Dynamics Of Water,Salt And Nutrients In Sandy Land Of Cultivated Wasteland And Changes Of Soil Carbon And Nitrogen Transport Based On Isotope

Soil Salinization and shortage of water resources are the important factors that restrict the development of agricultural production in Hetao Irrigation district of Inner Mongolia.The interlaced distribution pattern of cultivated land,wasteland and sandy land in irrigated area makes the salt wasteland become the salt drainage area of cultivated land,and the salinization and alkalization of cultivated soil develop in a positive direction in a certain period.Taking different land use types in Hetao irrigation area as research objects,this paper aims to explore the laws of temporal and spatial transport of water,total salt,salt ions and nutrients in cultivated land,wasteland and sandy land,the characteristics of salt accumulation in cultivated land and saline wasteland soil and the effect of Salt Ions on salt accumulation in soil were analyzed.At the same time,the migration of stable carbon and nitrogen isotopes in different types of saline and alkaline land was studied by stable isotope method.The main results were as follows:(1)HCO3-content was the highest in cultivated land and sandy land,the mean was 379.67mg/kg,281.9mg/kg,and SO42-content in wasteland was the highest,1252.51 mg/kg.The biggest variability of cultivated land and wasteland is Mg2+,the Coefficient of variability is about 44.68%～46.98%,the biggest variability of sandy land is Cl-,the coefficient of variability is 75.67%,the weakest variability of cultivated sandy land is HCO3-,the Coefficient of variability is about 13.18%～16.41%.The cultivated land and sandy land are mainly carbonate soil,and the waste land is mainly sulfate soil.The change of soil salt content of cultivated wasteland is more stable than that of sandy land.(2)After irrigation,Cl’ was most active and easily lost with water,up to 76.2%of that before irrigation.Cl-surface aggregation was inhibited to some extent in the later period of maize planting.From jointing to tasseling stage,maize decreased HCO3-content in root layer soil,and HCO3-content in 40～120cm soil layer increased with the increase of groundwater level.The SO42-in the soil is transported to the groundwater with the moisture content,and the surface layer descends greatly.The more the content of K+and Na+in soil,the faster the decrease speed after irrigation.The contents of Ca2+,Mg2+ and Na+ in difterent soil layers increased during the irrigation interval.(3)In 0-120cm soil layer,the salt accumulation of cultivated land is mainly concentrated at the beginning of growth,the salt accumulation is about 2.77t/hm2 and the maximum accumulation of salt is about 30.23t/hm2,70%of the table salt accumulation,which can explain that the waste land has the function of dry salt drainage,however,the salinity of sandy land increased with the depth of soil layer.There was a significant negative correlation between salt accumulation of cultivated land and that of uncultivated land,and a significant positive correlation between salt accumulation of sandy land and that of cultivated land.The prediction model of salt accumulation in cultivated sandy land was established by regression analysis.The adjusted R2 values were 0.405,0.941 and 0.784,respectively.The cultivated land was affected by irrigation,however,it can provide a theoretical reference for the prediction of salt accumulation in 1.2m deep soil in the study area.(4)The nitrate nitrogen content in cultivated land reached a peak value of 38.19 mg/kg after topdressing,and the change of nitrate nitrogen content in growing period of maize and sunflower was similar.The content of nitrate nitrogen in cultivated wasteland decreased with the depth of soil layer,but it was opposite in sandy land.The available phosphorus decreased with the depth of soil layer in sandy land.Nitrate nitrogen and available phosphorus migrated to deep soil layer with water after irrigation.The Correlation Coefficient between nitrate nitrogen and cultivated land was about 0.429～0.466.(5)In 0～80cm saline-alkali soil,the n isotope ratios decreased first and then stabilized,and the C isotope ratios increased first and then stabilized.The 15N values in the root zone of saline-alkali soil of different degrees were moderate soil>the light soil>the heavy soil the 13C values were mild soil>the medium soil>the heavy soil.The nitrogen isotope migration under drip irrigation was more uniform than that under flood irrigation.