Modelling Nitrogen And Phosphorous Balances In Agro-ecosystem In The North China Plain

This study combined experimental data analysis and systems modeling to investigate the crop productivity and nitrogen and phosphorous balances in the wheat-maize double cropping system in the North China Plain (NCP), and their responses to different management practices under the background of variable climate. Experimental data used include: 15 years of crop and soil data from Zhengzhou station in Henan province under the National Soil Fertility and Fertilizer Efficiency Long-term Monitoring Network, 5 years of data from a long-term fertilization experiments at Quzhou, Hebei province, and 2 years of data from Yucheng, Shandong province. The main findings are:(1) The Agricultural Production Systems Model (APSIM) was calibrated and validated using long-term and short-term experimental data and the performance of APSIM model was evaluated by its ability to predict: (1) biomass, grain yield, and N/P uptakes of both wheat and maize crop, (2) soil nitrogen and phosphorus dynamics, and (3) soil carbon dynamics, in response to different N/P application rates. The results showed that APSIM is able to capture the response of crop growth, yield and N/P uptakes to different fertilizer application levels. The determination values (r²) were all above 0.73 between simulation and measured data for wheat biomass and yield, r² also were above 0.61 and 0.81 for that of maize biomass and yield respectively. It also simulated the soil C, N and P changes with acceptable accuracy, their r were 0.70, 0.78 and 0.64 for total N, soil organic carbon and labile phosphrous in 0-20cm soil, respectively.Therefore, it can be used to conduct scenario analysis for different fertilization treatments.(2) Analysis of the long-term data at Zhengzhou showed that both N and P were limiting factors for crop growth. Without additional N and P fertilisation,only a very low yield level (ca 2t ha^-1 for wheat and 3 t ha^-1 for maize) could be maintained. To achieve the potential productivity (i.e. yield level free of water and nutrient stresses) of wheat (6.9 t ha^-1) and maize (8.3 t ha^-1), wheat would need, on average,170 kg N ha^-1and 32 kg P ha^-1, while maize would need 189 kg N ha^-1,34 kg P ha^-1.There was still a gaps of 0.7 t ha^-1 and 1.6 t ha^-1 for wheat and maize which can be increased in under the high fertilizer treatment depending on the improvement of management. On average the AE and REN of NPK treatment could be 4.26 and 11.2 times than PK treatment and the highest among all treatments. Treatments with nutrient inputs higher than the NPK treatment and treatments without combination of N and P have led to accumulation of N and P in the soil profile. Returning wheat straw to field the AE and RE would be significantly increased for maize.(3) At Zhengzhou, long-term (1961-2007) simulation results revealed that the optimal nitrogen application rates for wheat and maize are 180kgNhm-2 and 150kgNhm-2 respectively, above those change point yield did not increase anymore with increased N applications. These rates are significantly lower than the actual N application rates in many part of NCP. The probability of exceedance with 6t and 7t of yields of wheat and maize were 75% and 46% respectively applied 150 kgNha^-1fertilizer and the probability of exceedance with 6t yield of maize were 81% applied 120 kgNha^-1fertilizer.Furthermore, the simulation results also been showed that under the situation of returning all of straw into field the mineralization would be increased with more nitrogen fertilizer input. With the leavel of 150 kgNha^-1 nitrogen, the mineralization amount were 80^-132 kgNha^-1 with 75%～25% probability of exceedance. According to the simulation results, the yield, nitrogen and phosphrous uptake and mineralization could be variable and be changed with the varies climate.(4) Simulation results also showed that if N application rates were below the optimal rates, little N leaching would occur below the crop root zone. However,if N application rate was higher than the optimal rates,nitrate N starts to accumulate in the soil profile and significant leaching will occur subsequently. N leaching mainly occurred in the maize season due to the concentrated summer monsoon rainfall. N leaching events were significantly related to rainfall events. Combined 150kgNha^-1 and 40 kgPha^-1 for each crop were optimal fertilizer levels due to the balances of leaching risk and crop yiled. N leaching events were significantly related to rainfall events (correlation coefficient r = 0.65) although the farm was irrigated enough.(5) The APSIM model was also appled to simulate the wheat-maize cropping system in 298 counties of NCP. The correlation coefficient between the statistical and simulated yield were 0.74 and 0.62 for wheat and maize respecively. Under the current nitrogen application rate, the total nitrogen content was in 1755 8624 kgNha^-1, average 2417 kgNha^-1 in 0-40cm soil layers, NO3-N accumulation was 11 1839 kgNha^-1, average 214 kgNha^-1 in 0^-160cm soil. In NCP 60% of the areas with NO3 - N accumulation were more than100 kgNha^-1. Simulation results also showed that Unavailable-P content averaged 1193 kgP₂O₅ha^-1 in 0 - 40cm soil layer. The high labile P content was distributed in northern Anhui province, north-east of Henan province with high risk of environment.