Spatiotemporal Evolution Of Nitrogen Cycle And Grey Water Footprint Of Winter Wheat-summer Maize Rotation Fields In Hebei-Shandong-Henan Region

The widespread use of nitrogen fertilizer in agriculture poses a great threat to freshwater ecosystems,and the emissions of N₂O and NH₃ during this process can also cause greenhouse effects and harm human health.Grey water footprint（GWF）of crop production can evaluate the degree of pollution to water environment from the perspective of water quantity.However,most of the quantification of GWF in crop production sets the leaching-runoff rate of fertilizer as a fixed value,ignoring the effects of weather,soil,and agricultural measures.In addition,irrigation is usually the preferred strategy for farmers and policy makers to reduce risks and stabilize and increase crop yields.The environmental pressure of different irrigation techniques needs to be further explored.Therefore,this study takes the winter wheat-summer maize rotation system in the JLY region（Hebei,Shandong,and Henan provinces）from 2004 to 2020 as the research object,simulates the nitrogen cycling process of farmland during the crop growth period under different irrigation methods based on the DNDC model,and calculates the annual unit grey water footprint（UGWF）and total crop production grey water footprint（TGWF）of two crops at the city level,and analyzes their spatiotemporal evolution characteristics.The main research results are as follows:（1）The average nitrogen leaching-runoff rates of winter wheat and summer maize in the JLY region from 2004 to 2020 are about 8%and 26%,respectively,with inter-annual standard deviations of 2%and 6%,respectively.There are significant spatial differences in farmland nitrogen leaching-runoff rates of both crops in the same year,with high values in the southern part of Henan and the eastern part of Shandong.The nitrogen leaching rate in winter wheat fields is more sensitive to different irrigation methods than in summer maize fields,and the nitrogen leaching-runoff rate under underground drip irrigation is the lowest（7%）,which is 86%of the nitrogen leaching rate under surface irrigation conditions.（2）From 2004 to 2020,the N₂O emissions during the growth period of winter wheat and summer maize in the JLY region were approximately 3.32 kg/hm² and 0.87 kg/hm^2,respectively.The N₂O emissions during the summer maize growth period in Henan and Hebei provinces were higher than those during the wheat growth period,but the opposite was true in Hebei.In addition,there were significant spatial differences in N₂O emissions,with the high-value areas for winter wheat N₂O emissions located in the southern part of Henan,the northern part of Hebei,and the southern part of Shandong,while those for maize were concentrated in the southern part of Henan and the central-southern part of Shandong.During the growth period of both wheat and maize,underground drip irrigation resulted in the highest N₂O emissions,with the N₂O emissions from underground drip irrigation during the growth period of wheat and maize in Henan in 2020 being 24%and 2%higher,respectively,than those from surface irrigation.（3）During the period of 2004 to 2020,the NH₃ emissions during the growth period of winter wheat and summer maize in the JLY region showed a significant decreasing trend,with an overall decrease of approximately 64%.Winter wheat exhibited higher NH₃emissions during its growth period compared to summer maize,and there were significant spatial differences in NH₃ emissions between the two crops.The high NH₃ emission areas for winter wheat and summer maize were mainly located in the central and northern parts of Hebei Province.The study suggests that changes in irrigation methods have a relatively small impact on NH₃ emissions during the growth periods of winter wheat and summer maize.（4）The multi-year average of UGWF for winter wheat and summer maize were approximately 3513.88 m³/t and 9568.53 m³/t,respectively.Furthermore,high UGWF values were observed in regions such as southern Henan,northern Hebei,and central Shandong for both crops.The UGWF of wheat under the four irrigation methods mostly showed surface irrigation>sprinkler irrigation>surface drip irrigation>subsurface drip irrigation,and the UGWF of wheat under subsurface drip irrigation in Hebei,Shandong and Henan was 89%,88%and 84%of that under surface irrigation,respectively.Among the four irrigation methods,the UGWF for summer maize was highest under subsurface drip irrigation and lowest under surface irrigation,with a difference of about 1%.（5）In terms of TGWF,the multi-year average values of winter wheat and summer maize during their growth stages in the JLY region were 294.85×10⁹m³ and 596.80×10⁹m³,respectively.The TGWF of winter wheat and summer maize in 2020 increased by 75%and95%,respectively,compared to that in 2004.The high-value areas of winter wheat were mainly distributed in southern Henan,while summer maize was relatively scattered.The time and spatial heterogeneity of winter wheat and summer maize TGWF under the four irrigation methods were observed.Summer maize fertility was higher in TGWF under water-saving irrigation measures than surface irrigation.In summary,nitrogen leaching,N₂O emissions,NH₃ emissions,UGWF,and TGWF exhibit significant spatial variations,highlighting the need to enhance agricultural management and reduce inter-regional differences in agricultural production levels.Additionally,different crops and irrigation methods respond differently to atmospheric and water environmental impacts during crop production.Therefore,future research should comprehensively consider water resources and their impact on the environment,to provide methodological and data references for achieving the dual carbon goals of agriculture and implementing the strictest water resources management system.