Mechanism Of Nitrogen Transformations At Water And Soil Interface In Karst Area

Soil is an important medium for nitrogen cycle.All of nitrogen sources,including natural and anthropological input,transformed in pedosphere and then transported to aquatic system.Karst area in southwest China is one of the typical fragile ecosystems,which constrained by the thin soil layers and nutrient infertility.Coupling to the extensive agriculture and climate,soil erosion is severe and the quality of water environment gradually deteriorated.Previous studies mainly focused on understanding nitrogen sources in aquatic system,whereas few studies concern how nitrogen dynamic in soil system.To further explore the mechanism of nitrogen transformations among water-soil interface and how much nitrogen loss in karst soil,the controlling experiments,which considered fertilization and rainfall events,were designed to know the nitrogen transformations in karst soil.Leaching water samples and soil samples were collected and analyzed the concentration of nitrogen species and dual nitrate isotopes.Comparing the loss N flux of urea controlling experiment（UCE）and cow manure controlling experiment（CMCE）,the spatial and temporal variation of nitrogen transformation and dual nitrate isotopes in three varied soil depth were studied.After that,Chishui River samples were collected to understand nitrate sources and its spatial and temporal variation,which is helpful to further know how karstic agricultural impacted on catchment N pollution.The results are as follow:（1）The average rates of nitrification during intensify rainfall events experiment were 12.6 mg?N?kg^-1?d^-1 for UCE and 1.4 mg?N?kg^-1?d^-1 for CMCE.The of the stored inorganic nitrogen in soil during UCE（48.7%）is higher than CMCE（7.4%）which major nitrogen species is organic nitrogen（74.3%）.The N leaching flux is more greater during UCE（22.2%）than CMCE（1.4%）.These results indicated that the hydrolysis and nitrification during UCE are faster than CMCE.However,the leaching process from soil system is more slowly during CMCE than UCM,which suggested the fertility persistance using cow manure ferterlizer is advantage than urea ferterlizer,except the high risk of microbial pollution.（2）The avarage rates of nitrification under dry condition at surface（0cm）,10 and30 cm soil depth during UCM are 18.6,12.5 and 8.8 g?m^-2?d^-1.The minimaδ¹⁵N value during dry condition UCM is-35.8‰on the second day,thenδ¹⁵N value gradually increased.（3）The comprehensive simulation experiments indicated that low intensive rainfall events with dry condition favor N loss by volatilization up to 41%,while N loss pathway during wet condition would change to leaching process.The whole isotopic fractionation during nitrogen transformation can up to 29‰which resulted in the averageδ¹⁵N（6.0±1.0‰）at 30cm soil depth during UCM is higher than blank control experiment.This isotopic signature deviation caused high uncertainty of synthetic fertilization contribution quantification by isotopic approach.（4）The nitrate concentration in wet season was higher than dry season in Chishui River,which indicated that nitrogen in thin soil in Karst area is more likely to be lost.The dual isotopes of nitrate during the wet season suggested that the major sources of nitrate in the mainstream and tributaries were fertilizer and manure in the upper and middle reaches,while the sewage waste is major sources for downstream.During the dry season,the origination of NO₃^-showed obvious contribution by anthropology activities especially several tributaries located in developed area.In summary,the controlling experiments showed the transformations and loss of nitrogen in soil,while the samples result of Chishui River indicated that soil erosion is major sources of nitrate in Karst area.Therefore,controlling fertilizer input,increasing the ratio of farm manure and avoiding fertilization during frequent rainfall periods can effectively reduce nitrate input to rivers.