Major Nitrogen Loss Pathways In Citrus Orchard And Its Influence On Water Pollution In Agricultural Small Catchment

Citrus is one of the main sources of vitamins and dietary fiber for humans,and also one of the world’s major fruits.With the expansion of the citrus industry,excessive application of nitrogen fertilizer in orchards has become a global problem.Although the application of nitrogen fertilizer can increase yield,nitrogen loss through leaching,runoff,and gas emissions can damage air quality and increase the risk of nitrogen pollution in surrounding water bodies.China is the world’s largest producer of citrus,and 57.3%of traditional citrus orchards in the country suffer from excessive nitrogen application.In order to reduce the environmental risks caused by excessive application of nitrogen fertilizer,this study conducted a long-term location experiment in a citrus orchard in Dangyang,Hubei Province,and carried out a watershed monitoring experiment in the typical agricultural watershed where the orchard is located,quantifying the pathways and influencing factors of nitrogen loss in the orchard and deepening the understanding of the sources of nitrogen in the small watershed of the citrus orchard and its relationship with nitrogen loss in the orchard.Ten treatments were set up in this study,including four treatments that only applied chemical fertilizers（CK,NR,NP,CF）,two treatments that applied chemical and organic fertilizers（NRM,OM）,and four treatments that applied chemical and green manure（CKG,NRG,NPG,GM）.By continuously monitoring the nitrogen input and output pathways of CK,NR,NRM,and CF treatments,the nitrogen mass balance of the orchard was established,and the main nitrogen loss pathways and influencing factors of the orchard were quantified.At the same time,by continuously monitoring the runoff nitrogen loss and nitrogen component changes of different treatments,and combining the stable isotope technique of ¹⁵N-¹⁸O dual isotope labeling of nitrate,the characteristics,sources and influencing factors of nitrogen loss in the orchard runoff were quantified.In addition,by monitoring the changes in nitrogen components in runoff from different treatments during a single rainfall event,and combining the stable isotope technique of ¹⁵N-¹⁸O dual isotope labeling of nitrate,the changes in the sources and characteristics of nitrogen loss in the runoff during the rainfall process were clarified,providing a theoretical basis for the formulation of policies related to sustainable agricultural development and mitigation of non-point source pollution.The watershed experiment established a total of 34 lake sampling points,28 river sampling points,and 2 small watershed outlet sampling points in two citrus orchard agricultural small watersheds.Sampling was conducted on 34 lake sampling points before and after a single heavy rainfall event in 2019,and on 28 river sampling points during and outside of the rainy season in 2019.Continuous monitoring of 2 small watershed outlet sampling points was carried out for 100 days during the rainy season in 2019.The sources and influencing factors of nitrogen in the small watershed water were determined by combining the stable isotope technique of ¹⁵N-¹⁸O dual isotope labeling of nitrate and the three-dimensional fluorescence characteristic spectrum method with the changes in nitrogen in the small watershed water and nitrogen output.The main results of this study are as follows:（1）In citrus orchards of central China,leaching and runoff are the main pathways for nitrogen loss.By using a nitrogen mass balance table,we found that nitrogen loss caused by leaching accounts for 16.96%of the total nitrogen input,while nitrogen loss caused by runoff is 2.92%of the total nitrogen input.In addition,nitrogen loss caused by NH₃ volatilization and N₂O emission is 2.33%and 0.53%of the total nitrogen input,respectively.The nitrogen loss caused by leaching,runoff,and N₂O emission is significantly positively correlated with the soil NO₃^-content,while the nitrogen loss caused by NH₃ volatilization is significantly positively correlated with the soil NH₄⁺content.According to the fitting results of the SEM equation,we found that reducing nitrogen application rate can significantly reduce the content of NH₄⁺and NO₃^-in the soil,thereby reducing the nitrogen loss caused by leaching,runoff,NH₃ volatilization,and N₂O emission.In addition,increasing the application of organic fertilizer can significantly reduce the content of NO₃^-in the soil,thereby reducing the nitrogen loss caused by leaching and runoff.These findings are helpful in reducing agricultural pollution and providing important reference information for sustainable agriculture and pollution control policies.（2）In citrus agricultural watersheds,reducing nitrogen application rate significantly reduces nitrogen runoff loss.Replacing chemical fertilizers with green manure is an effective way to reduce nitrogen application rate.Green manure during the growth stage reduces nitrogen runoff loss,while directly covering the soil surface with green manure after harvest increases nitrogen runoff loss.In long-term runoff monitoring experiments,reducing nitrogen by 30%and 15%can respectively reduce total nitrogen runoff loss by 31±10%and 28±8%.Green manure during the growth stage reduces total nitrogen runoff loss by 15±11%,but directly covering the soil surface with green manure after harvest increases total nitrogen runoff loss by 21±2%.In addition,the results of the ¹⁵N-¹⁸O double isotope indicate that soil organic nitrogen is the main source of nitrogen loss in runoff.These findings provide valuable information for reducing agricultural pollution and developing sustainable agriculture and pollution control policies.（3）The results of the runoff monitoring experiment during the rainfall events indicate that with an increase in rainfall duration,the loss of nitrate and ammonium in the runoff significantly increases,while the loss of particulate organic nitrogen significantly decreases.In the treatment of single chemical fertilizer application and organic fertilizer application,the loss of soluble organic nitrogen in the runoff is positively correlated with the rainfall duration,while in the green manure treatment,the loss of soluble organic nitrogen in the runoff is negatively correlated with the rainfall duration.Planting green manure can significantly reduce the nitrogen loss in the runoff at each stage of rainfall.In addition,the experimental results of the nitrate 15N-18O double isotope indicate that the loss of nitrate in the early stage of rainfall runoff mainly comes from fertilizer nitrogen,while the loss of nitrate in the later stage of rainfall runoff mainly comes from soil organic nitrogen.The application of organic fertilizer and planting green manure can effectively reduce the nitrate loss in the runoff from fertilizer nitrogen.（4）Through the nitrogen source tracing experiment in the small watershed,we found that the main source of nitrogen in non-rainy season citrus agricultural small watershed water is fertilizer nitrogen,while the main source of nitrogen in rainy season citrus agricultural small watershed water is soil organic nitrogen.Therefore,increasing the planting area of citrus orchards will increase the concentration of various nitrogen components in citrus agricultural small watershed water,thus increasing the risk of nitrogen pollution in citrus small watershed water.（5）Long-term monitoring experiments of nitrogen runoff at the outlet of the small watershed show that the main source of nitrogen output in citrus agricultural small watersheds is soil organic nitrogen.Increasing the planting area of citrus orchards will increase the output of soil organic nitrogen in the small watershed.In conclusion,nitrogen loss in citrus orchards in central China mainly occurs through leaching and runoff.Soil NO₃^-content is a key factor affecting nitrogen loss through leaching and runoff.By reducing nitrogen application rates and increasing the application of organic fertilizers,soil NO₃^-content can be significantly reduced,thereby reducing nitrogen loss through leaching and runoff in citrus orchards.During the study period,nitrogen loss in runoff mainly came from soil organic nitrogen,particularly nitrate nitrogen.However,the use of green manure during the growing season can significantly reduce nitrogen loss in runoff during subsequent rainfall events.In addition,the proportion of citrus orchard area is a key factor affecting nitrogen pollution and output in agricultural small watersheds.Increasing the area of citrus orchards will increase nitrogen pollution in small watersheds,while reducing the proportion of citrus orchard area can significantly reduce soil organic nitrogen loss during rainfall events,thereby reducing nitrogen pollution and output in agricultural small watersheds.Therefore,studying the impact of rainfall-induced soil organic nitrogen loss on nitrogen pollution in citrus agricultural small watersheds in central China is of great significance.