Study On Soil PH Change And Dynamic Change Prediction By Afforestation

Afforestation is a key measure to combat climate change as it can slow down the emission of the greenhouse gas carbon dioxide.Although afforestation plays a key role in the biochemical processes of terrestrial ecosystems,its effect on soil pH remains unclear.In this study,a literature search resulted in a dataset of 166 articles with 1082 experimental sites.This dataset was used to resolve the characteristics of soil pH changes before and after afforestation and to fit the relationship between soil pH change values and the main influencing factors.On this basis,a predictive model of soil pH change after afforestation in relation to climate,topography and soil factors was developed using random forests.We predict the dynamic spatial distribution of soil pH change after 5,10 and 20 years of afforestation on forested land,and analyse regional differences in soil pH change.The main findings are as follows:(1)Soil pH decreased significantly by 0.23 after afforestation.soil pH decreased more in the boreal and temperate zones than in the tropics,with soil pH decreasing by 0.28,0.29 and 0.08 respectively.soil pH decreased significantly in humid areas,while there was no significant change in soil pH in arid areas.All four afforestation types showed significant decreases in soil pH,with the greatest decrease in soil pH occurring when cropland land was converted to secondary forest.The initial soil pH is an important factor influencing the soil pH change.The initial pH range of 6-7 showed the greatest decrease in soil pH after afforestation,with a decrease of 0.45.The degree of decrease in soil pH after afforestation increased with the number of years of afforestation and decreased with the depth of the soil layer.(2)Single factor fitting results showed that annual mean temperature(MAT)was positively correlated with soil pH change value(ΔpH)after afforestation,and the fitting expression was ΔpH=0.02×MAT-0.19.The mean annual precipitation(MAP)is convex toΔpH,and the fitting expression is ΔpH=-3.77×10-7×MAP2+1.06×10-3×MAP-0.55.The initial soil pH(ipH)is concave to ΔpH,and the fitting expression is ΔpH=0.002×ipH2-0.21×ipH+1.15.Soil depth(sd)is negatively correlated with ΔpH,and the fitting expression is ΔpH=0.006×sd-0.17.Stand age(year)is negatively correlated with ΔpH,and the fitting expression is ΔpH=-0.004×year+0.17.(3)A predictive model for the change in soil pH after afforestation was developed based on a random forest,using nine variables: mean annual temperature,mean annual precipitation,slope,aspect,soil organic carbon,clay content,initial soil pH,soil depth and forest age,with an explanation of 71%.The importance of the factors was ranked as mean annual precipitation > initial soil pH > forest age > mean annual temperature > slope > soil organic carbon > clay content > soil depth > aspect.Using the prediction model,the predicted spatial distribution of soil pH change after 5,10 and 20 years of future afforestation was mapped.(4)The prediction results showed that soil pH value decreased by 0.049,0.082 and 0.24 after 5,10 and 20 years of afforestation,respectively.There were significant regional differences in soil pH changes after afforestation of suitable forestland.After afforestation of suitable forestland for 5,10 and 20 years,European soil pH decreased the most,with the decrease values of-0.054,-0.089 and-0.27,respectively.In North America,the decrease values were-0.049,-0.083 and-0.25,respectively.South America showed the least decrease,with the decrease values of-0.042,-0.069 and-0.20,respectively.There is no difference between Oceania,Africa and Asia.