| High salt content is one of the main limiting factors of agricultural development in the delta region all over the world,which can cause a series of problems such as land degradation and groundwater pollution.Farmland reclamation was the common method for land repair and farmland expansion in the Yellow River delta,but as a result of sensitive environment and human disturbance in reclamation period,under the influence of groundwater,irrigation,farming,soil conditioner and chemical fertilizers,soil solute gradually showed abnormal enrichment,large area of cultivated farmland had different level of soil salinity,soil degradation,seriously affected farmland production efficiency.Therefore,revealing the changes mechanism of the soil physical and chemical properties of the farmland can offer crucial view for farmland management and food security.In this thesis,plots reclaimed for different years(wasteland,1 a,5 a,10 a,20 a)in the Yellow River Delta were selected as the research objects,and the soil salt content,ion concentration and physical and chemical indexes at different depths of plots were measured to clarify the vertical changes of soil salt in different plots,and to explore the main reasons for the change of salinization process.Transport simulation tests were set up to monitor and compare the similarities and differences of water and salt transport laws of various soils under the condition of natural precipitation.Using the HYDRUS-1D model to simulate the process of soil water and salt migration of five types of farmland,the adaptability of the model to soil water and salt migration in different farmland in the study area was also explored,and the following conclusions were drawn:(1)Soil in the study area was facing serious salinization as a whole,and the salt content of all kinds of farmland gradually decreased with the extension of reclamation time.Cultivation,irrigation,soaking and other artificial reclamation can help to mitigate the surface accumulation of soil salt.The peak salt value gradually moved down and the salinization level gradually weakened.20 years later,the overall soil electrical conductivity decreased by 74%,the salt peak moved down to the 80-100 cm layer,and the overall salinity classfication shifted from moderate to mild or non-salinization soil.However,the salinization of deep soil showed a slight rebound after 20 years.(2)In the farmland reclaimed for a long period,the similarity of soil chemical components and soil particle gradation was high,the balance of farmland water-salt system was gradually formed,the composition of base ions was basically stable,and the main chemical composition of soil samples was always Na-Cl type during the whole research period.With the emergence of artificial reclamation,the sand content was greatly reduced and the silt content was greatly increased.The results of soil classification changed from diversified to simple and from sandy loam to silty loam in 20 years.(3)After 20 years of reclamation,soil has had strong water and fertilizer retention ability,and the soil water and salt migration rate has decreased.and the water retention ability was about 7 times higher than that of wasteland.The soil bulk density of 0-10 cm soil layer was significantly lower than that of other farmlands.Salt content variation showed an“A”shape with sharp increase and then sharp decrease during large-scale precipitation.At 20 cm,the salt content variation showed an“n”shape with continuous fluctuation near the peak value.At 40 cm,the“A”shaped trend was similar to that at 10 cm.(4)It is suggested that farmland in the study area should be deeply ploughed or irrigated for salt washing after 10-15 years of reclamation,and the water consumption for salt washing should be more than 2.0×10~3m~3/hm~2(200 mm)to ensure the combination of utilizing and maintaining and avoid secondary salinization in deep soil.(5)The HYDRUS-1D model showed good adaptability in farmland reclaimed for 10-20 years in the study area,and the simulation error was large in wasteland.It showed good fit at 40-80 cm,and the simulation error was large at 10-20 cm.Therefore,it is suggested that the simulation error calculation should be carried out first and other models should be selected if necessary for the studies of water and salt migration simulation in primary soil in the Yellow River Delta. |
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