| With the increase of global population and the acceleration of urbanization,cultivated land resources are increasingly scarce,and agricultural production is faced with many difficulties and challenges.At the same time,climate change,environmental pollution and other issues have also brought new challenges and threats to food security and farmland protection.Therefore,in order to ensure food security and sustainable utilization of cultivated land resources,effective measures must be taken to protect and manage cultivated land.Among them,the protection and improvement of soil organic matter content is one of the important measures to ensure food security and farmland protection.The study on the temporal and spatial distribution characteristics of soil organic matter can not only provide scientific basis for ecological environment protection and agricultural sustainable development,but also help to improve the efficiency of land resource utilization and precise fertilization,and further promote the process of agricultural modernization.On the other hand,it can also make scientific policies on the use of cultivated land and land protection,improve soil organic matter content,protect the quality of cultivated land and ensure food security.As an important part of soil,soil organic matter plays an important role in soil fertility,water retention ability and ecosystem stability.In this paper,the spatial distribution information of soil organic matter content was retrieved using remote sensing technology,and the temporal and spatial changes of soil organic matter content in different periods were analyzed.It is of great significance to evaluate soil fertility,guide agricultural production,protect cultivated land and maintain ecosystem stability.This study takes the Bianshi River Basin of Youyi County as the research area.The soil erosion modulus in 1993,2003 and 2020 were calculated by ULSE,and the change of soil erosion intensity and its causes were analyzed.Using Landsat series satellites,rainfall erosivity factor was introduced as auxiliary variable.Quantitative inversion of soil organic matter content in cultivated land in 1993,2003 and 2020 was conducted using stochastic forest model.The temporal and spatial changes of soil organic matter content were analyzed from the aspects of cultivated land area,precipitation,river distribution,soil erosion,topography,soil texture,human factors and policy factors.Based on the research results,some suggestions were put forward to improve soil organic matter content from the aspects of rational fertilization and soil erosion control.The main conclusions of this paper are as follows:(1)The Soil erosion modulus of cultivated land in 1993,2003 and 2020 was calculated by using the Universal Soil Loss Equation(USLE),and the soil erosion modulus was classified according to the Classification and Classification Standard of Soil Erosion(SL190--2007).The grade maps of cultivated land erosion intensity in 1993,2003 and 2020 were obtained.Through the comparative analysis of soil erosion intensity in the past 30 years,it is concluded that the overall soil erosion intensity in the Bianshi River watershed presents a trend of continuous enhancement.From 1993 to 2003,the area of mild erosion increased by 3474.81 hectares,the area of mild erosion increased by 116.28 hectares,the area of moderate erosion increased by 6.84 hectares,the area of strong erosion increased by 0.72 hectares,the area of very strong erosion increased by 1.98 hectares,and the area of severe erosion increased by 0.27 hectares.From 2003 to 2020,the area of mild erosion decreased by 4067.73 hectares,that of mild erosion increased by 2916.18 hectares,that of moderate erosion increased by 275.4 hectares,that of strong erosion increased by 73.89 hectares,that of very strong erosion increased by 34.02 hectares and that of severe erosion increased by 27.45 hectares.Based on the analysis of the variation characteristics of soil erosion intensity from the perspectives of natural factors and human factors,it is concluded that the overall intensification of soil erosion intensity in the Bianshi River watershed is caused by natural factors such as precipitation,terrain,soil type,irrational land use and human factors such as reuse of light cultivation habits.(2)In order to improve the correlation between spectral reflectance and Soil Organic Matter(SOM),this study carried out mathematical transformation and algebraic operations such as addition,subtraction,multiplication and division for different bands.Based on Landsat 8 OLI image data,A total of 88 spectral indexes of 9 types were constructed,including reciprocal,logarithm,logarithm of reciprocal,reciprocal of logarithm,difference between two bands,sum of difference between two bands,ratio of two bands,slope of two bands and tangent value of inflection point of band(tanα).The correlation analysis of the constructed spectral index shows that the band with the highest correlation between the original spectral reflectance and soil organic matter in 2020 is the red band,with the correlation reaching-0.618.After mathematical transformation of the reflectance of each band,the correlation decreases,among which the logarithm and reciprocal logarithm of the red band have the highest correlation.Are-0.615 and0.615,respectively.Among all the spectral indicators,the correlation between band slope and band difference indexes constructed by blue and green bands and blue and red bands is the highest,reaching-0.640.The spectral index constructed in this study greatly improves the correlation of soil organic matter,which can provide ideas for future research.(3)The formation and accumulation of soil organic matter is closely related to precipitation and soil erosion.In order to make the inversion closer to the actual process of soil organic matter accumulation and formation,the rainfall erosion force was introduced as the input,and combined with the constructed spectral index,the stochastic forest prediction model was established.The results show that after the introduction of rainfall erosivity factor,the validation R2 and RMSE of the model are increased by 0.107 and 0.091,respectively,which significantly improves the accuracy compared with the spectral index modeling only.After comprehensive analysis,the spectral index model with rainfall erosion factor is the best model to invert soil organic matter content.(4)Input values of Landsat 8 OLI data and Landsat 5 TM data after cross-calibration were extracted and input into the model.Inversion maps of soil organic matter content in 1993,2003 and 2020 were made.Based on the inversion results,the change trend of soil organic matter in the small watershed of Bianshi River in the past 30 years were analyzed.The results showed that the average soil organic matter content decreased from 4.86% to 4.22% from 1993 to 2003,and from4.22% to 3.81% from 2003 to 2020,showing a continuous decreasing trend.Although the rate of decline slowed down,the quality of cultivated land was still worrying.(5)According to the spatio-temporal variation characteristics of soil organic matter content in the small watershed of Bianshi River,the countermeasures and suggestions for improving soil organic matter content were put forward.On the one hand,based on the long-term reuse and light cultivation habits in the small watershed of Bianshi River,suggestions were put forward from the aspect of rational fertilization.By increasing the proportion of organic fertilizer,variable fertilization was carried out according to the actual nutrient status of soil and the growth characteristics of crops to improve the soil fertility and structure,so as to increase the content of soil organic matter and promote the sustainable development of agricultural production.On the other hand,through the analysis of slope and soil erosion intensity of the Bianshi River watershed,it is found that the slope below 6° is in mild erosion,the cultivated land between 6° and 15° is in mild erosion,the cultivated land between 15° and 25° is in strong erosion,and the area above 25°is in severe erosion.Combined with the slope and soil erosion status of different regions,In order to improve the content of soil organic matter,enhance the productivity of cultivated land and ensure food security,the corresponding prevention and control suggestions of soil erosion were put forward. |
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