| The western of Jilin Province,China,is located in the arid and humid East Asian monsoon region.It was affected by geological structure and climatic factors and underwent positive and negative evolution of salinization and desertification,which has been the largest saline soil area in China.In order to increase the utilization rate of salinized soil,water conservancy projects was used to decrease salt by irrigation.However,the residual nitrogen remains in the vadose zone due to the poor permeability of the salinized unsaturated zone in the area,and it will cause varying degrees of contamination of groundwater with the increase of infiltration volume.This paper was finacially supported by the Natural Science Foundation Project of Jilin Province and Natural Science Foundation of China.Da'an city in western Jilin Province is taken as the study area.Based on meteorological data,remote sensing imagery data,monitoring data of field sampling and indoor simulation testing data and using statistical analysis method,stochastic method,and Hydrus model technique,the study of the water and nitrogen transport in saline soil in the western of Jilin Province and the simulation and prediction of nitrogen under the conditions of variable groundwater levels are carried out.The main contents include:1?The characteristics and spatial variation of water and nitrogen in the saline soilOn the basis of field investigation,the salt,water,p H,nitrate and ammonium nitrogen of soil were determined to analyse the statistical characteristics,spatial variability and correlation.The results showed that the variation coefficient of salinity was significantly greater than that of water content and p H.The salinity was influenced by random factors,and there was significant change in vertical and lateral space.The p H is more consistent in vertical and transverse spatial distribution,and is mainly affected by structural factors.The spatial distribution regularity of nitrate and ammonium nitrogen is obvious.From east to west,the nitrate and ammonium nitrogen content decreased gradually.From north to south,the ammonium nitrogen gradually increased and the nitrate nitrogen gradually decreased.The spatial distribution of nitrogen was affected by structural factors in small scale.On this basis,the spatial correlation between nitrate and ammonium nitrogen and soil water salt parameters was analyzed.When soil salinity increased from 0.2% to 0.6%,soil nitrogen content increased.When the p H value drops to the range of 8.5-9,the content of nitrate and ammonium nitrogen in the soil is larger,and the soil nitrogen decreases as the p H value increases or decreases.The soil moisture content was less than that of the soil,and when the soil water content increased to 20%,the soil nitrate decreased,and the ammonium nitrogen increased,indicating that the increase of water increased the leaching of nitrate.2? Study on nitrogen transport and transformation of vadose zone in salinized soil(1)According to the distribution characteristics of saline soil,selection of typical saline soil area of field experiment,through the adsorption kinetics and isothermal adsorption experiment,the analysis of nitrogen in saline topped the adsorption characteristics of gas with medium.The results showed that the adsorption nitrogen rate of saline soil was slower than that of non-saline soil,and the maximum adsorption capacity was 18.75% less than that of non-saline soil.The adsorption performance of different soil depths is different,and the deep soil adsorption is strong.When the initial concentration of solute was large,the change of temperature had a significant effect on the adsorption of ammonium nitrogen in saline soil.(2)The research area is located in the northeast seasonal frozen region,with obvious seasonal freezing-thawing phenomenon.In this paper,the effects of freezing-thawing cycle on nitrogen conversion in saline soil zone were studied.The data of indoor freezing-thawing test showed that,with the increase of the number of freeze-thaw cycles,the content of NH4+-N in soil showed a significant increase trend,and the soil NO3--N showed a significant decline trend.This is due to the mineralization of residual nitrogen in the soil after crop harvest.As the number of freeze-thaw cycles increased,the nitrification of nitrogen was inhibited,and the soil NO3--N decreased.In the short term,the freezing and thawing cycle can promote the mineralization of nitrogen,but many long-term freeze-thaw cycles can inhibit the mineralization reaction of nitrogen.Through experiments,it can be found that when the soil moisture content is high and the fertilization amount is large,the soil nitrate nitrogen is easy to be lost after repeated freeze-thaw action.(3)Combined with field in situ test,the nitrogen migration patterns of crops in different growth stages were studied.The changes of soil nitrogen,salinity and p H value under different fertilizer application showed that the change of soil ammonium nitrogen was inconsistent in the vertical direction.The ammonium nitrogen difference of rice during the period of redevelopment and the stage of the booting period was large,and the soil ammonium nitrogen increased with the increase of fertilizer.In different growth periods,soil nitrate nitrogen was consistent with the change law,and the topsoil nitrate nitrogen content was the largest,decreasing with the increase of soil depth.The effect of fertilizer increase on the nitrate and ammonium nitrogen in the surface of soil is obvious,which has little effect on the deep content.The distribution characteristics of soil salinity were not obvious during crop growth period,and the salt content was relatively small when the soil was 100 cm deep.After the harvest,soil salt clear difference with the increase of soil depth,and soil salinity reached maximum,in surface soil salt deposition occurs after the rice harvest,and increased fertilizer use is large soil salt,soil salt is closely related to the size and the fertilizer.3?Uncertainty analysis and construction nitrogen transport model in vadose zone(1)Based on the data of field experiment,a model of water nitrogen transport was established based on Hydrus.According to the model evaluation standard,the model has a good simulation effect,which can be used to simulate the water and nitrogen transport in vadose zone.Through the analysis of water and nitrogen balance,it can be seen that the water loss is evapotranspiration,and the water utilization at high fertilization level is slightly higher than that of low nitrogen.The amount of nitrogen applied increased from 180 kg/ha to 220kg/ha,and the mineralized solid content of nitrogen increased by 21.6%.The amount of nitrogen uptake increased by 12kg/ha.The cumulative leaching of nitrogen in the depth of 1m increased by 8.59 kg/ha.The increase of fertilization increased the mineralization capacity of soil nitrogen,but it also increased the leaching loss of nitrogen.Therefore,the effective utilization rate of nitrogen should be improved by combining with irrigation fertilization.(2)The uncertainty of the parameters is identified by using the probability uncertainty estimation and the Latin hypercube stochastic simulation method.And the influence of parameter uncertainty on the cumulative leaching of nitrate nitrogen.The soil saturated moisture content,dispersion degree and adsorption coefficient have better recognition,and the denitrification rate and saturation conductivity have great influence on the uncertainty of model results.The effects of two schemes,which were considered and not considered for parameter uncertainties,were studied on the soil nitrate leaching loss of 100 cm deep under rice growing conditions.The predicted range of cumulative leaching of nitrate nitrogen in the 0-100 cm soil under different schemes was 10.34-13.63kg/ha,9.47-12.71 kg/ha.Allow prediction parameter uncertainty range to reduce the uncertainty,improve the simulation accuracy of vadose water nitrogen migration rule,accumulated in nitrate leaching risk quantitative evaluation,and improve the accuracy of the results.4.The influence of groundwater depth changes on the nitrogen transport in vadose zone.Based on the least square support vector machine(LS_SVM),the groundwater depth simulation prediction model is established,and the dynamic change trend of groundwater depth in the research area is analyzed.By introducing a 95% confidence interval,the groundwater depth change was predicted,and the upper and lower limits were respectively(5.98-6.85)m,(5.15-6.07)m.The groundwater depth was used as the lower boundary of the model of the nitrogen transport of the gas belt,and the influence of the change of the groundwater level on the nitrogen transport law of the gas belt was simulated.The effect of groundwater depth on soil water content,soil nitrate and ammonium nitrogen was less than that in 5.15-6.85 m.When the local water was buried deep down to 1.49-1.71 m,the leaching risk index of soil nitrate was promoted from level I to grade II,which increased the pollution risk of groundwater ecological environment.Therefore,it is necessary to control the average groundwater depth of the month to be greater than 1.7m,which can effectively reduce the risk of nitrogen leaching in study area.In conclusion,this study not only provides theroretical support for the mechanism of nitrogen migration in salinization vadose zone-saturated,but also significance for nitrogen pollution prevention in groundwater. |
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