| Soil acidity and alkalinity are comprehensive reflection of many physical and chemical properties of soil.Changes in soil acidity and alkalinity have important effects on soil nutrient maintenance and supply,crop growth,and agricultural product quality.Soil pH is an indicator of soil acidity and alkalinity.The unique natural environment and human activities under different land use types make the soil pH significantly different.Exploring the spatiotemporal variation of soil pH and its influencing factors under land use changes is the basis for understanding the trend of soil acid-base changes and then formulating targeted acid-base regulation measures.This study took Xunwu County as the research area.Based on 595 soil samples collected in the second soil survey in 1984 and 580 collected in 2018,the spatiotemporal variation characteristics of soil pH in Xunwu County over 35 years were investigated by Ordinary kriging(OK)on the one hand.On the other hand,the Random Forest(RF)method was used to supervised and classified the land use types in 1984,1995,2006 and 2018 in Xunwu County,in order to clarify the changes of land use types in 35years.Then,the spatial nonstationarity of the effects of altitude,slope,distance to rivers,distances to roads,and nitrogen fertilizer application on the spatiotemporal variation of soil pH were investigated using a Geographically Weighted Regression(GWR)model.Finally,the response of the spatiotemporal variation of soil pH to changes in land use was analyzed,and the differences in the factors affecting the spatiotemporal variation of soil pH under different land use conditions were clarified.The main conclusions of the study are as follows:(1)The soil sample pH in Xunwu County in 1984 was between 4.30 and 6.70,with an average value of 5.35,and the coefficient of variation was 6.54%.In 2018,the soil sample pH was between 4.32 and 5.83,with an average value of 5.05,and the coefficient of variation was 4.95%.The results of semi-variance analysis showed that the optimal fitting model for the spatial variation of soil sample pH in 1984 was a linear model,the fitting coefficient was0.787,the nugget effect was 85.83%,and the variation range was 36.73 km.In 2018,the optimal fitting model of soil sample pH spatial variation was an exponential model with a fitting coefficient of 0.876,nugget effect of 31.67%,and variation range of 7.90 km.The results of ordinary Kriging interpolation showed that the high and low value areas of soil pH were distributed continuously in 1984.The high value areas were mainly distributed in Danxi Township in the southeast,Wenfeng Township and Changning Town in the middle,Sanbiao township and Shuiyuan township in the north,and the low value areas were mainly distributed in Chenguang township and Guizhumao township in the west.In 2018,the high pH values were scattered in Chengjiang town in the north,Xiangshan township in the east,Danxi township in the southeast and Guizhumao town in the west,while the low pH values were mainly distributed in Wenfeng township and Liuche town in the middle.During the past 35 years,the change of soil pH in Xunwu County ranged from-0.91 to 0.32,with an average decrease of 0.30 units.91.72%of the soil in the county showed a trend of acidification,and the most obvious acidified areas were mainly distributed in Sanbiao Township,Wenfeng Township,Jitan Township,Nanqiao Township,Longyan Township,and Danxi Township,but Chengjiang Town had a tendency to alkalize.(2)In 1984,the area of cultivated land,garden land,woodland and other land was315.94 km~2,11.71 km~2,1961.03 km~2 and 63.89 km~2,respectively.In 2018,the area of the four land use types was 143.02 km~2,298.19km~2,1805.08 km~2 and 106.28 km~2.During the past 35 years,the area of the garden and other land showed an upward trend,with a net area growth of 286.48 km~2 and 42.39 km~2,and the growth rate was 2447%and 66%,respectively.However,cultivated land and woodland areas showed a downward trend,with a net area reduction of 172.92 km~2 and 155.95 km~2,and the reduction rate was 55%and 8%,respectively.During the past 35 years,the main change of land use types was the transformation of cultivated land and woodland to garden land and other land types.In 2018,70%of garden land came from forest land,27%from cultivated land,while 35%of other land came from forest land and 28%from cultivated land.(3)During the past 35 years,among the 16 transformation types for the conversion of4 kinds of land use types(cultivated land,garden land,forest land and other land types),the soil pH of 15 transformation types has decreased.The pH of the non-converted forest land had the most serious decline,which decreased 0.41 units,while soil pH increased 0.32 units during the conversion of cultivated land to garden land.Soil pH changes influenced by altitude,slope,distance to river,distance to road and nitrogen fertilizer application.Geographically Weighted Regression results showed that the average coefficients of five impact factor of regression were 0.02,0.02,0.16,0.26 and 0.20.The distance to river and the distance to road has obvious positive effect,while nitrogen mainly played a negative impact.In different transformation types of land use types,the distance to road played a leading role in the change of soil pH under 12 transformation types of land use types,nitrogen fertilizer application played a leading role under 3 transformation types and slope played a leading role only in the conversion of other land types to farmland.(4)Based on the spatiotemporal pH variability of the soil of Xunwu County and its response to land use changes.The study proposed that the acidity and alkalinity of Xunwu County’s soil should be controlled from the perspective of reasonable reduction of nitrogen fertilizer and increase of organic fertilizer;appropriate application of soil acidic amendments;promotion of fallow crop rotation and reasonable crop set;strengthening the ecological restoration of rare earth mining areas;these four angles to ensure the production and ecological functions of the soil. |
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