Influence Of Typical Soil-rock Structure Composition On Nitrogen Leaching Loss In Southwest Karst Area

In karst area with a unique dual hydrological system and shallow soil thickness,nutrients in the soil are easily leached during rainfall,resulting in groundwater pollution.Previous studies have shown that different rock-soil structures in karst area have significant effects on the transport of water and nutrients,but the extent and mechanism of the effects are still unknown.In this study,column experiments were used to simulate different soil-rock structures of karst area to explore the transfer and transformation processes and leaching characteristics of nitrogen in different soil-rock structures of karst area.In this study,the column I structure(from top to bottom,same below)is lime soil layer(23 cm),soil-rock mixed layer,limestone layer,and filter layer.The column II structure is lime soil layer(20cm)and dolomite layer.The column III structure is lime soil layer(20 cm),yellow loam soil layer(70 cm)and limestone layer.Combined with the results of the column experiment and the detection results of water samples,the migration and transformation process of nitrogen and the source of nitrate in the typical karst river under different rainfall events were discussed.The results are as follow:(1)During the column experiment,the concentrations of Ca²⁺,Mg²⁺and HCO₃^-in the leaching solution indicated that column I was less affected by dissolution and column III was strongly affected.The significant positive correlation between[Ca²⁺+Mg²⁺]and NO₃^--N in each column leaching solution(p<0.0001)indicated that the chemical dissolution of NO₃^-was an important reason for the high concentration of Ca²⁺,Mg²⁺and HCO₃^-.During the increase of NO₃^--N concentration in each column leaching solution,there was a significant negative correlation between HCO₃^-and NO₃^--N(p<0.0001),indicating that H⁺produced in the nitrification process reacted with HCO₃^-.This is an important reason why the concentration of HCO₃^-is lower than the theoretical value,especially column III.This explains the different responses to nitrogen in different soil-rock structures.(2)During the experiment,During the simulation experiment,the nitrogen conversion process(hydrolysis-nitration)in column I and column II was rapid,and column III was slow.The isotopic composition of nitrate in column III leaching solution shows the denitrification process,especially after the fourth rainfall event.During the migration process,the vertical soil-rock interface in the soil-rock mixed layer and the fissure channel in the limestone layer in the column I leaching solution are the main paths for the migration of a large amount of nitrogen-rich soil water,which is the main reason for its rapid nitrogen migration.The shallow soil and the blocking effect of the dolomite layer in the column II resulted in the obvious evaporative fractionation of its water isotope.The deep soil layer(90 cm)and the low porosity limestone layer(12.5%)in column III extended the nitrogen retention time.The analysis of rainfall event and nitrogen leaching loss showed that the maximum nitrogen leaching loss occurred within 6 h after the rainfall event.This explains that soil-rock structure is an important factor affecting nitrogen leaching loss.(3)Taking the Nanming River Basin as an example,there is agricultural area in the upper reaches of the study area,and the soil-rock structure is similar to column III.And the midstream flows through the urban area,and the structure is similar to column I,with a multi-karst pipeline structure.The concentration analysis of various forms of inorganic nitrogen in the samples indicated that NO₃^--N was the main form of soluble inorganic nitrogen in the River.Nitrate isotopic composition indicated that NO₃^--N in rivers is mainly derived from animal manure and sewage.During different rainfall events,NO₃^--N concentrations and nitrate isotopic compositions in water samples indicated that volatilization and denitrification during low-intensity rainfall events were important reasons for the enrichment of heavy isotopes(?¹⁵N-NO₃^-and?¹⁸O-NO₃^-).And nitrogen was less affected by biogeochemical processes during high-intensity rainfall events.Analysis of water samples from different areas in the basin showed that nitrogen in the stagnant soil in agricultural areas continued to contribute during low-intensity rainfall events,while the contribution from urban areas was relatively limited.And during the intensity drop event,nitrogen input in urban areas increased the nitrogen load in the Nanming River.And this also explains that the soil-rock structure type plays an important role in the hydrology and solute biogeochemical processes.