Nitrogen Loss Estimation And Spatial Optimization Of Main Rice Cropping Systems In Hubei Province

Approximately 25% of the total annual nitrogen input to the Earth is attributed to agricultural fertilizer usage,but this practice also leads to significant nitrogen emissions into the environment,resulting in the degradation of water,air,and soil quality.The rice field accounts for a large proportion of the national sown area and showed a large N loss.Rice is the largest crop in Hubei Province,with a diverse range of rice cultivation systems.Therefore,quantifying nitrogen loss in rice fields,understanding influencing factors,and taking measures to reduce nitrogen loss are of great significance in promoting sustainable agricultural development.In this study,the main rice cropping systems in Hubei Province were taken as the research object,for example rice-fallow system,rice-crayfish system,rice-rapeseed system and rice-wheat system,then the Hubei Province map is divided it into a 10km×10km grid map,the grid was used as the minimum research unit,and the DNDC model was used to estimate the nitrogen loss of distinct forms.Then,the spatial distribution characteristics and factors contributing to formation of distinct forms of nitrogen loss were studied.Finally,we explored the spatial optimal allocation scheme of rice cropping systems and calculated the environmental cost under different optimization scenarios.The main results show that:（1）Four rice cropping systems experienced the greatest nitrogen loss per unit area through NH₃ volatilization,followed by nitrogen leaching and runoff,and N₂O emission accounted for the smallest proportion.The proportions of the four forms of nitrogen loss account for approximately 80.90%～87.34%,3.93%～9.00%,5.12%～12.40%,and 0.16%～0.39%of the total nitrogen loss,respectively.Except for nitrogen runoff,other nitrogen loss forms under the two rice-upland systems were greater than that under the rice-fallow system.Additionally,N₂O emission and nitrogen leaching were significantly lower than the rice-fallow system,but nitrogen runoff and NH₃volatilization under the rice-crayfish system were greater than that of the rice-fallow system.（2）Different forms of nitrogen loss exhibited marked spatial clustering patterns in different cropping systems,with varying cold and hot spot distribution areas,and significant differences in emission values between the hot and cold spots.The emission of N₂O was significantly different between the two rice-upland rotation systems and that of single-season rice.The cold spots of N₂O emissions in single rice system were mainly distributed in western Hubei,but those of rice-upland rotation systems were distributed in Jianghan plain.The NH₃ emission in the rice-crayfish system was completely different from the other three systems,showing low emission in the west and high emission in the east areas.In the four cropping systems,the corresponding locations of cold hot spots of nitrogen leaching were basically the same,and the cold spots emitted in southeast and north regions of Hubei,and the hot spots emitted in northwest（south）and southwest（north）regions of Hubei.The nitrogen runoff showed low emissions in northern Hubei,and high emissions in southeastern and part of southwestern Hubei in all rice cropping systems.（3）The maximum clay content and minimum bulk density were the most important factors affecting N₂O emissions,while in the rice-upland system,the lowest p H value was the most influential.The main factors influencing NH₃volatilization were the maximum clay content and the minimum bulk density in rice-upland system,while in the single rice system,the distribution area and temperature are the most influential.The factors affecting the amount of nitrogen leaching include bulk density,clay content,and soil organic carbon content,and the explanatory power of these factors varies under different rice cultivation systems.The factors influencing nitrogen runoff were the same in all four rice cultivation systems,which were precipitation,precipitation intensity and zoning.（4）When considering environmental impact,it is recommended to implement the rice-fallow system in southeastern and central Hubei,the rice-crayfish system in western Hubei,the rice-rapeseed system in northeastern Hubei,and the rice-wheat system in northwestern Hubei（northern area）,the Jianghan Plain,and the eastern Jianghan Plain.Under the goal of maximizing yield,it is recommended to implement the rice-fallow in central Hubei,the rice-crayfish system in southeastern Hubei,the rice-rapeseed system in the central area of western and northeastern Hubei and the Jianghan Plain,and the rice-wheat system in the northern central area of Hubei.（5）It is found that the scenario that only considers maximizing yield has the highest environmental cost,and lowest environmental cost is achieved by considering both environmental impact and yield.The study found a 1,300 yuan difference in average environmental cost per unit area between the two scenarios,resulting in a total 3 billion yuan difference in environmental cost for rice paddies in the study area.The nitrogen loss pathway of NH₃ volatilization needs to be focused on in the rice planting system,and measures to alleviate NH₃ emissions,such as optimizing fertilizer management,should be actively promoted.Nitrogen loss exhibits significant spatial clustering,so reducing nitrogen loss should be based on larger areas as management units,and focus on nitrogen loss hotspots.The distribution of cold and hot spot emissions of different nitrogen loss forms varies under different rice planting systems.Spatial optimization methods have great potential to mitigate the environmental impact of the rice planting system.In actual production,spatial optimization configuration and recommended emission reduction measures should not only pursue economic benefits but also focus on ecological benefits.