution.4. NO₃^--N content below the 60 cm soil layer under conventional N application was significantly enhanced due to excessive N application, showing that most of nitrogen not utilized by crops under conventional N application was at the risk of leaching into deep soil. Optimizing N application improved cumulative amounts of NO₃^--N in 0-20cm soil layer at crop maturity, but reduced those in the 20-100cm soil layer, which led to the apparent N loss to be the lowest under optimizing N application.5. Micro-plot experiment with ¹⁵N tracing showed that N recovery for winter wheat and summer corn under optimizing N application was improved by percentage points 16.7 and 17.3, respectively, The residual rate in soil increased by percentage points 3.1 and 4.7, respectively, the loss rate of fertilizer ¹⁵N decreased by percentage points 19.9 and 22.0, respectively, in comparison with conventional N application. Cumulative AV amounts from fertilizer ¹⁵N under optimizing N application reduced by 48.4% and 51.9%, and denitrification losses reduced by 40.9% and 58.1% during winter wheat and summer corn growing seasons, respectively, compared to conventional N application.6. 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 in Hengshui under different nitrogen fertilizer management measures. According to sensitivity test on the DNDC model, soil ammonia volatilization, N₂O emission and NO₃^--N leaching were observed to be sensitive to the changes of the amount and time of N application, soil initial inorganic nitrogen and soil texture, in which the amount of N application was the most sensitive factor, while other factors except no N application were insensitive to plant N uptake.In conclusion, optimizing N application associated with increasing ratios of dress N to base N fertilizer and split application based on crop N uptake at different growth stage was a satisfactory pattern of N fertilizer application, which could enhance plant N uptake, improve N fertilizer use efficiency, reduce ammonia volatilization and denitrification, decrease residual N amounts in soils, and gain a higher grain yield as well. These results suggested that 30% of nitrogen fertilizer could be saved by optimized N fertilization.