Modeling The Critical Nitrogen Concentration,Root Distribution Of Winter Wheat And Its Eco-efficiency In North China Plain

The winter wheat-summer maize rotation is the main cropping system in the North China Plain (NCP). While its productivity has been constantly increased, the sustainability of the double cropping system has been increasingly questioned for its high resource input. Future development of sustainable agricultural systems relies on improved understanding of the dynamic processes in the soil-plant-climate system to develop cropping systems and management strategies for both increasing productivity and reducing the negative impact on the environment. In recent years, the farming systems model APSIM has been intensively used as an effective tool to analyse the yield and resource use efficiency of the wheat-maize system in NCP for the purpose of optimizing management practices like irrigation and N applications. Based on field experiment data at Wuqiao site, this study focused on improved modelling of the wheat-maize double cropping rotation system in the NCP with APSIM version7.5. This includes improvement in modelling crop physiological processes (dynamics of crop biomass, nitrogen uptake and root distribution), model calibration and testing, as well as use of the improved model to explore best management practices that maintain or enhance productivity while reducing the negative impact on the environment (e.g. reducing N leaching, non-productive water loss etc). Main results of the study were as follows:1. In general, APSIM version7.5simulated above-ground biomass and grain yield well with the observations. Across all the experiments where data were used for model testing, APSIM model was able to explain more than88%of the variations in crop above-ground biomass with RMSE of1.1, and more than83%of the variations in grain yield with RMSE of0.73.2. APSIM version7.5underestimated the rooting front advance and final rooting depth of winter wheat, but overestimated the root biomass and root-shoot ratio at maturity by100-200%. Modifications to root growth parameters in the model led to improved simulations of root depth and biomass dynamics. The modifications had little impact on simulated shoot biomass and grain yield under conditions of sufficient nitrogen supply, but led to higher simulated grain yield when nitrogen was deficient. They also resulted in reduction in simulated soil organic carbon in the top (0-20cm) soil layer by0.02%.3. This study provides data from NCP to re-exam the Ncc-stage relationships used in two widely used wheat models and to compare the Ncc-biomass vs. Ncc-stage relationships. The results revealed significant higher maximum and critical N concentrations in leaves of wheat in NCP than the values used in the APSIM-wheat model. Recalibration of the APSIM model with the new N concentrations led to improved simulations for wheat biomass and N uptake, particularly under low N input. Our results also show that the Ncc-stage relationship appeared to be more robust than the Ncc-biomass relationship, and it helped explain the variations in wheat Ncc-biomass curves from different regions. This likely reflects the fact that Ncc-stage curve captures the stage-driven formation of structural biomass and carbohydrate reserves of wheat, which is the main cause for N dilution. The implications of the findings on modelling of wheat-nitrogen relationships and on nitrogen management practices are also discussed. 4. APSIM version7.5overestimated early development and growth of winter wheat and underestimated the yield of maize. Changes were made to APSIM for temperature responses of phenology and RUE based on data from field experiments and literature. These modifications improved APSIM performances on modelling dynamics of crop biomass and grain yield. Combined field data with APSIM scenario modeling, results show that180kg/ha/year of nitrogen could be considered as the optimal N rate for summer maize. For wheat-maize double cropping system,225mm of irrigation water for wheat and330kg/ha (150for wheat and180for maize) of nitrogen are required to maintain the potential productivity of grain yield at18t/ha and to have minimum impact on environment in terms of N leaching to the groundwater and N loss due to denitrification. At such management level, the simulated annual totals of drainage, N leaching and denitrification N loss were117mm,12kgN/ha and28kg N/ha respectively.