Study On Productivity Degradation Risk Of Small Watershed Based On Soil Loss Tolerance Of Different Land Types

As the main grain production base in China,with the rapid development of economy,the pressure of land output increases.Under the high-intensity input and output,soil productivity faces the risk of degradation.Exploring whether there is a risk of soil productivity degradation in red soil region is very important for the development of agricultural economy in China.Among them,the soil loss tolerance（T value）is an important index for the evaluation of regional soil productivity degradation.After years of soil and water loss control,the trend of soil and water loss in red soil area has been reversed.However,at present,the standard for the soil loss tolerance in red soil hilly region in South China is the unified value of 500t/km²·a issued by the Ministry of water resources.So far,many studies have shown that there is a gap between the T value of specific areas in red soil region and the unified value issued by the state.Therefore,exploring the T value of different land types in red soil hilly region is the key to evaluate the risk of local soil productivity degradation.In this study,the Tangbei River watershed,a typical small watershed in the red soil region of South China,is taken as the research object.Based on the composite fingerprint technology,the main sediment sources of the rainstorm process in the study area are calculated,and the soil and water loss in the small watershed is preliminarily understood;The Modified Productivity Index（MPI）model was used to calculate the T value of different land types in the study area;At the same time,combined with The Revised Universal Soil Loss Equation（RUSLE）,according to the relationship between T value and actual soil erosion intensity in the study area,the degradation risk of soil productivity of different land types is analyzed.On this basis,according to the main sources of sediment in the process of rainstorm erosion in the study area,the areas with high degradation risk of soil productivity in small watershed are identified.The main conclusions are as follows:（1）The main land types in the study area are forest land,orchard and dry cultivated land,which account for 65%,13.2%and 14.6%of the total drainage area respectively.Through the nonparametric Kruskal waillis H test,six fingerprint factors such as TC,TN,TP,CA,Cr and Dy were preliminarily screened.After multivariate discriminant analysis,Ca and Dy were the best combination of fingerprint factors.The sediment contribution rates of forest land,orchard and dry cultivated land calculated by multivariate mixed model are 0.9%～1.9%,97.5%～98.9%and 0.2%～0.6%respectively,indicating that the main sediment source of rainstorm process in the study area is orchard.（2）Aiming at the two main types of production land,orchard and dry cultivated land,the T value is calculated by MPI model.The results show that the T value of the whole small watershed is 68～604t/km²·a,and the average value is 176t/km²·a;The T value of dry cultivated land is 79～604t/km²·a,and the average value is 275t/km²·a;The T value of orchard is 68～197t/km²·a,and the average value is 102t/km²·a.The soil productivity index MPI₀in the study area is 0.17～0.49,with an average value of 0.33;The soil erosion vulnerability index V is-0.0082～-0.0007,and the average value is-0.0042.The correlation between T,MPI₀and V was analyzed.It was found that T and V showed a very significant positive correlation（p<0.01）,and the correlation coefficient was 0.7,The correlation between T and MPI₀is weak.（3）The results show that the average soil erosion modulus of Tangbei river small watershed is 427t/km²·a,among which the average erosion modulus of forest land,orchard and dry cultivated land are 217.3t/km²·a,1742.5/km²·a and 405.2t/km²·a respectively,and the erosion intensity of orchard is the largest.When the actual soil erosion intensity in the study area exceeds the T value,there will be a risk of soil productivity degradation.Through the comparison between the T value and the current soil erosion modulus,the area with productivity degradation risk in the study area is3.96km²,accounting for 24.2%of the total area of the study area.Among them,the area with degradation risk in forest land accounts for 18.6%of the total area of forest land,the area with productivity degradation risk in orchard accounts for 51.4%of the total area of orchard,and the area with productivity degradation risk in dry land accounts for 36.4%of the total area of dry land.Orchards are high-risk areas for soil productivity degradation.