Nitrogen Loss And The Key Process Of Nitrous Oxide In Tuojia Watershed

Human activities strongly interfere with the nitrogen cycling processes in agricultural watersheds,causing a series of ecological and environmental problems such as watershed nitrogen loss,emission of nitrous oxide（N₂O）from soil,nitrate pollution and emission of N₂O in water body.Currently,there are few studies that comprehensively consider nitrogen cycling processes at the watershed scale,and the mechanism of nitrogen cycle processes in the watershed is still unclear.This study takes the Tuojia River Watershed in a typical subtropical region as the research object.Through stable isotope technology,nitrogen-oxygen dual labeling culture experiments,headspace equilibrium-dual layer diffusion models,long-term field monitoring is conducted to analyze the spatiotemporal changes in N₂O emissions from rivers,identify the main influencing factors and production processes,quantify the sources of soil organic matter loss in the watershed,trace to the key pathways of soil N₂O production based on oxygen exchange,and establish a research system for the generation mechanism and reduction pathway of nitrous oxide in the watershed.The main research results are as follows:（1）Spatio-temporal variation and main controlling factors of N₂O emissions in the watershed river.The average dissolved concentration,saturation,diffusion flux,and annual cumulative emission of N₂O were 0.06±0.003μmol L^-1,699.47%±41.16%,16.53±1.61μg N m^-2 h^-1,1.45kg N ha^-1 yr^-1,respectively,and the emission factor was 0.28%±0.02%.The spatial variation of N₂O emissions was not significant,but there were differences between years and seasonal variations.The wind speed was the most important driving factor for N₂O diffusion flux,while dissolved oxygen（DO）had a negative effect on it.pH,river reaches and seasons indirectly affected it by influencing other environmental factors（such as DOC and DO）.Denitrification might be the main source of N₂O in the Tuojia River,accounting for about 60%of all N₂O.（2）Traceability of nitrate nitrogen in the watershed river.The concentrations of NH₄⁺-N,NO₃^--N and NO₂^--N in the Tuojia River water body were 1.37,1.28,and 0.12 mg L^-1,respectively;and their distribution characteristics showed significant temporal and spatial features,with NH₄⁺-N concentration at S1 being significantly higher than at S3 and S4,NO₃^--N being significantly higher in autumn and winter than in spring and summer,and NO₂^--N being significantly higher in winter than in other seasons.The sources of nitrate varied among different river reaches:S1 mainly came from atmospheric nitrogen deposition and soil organic nitrogen;S2 and S3 were ammonium nitrogen fertilizer,human and animal manure,domestic sewage and atmospheric nitrogen deposition;S4 was human and animal manure and domestic sewage.Inorganic nitrogen in water underwent significant nitrification and denitrification processes,with the degree of nitrification possibly higher in S1 and S2 than in S3 and S4.（3）Traceability of stable isotope fingerprints of sediment organic matter in the watershed.Significant spatial variations were observed in theδ¹³C,δ¹⁵N and C/N of sediments and their potential source soils.Theδ¹³C（-26.2‰to-23.8‰）andδ¹⁵N（3.9‰to 7.9‰）of sediments were within the range of source soils,andδ¹³C andδ¹⁵N values of five source soils（forest,paddy,vegetable,residential and tea）were ranging from-29.3‰to-20‰and-1.2‰to 9‰,respectively.The average C/N（11.1±0.52）of sediments was significantly higher than that of all source samples.The organic matter in sediments mainly originated from sewage,and residential soils were the main contributor to the loss of organic matter in the basin soils（75.8%）,followed by agricultural land（paddy and vegetable soil）,with a combined contribution of 12%.（4）Quantification of key production pathways of N₂O based on oxygen exchange.Oxygen exchange occurred during the N₂O production process in paddy,vegetable and forest soils,and the degree of occurrence in agricultural soils（average 76.89%）was higher than that in forest soil（41.39%）.Fertilizer denitrification was the main source pathway for N₂O;hydroxylamine oxidation contributed almost nothing to N₂O in the three types of soils.The proportion of N₂O originating from fertilizer denitrification in vegetable soil（51.13%）was higher than that from nitrifier denitrification（25.32%）and nitrification-coupled denitrification（23.55%）.Oxygen exchange could reduce the natural stable isotope abundance of ¹⁸O-N₂O in soil,resulting in an underestimation of the contribution of denitrification to N₂O.In conclusion,human life and agricultural production have an important impact on the nitrogen cycle in the Tuojia River Watershed.Improving rural living environment and optimizing agricultural nutrient management measures may play a key role in controlling nitrogen loss and N₂O emissions in subtropical agricultural watersheds.Meanwhile,when using ¹⁸O-N₂O to estimate the source of N₂O,oxygen exchange should be included in the correction of ¹⁸O-N₂O.