| China propogates "one village,one product" characteristic agriculture to help alleviate poverty and revitalize rural areas.The planting area of cash crops is rapidly increasing.The development of sloping orchards,high-density single-variety planting and high-intensity fertilization have caused ecological and environmental issues such as soil degradation,water pollution,and eutrophication,threatening the sustainable development of these regions.However,the current understanding of the sources,migration and transformation of non-point source pollution in sloping agricultural watersheds is still very limited,especially the lack of systematic understanding in the nitrogen export processes,fluxes and mechanisms the soil-groundwater-river continuum.In this study,a typical subtropical agricultural small watershed-the Pomelo Subwatershed(PSW)in Pinghe County,Fujian Province,was investigated.By establishing a soil-groundwater-river continuum observation system in the watershed,we carried out research plans focusing on:(1)the sources and temporal and spatial distribution of N nutirents in PSW,(2)seasonal variations of soil water-groundwaterriver system,and(3)the export processes and fluxes of N from PSW during flood and base flow periods.The main results and research conclusions are as follows:(1)Orchard fertilization is the main source of nitrogen nutrient in PSW,and orchard soil and groundwater are important nitrogen reserves.The annual total nitrogen(TN)input in the small watershed reached 814 kgN ha-1 a-1,of which 98.4%of nitrogen fertilizers were applied to orchards.Orchard soil is an important place for nitrogen nutrient storage,transformation and migration.Biochemical transformation takes place in the surface soil with the application of nitrogen fertilizer,most of which is converted into organic nitrogen stored in the soil organic matter.The estimated TN storage in surface soil is 3.5 times of the annual TN input in the small watershed.Dissolved inorganic nitrogen(DIN,including ammonia nitrogen(NH4-N)and nitrate nitrogen(NO3-N))that can be directly utilized by plants accounts for only 18%.As the soil depth increases,the content and proportion of NH4-N and NO3-N also change accordingly.The proportion of NO3-N/DIN increased significantly,from 56%in the surface soil to more than 90%at the soil-groundwater interface(the highest observed concentration reached 91 mg L-1),NH4-N is almost completely converted or adsorbed during the longitudinal migration process.The 15N and 18O of nitrate characteristics of soil water at different depths indicate that there exists denitrification process in deep soil,which reduces the nitrogen load of groundwater to a certain extent.However,due to the high amount of fertilization,average NO3-N concentration at the orchard soilgroundwater interface is still up to 35.9 mg L-1,which is an important NO3-N storage in addition to soil.Groundwater excretion is an important channel for nitrogen output during the base flow period of PSW.(2)Land use type,fertilization cycle,hydrological and meteorological conditions are the main factors forthe nitrogen temporal and spatial changes of soil-groundwater-river system in PSW.The concentration of NO3-N and NH4-N in soil water is affected by nitrogen input,migration and biological transformation.The concentration during the coupling period of fertilization and precipitation is significantly higher than that during the non-coupling period.The concentration of NO2-N in the humid period was significantly higher than that in the dry period,reflecting the active soil microbial nitrogen conversion during the humid period.The drought-humid hydrological cycle inhibits/activates soil microbial activities,and extreme weather can accelerates the nitrogen output from orchard soil to rivers and groundwater.The nitrogen concentration in shallow groundwater increases significantly when the fertilization period and long-term rainfall coincide.The changes in the nitrogen concentration and form in the mainstream and tributary are mainly affected by the agricultural non-point source pollution load,the inflow of tributaries and the discharge of domestic sewage.Groundwater recharge during the base flow period controls nitrogen components change of river.(3)The export flux of NO3-N in rivers is mainly controlled by soil moisture,fertilization practice and runoff.With little seasonal variation in NO3-N concentrations,NO3-N export flux is mainly controlled by total runoff,accounting for more than 90%of DTN export in rivers.During months with high total runoff(>106 m3),NO3-N export flux per unit of total runoff would be higher or lower,which is mainly controlled by orchard fertilization activities and soil moisture conditions.The high-flow flood period is key window for NO3-N output in PSW(with 10%duration and 38%NO3-N export flux).During heavy rains,a large amount of NH4-N and NO3N in the surface soil were washed off,and laterally transported to surface runoff in the form of soil interflow,causing river water N nutrient output pulses,especially during extremely high flows(>9.6 m3 s-1)which significantly enhanced the enrichment of NO3-N.The low flow period(<2.48 m3 s-1),which lasted for 69%of the time,exported 34%of the NO3-N load,which is also a key controlling period for NO3-N export.The multiple linear regression analysis of the NO3-N export parameters and the meteorological and hydrological parameters of multiple rainstorms shows that"maximum precipitation intensity","precipitation duration","cumulative precipitation 1 day before the flood event" and "average temperature of the 14 days before the flood event" are the main driving factor of river NO3-N concentrations and export fluxes during storm events.In summary,the source,spatio-temporal changes and export mechanism of nitrogen in the soil-groundwater-river continuum of PSW were prelimilarily explained,as well as the relative contribution of baseflow and stormflow to river nitrogen export.In order to effectively control the nitrogen pollution of rivers and groundwater,in addition to the traditional fertilizer reduction methods,it is necessary to focus on:(1)Establish a local weather forecast system and a soil-groundwater-river hydrological nutrient intelligent monitoring system to serve the refined management of fertilizer and water,increase the residence time of fertilizers in the soil,and thus reduce the loss of nutrients due to the coupling of fertilization and precipitation.(2)In view of the importance of storm runoff and shortage of local land,it is recommended to build ecological engineering facilities with less area,such as artificial pond systems,for interception,biological treatment and recycling of storm runoff. |
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