Spatial And Temporal Variation Of Soil Organic Matter And Its Driving Factors In Hainan Island

Soil organic matter?SOM?content is an important indicator for evaluating soil fertility and soil carbon pools.On the one hand,Models different in simulation mechanism are used to predict distribution of SOM contents,because different regions have soils of different soil properties and different environmental factors affecting soil properties.It is,therefore,essential to choose an optimal suitable model site-specifically for the prediction,this paper is oriented to explore for a model that is capable of predicting spatial distribution of SOM content in a tropical region complicated in relief,like Hainan Island.On the other hand,local studies also show differences in spatio-temporal variability of SOM in different regions.However,researches on spatio-temporal variation of SOM in China are mostly concentrated in temperate and subtropical regions.There are few studies on tropical regions in China?such as Hainan Island?.Existing studies have focused on specific farmland such as paddy fields and rubber fields,and lack of spatio-temporal variability of SOM involving different types of land-use throughout Hainan Island.Therefore,this paper takes Hainan Island as a research area,using three methods:ordinary kriging?OK?,regression-kriging?RK?,and randomForest?RF?,combining topographic factors,normalized vegetation indices,land-use types,soil types,and other auxiliary variables for the region's 128 samples in 2012.The spatial distribution of SOM contents in four soil layers from 0⁵,0²0,20⁴0,and 40⁶0cm was predicted,and accuracy evaluation was performed using 32 verification points.The best fitting model for different soil layers in this area was selected;Comparing field sampling data?2012?and historical data?1980s?,combined with agricultural statistic datas,the spatio-temporal variability and driving factors of SOM in this area were studied.So as to provide a reference basis for the selection of spatial variation mapping methods of SOM contents and the adjustment of agricultural planting structure in Hainan Island.The conclusions are as follows:?1?The best fitting models for 0⁵,0²0,20⁴0cm soil layers are RF,while the best fitting model for 40-60 cm soil layer is OK;RK and RF can describe the details of the local variation of SOM contents.?2?From the spatial prediction map,the SOM content in the four soil layers shows a spatial distribution trend that SOM contents are higher in the southwest and northeast parts and lower in the west and southeast parts of the island.?3?In the recent 30 years,the contents of SOM in this area showed a decreasing trend overall.In2012,the SOM content was reduced by 4.68 g kg^-1 compared with 1980s.The decrease of paddy field was the most obvious,with a decrease of 6.42 g kg^-1,followed by the garden,with a decrease of 2.65 g kg^-1,while the dry land reduction was the smallest with 1.28 g kg^-1.?4?From the spatio-temporal variability map of SOM contents,the SOM contents in Hainan Island showed an overall decreasing trend in the recent 30 years.The contents of SOM decreased mainly in the range of-5^-10 and-5^-1 g kg^-1,distributing in most of parts of the island except for the eastern and northeastern parts,accounting for 60.23%of the total island area.The SOM contents in the northeastern and eastern coastal areas increased,mainly in the range of 1⁵ and 5¹0 g kg^-1,accounting for 19.47%of the total island area.?5?Rice crop rotation,land reclamation and increasing multiple crop indexes are the main reasons causing the decrease of SOM contents in Hainan Island.