Spatial And Temporal Characteristics Of Grassland Dynamics And Soil Erosion In Ningxia Before And After Grazing Prohibition

Grasslands have powerful soil and water conservation functions.The main cause of grassland degradation is overgrazing.Since the implementation of the grassland ecological protection policy of grazing prohibition in natural grasslands throughout the territory,the growth of grassland in Ningxia has improved.Accurately assessing the dynamics of grassland soil erosion before and after grazing prohibition and revealing its driving mechanism are the basis for risk management of grassland soil and water loss.In this study,the spatio-temporal information of Ningxia was reconstructed based on satellite remote sensing data,meteorological station data and hydrological station data.Investigated the spatial differentiation mechanism of grassland normalized vegetation index（NDVI）in Ningxia from 1988 to 2018,analyzed the relative contributions of climate change and human activities to NDVI changes,and projected the future trend of grassland variations.The long term soil erosion in Ningxia grassland was simulated by integrating and calibrating the transport limited sediment delivery（TLSD）function with the revised universal soil loss equation（RUSLE）model.The differential mechanisms of soil loss were explored and the relative effects of precipitation changes and human activities on grassland soil erosion were investigated using double mass curves.The Revised Wind Erosion Equation（RWEQ）was applied to quantitatively evaluate the spatio-temporal variations of soil conservation service function during 31 years before and after grazing prohibition in Ningxia.（1）Characteristics of vegetation restoration before and after grazing prohibitionExcept in winter,the annual,seasonal and monthly average of NDVI values after grazing prohibition were higher than those before grazing prohibition.The level of change trend was mainly a slow increase.The proportion of medium coverage increased by 16.15%after grazing prohibition,which mainly occurred in the path of“lower coverage grassland→medium coverage grassland”.The transformation trend is dominated by positive evolution,and the ecological construction project has achieved remarkable results.After grazing prohibition,vegetation restoration in some areas showed negative feedback.After grazing prohibition,the growth rate decreased by17.91%,but the degradation rate increased by 3.92%.After the grazing prohibition,53.22%of the total area were affected by human activities and climate variations,and the relative contribution of human activities decreased in the NDVI improved area,but increased in the NDVI degraded area.The main factors affecting NDVI differentiation of grassland in Ningxia are snow depth,potential evapotranspiration,radiation and precipitation.After grazing prohibition,the explanatory power of each factor and the interaction between the factors decreased significantly,but the explanatory power of wind speed improved greatly.The suitable range or types of NDVI of some factors were different before and after grazing prohibition,mainly reflected in wind speed,temperature and soil type.（2）Characteristics of water erosion before and after grazing prohibitionThe measured sediment discharges from six hydrological stations verified that the RUSLE-TLSD model can reliably simulate water erosion in Ningxia.From 1988to 2018,the water erosion rate of grassland in Ningxia ranged from 14.98 to 74.98t·ha^-1·a^-1,showing an overall downward trend.July to September is the period with the highest risk of water erosion.Slope is a key factor affecting the spatial distribution of water erosion.After grazing prohibition,the explanatory power of the combination of slope and NDVI is enhanced.The combined effects of climate change and human activities after grazing prohibition led to an increase of 24.49%in the risk of net water erosion,mainly distributed in the ecological extension area of Helan MEEA.The increase in the risk of net water erosion caused by human activities was reduced by25.82%,mainly distributed in the Liupan MEEA.With the implementation of the"Grazing Prohibition"plan,Ningxia’s grasslands,especially the Liupan MEEA,have recovered significantly,and their ability to resist erosion has been enhanced.In Ningxia,the management and control of water erosion focuses on the prevention and control of soil erosion in different landforms and the protection of grasslands with slopes greater than 10°.（3）Characteristics of wind erosion before and after grazing prohibitionBefore grazing prohibition（from 1988 to 2003）:the average soil wind erosion modulus was 5.59 kg·m^-2.After grazing prohibition（from 2003 to 2018）:the average soil wind erosion modulus was 1.45 kg·m^-2.After grazing prohibition,the area of the intensity of soil wind erosion（6 levels in total）in micro and slight decreased by14281 km²,the proportion of which increased to 73.52%,wind erosion intensity decreased.Before grazing prohibition,the average sand-fixing function was 25.49kg·m^-2·a^-1.After grazing prohibition,the average wind prevention and sand-fixing function was 14.3 kg·m^-2·a^-1.After grazing prohibition,the level of windbreak and sand fixation in grassland was mainly medium,with an area of 7382 km²increased to23.65%.From 1998 to 2018,the retention rate of windbreak and sand fixation in Ningxia grassland showed an increasing trend,ranging from 73.2%to 96.7%.Before grazing prohibition,the average retention rate of windbreak and sand fixation function was 81.24%,after grazing prohibition,the average was 91.9%.It shows the contribution of grassland to wind prevention and sand fixation in Ningxia was gradually being improved and the ecological construction project had achieved remarkable results.Therefore,regional governance should be strengthened and differentiated schemes should be implemented in the future.