Spatiotemporal Inversion Research Of Total Organic Carbon In Surface Soil Based On Sentinel Image Fusion

Refined inversion of total organic carbon（TOC）content in surface soil provides important data for promoting the development of precision agriculture.The inversion of organic matter content in previous studies were mostly based on a single scene or a few scene images.The estimation of organic matter tended to be static,which was difficult to capture its spatiotemporal changes.The organic matter is sensitive to the influence of climate,vegetation,fertilization,farming and other factors.Therefore,the degree of changes in different time periods cannot be ignored.In this study,the time resolution is improved under the premise of ensuring the accuracy of the inversion.Fusion of multispectral data Sentinel-2 and Sentinel-3 can take advantage of Sentinel-2’s high spatial resolution information（10m）and Sentinel-3’s rich spectral information（21 bands）.The study produced a 52-phase image of soil TOC content inversion from Nov.2016 to Nov.2018 in Donghai county.The model was determined,estimated and verified by combining the measured data of 2017 and 2018.A multiple stepwise linear regression model was established based on the RVI(B₁₈/B₉)method:TOC=1.4+0.523×（B18/B9）-1.448×B20.The main conclusions of this study are as follows:（1）The model was established by combining RVI（B18/B9）and B20,modeling sets R²=0.6396,RMSE=0.4018%;verification sets R²=0.6595,RMSE=0.1495%;modeling sets and validation sets are evenly distributed on both sides of the 1:1 line,and the model fitting effect is good.（2）The TOC content of soil in Donghai county has an increasing trend from the 1980s（0.59%）to 2017（1.29%）and 2018（1.59%）.The average value of 52 images in the past two years is 1.49%.The smashing of straw on the spot and the extensive use of organic fertilizers have promoted the gradual improvement of soil fertility;soil TOC content showes seasonal differences,the lowest level is observed in winter（0.78%）,which is lower than that of spring（2.11%）and autumn（1.55%）,the highest level is observed in summer（2.17%）,which are affected by temperature and humidity.The TOC content in the spring and summer which are warm and humid is higher than that in the autumn and winter which are cold and dry.The TOC content in the eastern soil is higher than that in the western soil in spring,summer and autumn.The organic matter content is generally high in summer,which is generally low in the winter.The TOC content is unevenly distributed in spring and autumn.It is mainly affected by agricultural measures such as straw returning and fertilization.The rate of straw returning in the eastern region is higher than that in the western region.The large walnut bases in the western region apply a large amount of organic fertilizer in the winter,and four major livestock and poultry breeding bases in the western region have increased the TOC content in winter.（3）TOC increases from northwest to southeast.Soil texture affectes soil fertility.TOC was also affected by topographic factors such as elevation,slope and aspect.The TOC of Hot Spring town,Linian forest farm and Linian township have the fastest rise,which is affected by factors such as land remediation,greening and afforestation.Among the different land-use types,the highest TOC content is observed in garden land（2.01%）,which is higher than that of paddy field（1.71%）and forest land（1.65%）,the lowest level is observed in grassland（1.37%）,which is higher than that of dry land（1.48%）.The highest coefficient of variation was in paddy field（15.13%）,which is higher than that of woodland（14.61%）,garden field（10.08%）and dry land（9.77%）,and the lowest is grassland（4.29%）,which is closely related to soil texture,topographic factors,fertilization and human disturbance.In addition to high organic background value content in gardens and paddy fields,fertilization will be applied to increase yield;the organic matter of the forest land comes from the decomposition and accumulation of the litter layer on the surface,but the soil erosion is more serious due to the higher elevation of the forest land,so the TOC content is at a medium level;the dry land soil is dominated by brown soil,which is very sandy,the nutrient content is low and the soil and water loss in the dry land is serious.In addition,the influence of man-made disturbances such as farming activities in the dry land accelerates the mineralization of organic matter,so the TOC content is low.Among the environmental factors such as topographic factors,land use types,soil types and soil genus,elevation is the main driving factor affecting TOC content.