| With the rapid development of the global economy and the rapid increase of population,the contradiction between people and land is becoming more and more serious.The development and protection of cultivated land resources and the management of land degradation has become a hot issue of global concern.The Yellow River Delta is one of the biggest estuary deltas and important reserve cultivated land resources in China.Due to the influence of the Yellow River diversion,seawater invasion and infiltration and other factors in the process of land formation,different degrees of salinization occur over nearly 50% of the land.However,there is still a lack of systematic research on soil salinization in this area,which needs further discussion.In this paper,Kenli County,Wudi County and "Bohai Granary" model area were selected as the study areas.We measured soil moisture in surface soil and soil salinity in 0-15 cm,15-30 cm,30-45 cm and 45-60 cm soil layers by field surveys in typical time in spring,summer,autumn and winter,which were used to study the spatial and temporal variability of soil moisture and salinity.Then,the main influencing factors of salinization were studied qualitatively and quantitatively,and the intensity of each factor was calculated by grey relational analysis.Next,the main influencing factors were screened and the multiple linear regression analysis was used to construct the forecast models of soil salinity in different soil layers.Then this paper put forward the measures of soil salinization improvement from county,region and micro scales.The main conclusions were as follows:(1)Spatial and temporal variability of soil moisture and salinitySoil moistures in Kenli County,Wudi County and “Bohai Granary” model area were moderate variability.Areas with high soil moisture in Kenli were mainly distributed in the east near the Bohai Sea and the southwest where was the major wheat-growing areas.Areas with high soil moisture in Wudi were mainly distributed in the north near the Bohai Sea.From February to May in next year,soil moisture in “Bohai Granary” model area showed the monthly dynamic pattern of increasing firstly and then decreasing,and then increasing again,which were the highest in August and lowest in February.Soil moistures in both Kenli and Wudi were highest in summer,the second in spring and autumn,and the lowest in winter.Thedifferences among different years were not significant.Soil salinities in Kenli,Wudi and the model area were mainly 2.0-5.0 g·kg-1,2.0-4.5 g·kg-1 and 2.0-6.0 g·kg-1,respectively.In vertical direction,soil salinity gradually increased with increasing soil depths.In horizontal direction,soil salinities in different soil layers in Kenli gradually increased from west to east,and they increased from southwest to northeast in both Wudi and the model area.The monthly variability of both soil salinity and soil salinization in surface soil were strong,but with increasing soil depth,the variability gradually reduced.Soil salinity in Kenli was the lowest in summer,second in spring and autumn,the highest in winter but with only 0.1-0.3 g·kg-1 higher.Soil salinization in each soil layers in Kenli showed the seasonal pattern of significantly decreased to northeast from spring to summer,then significantly deteriorated to southwest from summer to autumn,then slightly deteriorated to southeast from autumn to winter,and then slightly decreased to northeast from winter to next spring.Soil salinity in Wudi was the lowest in summer,second in winter,the highest in spring and autumn.Soil salinization in each soil layers in Wudi showed the seasonal pattern of slightly deteriorated to northeast from autumn to winter,then slightly decreased to southwest from winter to spring,significantly decreased to southeast from spring to summer,and then significantly deteriorated to northwest from summer to next autumn.Soil salinity and salinization in both Kenli and Wudi were internal differences,but the differences were not significant,and with increasing soil depth,the differences gradually weakened.Soil moisture and salinity were positively related,but not significant.The correlation coefficients in spring and autumn were greater than that in summer and winter.(2)Qualitative and quantitative analysis of influencing factorsThe main influencing factors of soil salinization in the Yellow River Delta were climate condition(evaporation ratio as quantification),vegetation(vegetation coverage),distance from the Bohai Sea or the Yellow River,topography(relative elevation),soil texture and soil profile(soil clay content),groundwater depth,mineralization of groundwater and human activity.Among them,evaporation ratio and mineralization of groundwater were significantly positively correlated with soil salinity;the other influencing factors were significantly negatively correlated.Meanwhile,soil salinities also varied with microtopographics,such as distance from the roadside in cotton field,ponding depression in maize field,from the inlet to the outlet in paddy field,slope in raised fields,different parts in bare land or slopes in dry ditch.In 0-15 cm,15-30 cm,30-45 cm and 45-60 cm soil layers,the rank of the grey correlation degree of each factor was all groundwater mineralization> vegetation coverage > groundwater level > soil clay content > distance for the sea > relativeelevation.(3)Forecast model of soil salinity and the improvement measuresThe main influencing factor of soil salinity in 0-15 cm,15-30 cm,30-45 cm and 45-60 cm soil layers in Kenli County were all groundwater mineralization,vegetation coverage,groundwater level and soil clay content.Using the four factors as independent variables,4 multivariate linear regression models for forecasting soil salinity in different soil layers were constructed.The adjusted R2 of the models were 0.7632,0.8216,0.8156 and 0.7727,respectively.The fitting accuracies of the predicted and measured values were all greater than 0.78 and the accuarcy of distribution maps were all greater than 75%.The accuarcy of the models were high,the fitting effects were good and they could better reflect the spatial distribution characteristics of soil salinity in different soil layers in Kenli County in spring.On the county scale,the improvement and utilization maps of soil salinization in Kenli County and Wudi County were divided respectively and corresponding measures were put forward varied with the natural condition and existing problem of each district.On the regional scale,according to the pattern of soil water and salt movement,the regulation models of soil salinization in different periods in the model area were put forward.On the micro scale,three measures of strengthening the soil tillage management,improving soil fertility and the ecological environment were also put forward.In this paper,we systematically studied soil salinization in typical areas in the Yellow River Delta,found out the dynamic pattern of soil moisture and salinity and the influencing factors,put forward the forecasting models of soil salinization,which provide scientific basis and technical support for sustainable use and management of soil salinization.At the same time,the method of combined field investigation,GIS and quantitative models provided an effective method for quantitative analysis of salinized soil.Finally,some targeted measures for improvement and utilization in different scales were put forward,which would be reference and guidance for soil salinization utilization. |
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