Formation Mechanism And Trade-offs/Synergies Of Ecosystem Services In The Zuli River Basin

Climate change,underlying surface and human activities combine to drive the hydrological resources of global ecosystems,causing the evolution of runoff and soil erosion in river systems.Sustainable regulation of ecosystem services is important for water resources utilization,soil erosion control,ecological conservation and human well-being.The Zuli River basin is a first-order sediment-laden tributary of the upper Yellow River,and the soil erosion was 4394.80 t.km^-2.a^-1 before 2000;Serious soil erosion,very fragile ecological environment and incongruous runoff-sediment relationship in the basin;In addition,sustainable and high-quality economic and social development faces serious ecological and environmental problems.Entering a new era,with the implementation of the national strategy for ecological protection and high-quality development of the Yellow River basin,the Zuli River basin is also bound to step into the pace of the era of great governance.Since 2000,with an erosion of 1176.57 t.km^-2.a^-1;the ecological service function has been improved substantially,which provides a favorable opportunity for a comprehensive and systematic study of the formation mechanism and trade-off/synergistic effect of ecosystem services in the Zuli River basin.This study takes the Zuli River Basin as the research object.Based on the long-term observation data of runoff plots in a typical small watershed of Anjiagou,the internal driving mechanisms of climate and underlying surface properties on runoff and soil erosion was discussed by using a combination of regression analysis and path analysis.Based on the principles of water balance,generalized soil loss equation and combined with spatial analysis,the mechanism of coupled effect of runoff and soil erosion was found to be closely related to the runoff service,soil conservation service and carbon storage service of the ecosystem.The ecosystem perspective expands the research of service formation mechanism,revealing the trade-off/synergistic effects and driving mechanisms of ecosystem services.The main research results are as follows.（1）The mechanism of the internal effects of runoff and soil erosion of typical plant measures driven by climate and underlying surface.Rainfall types in the Anjiagou runoff plot were dominated by moderate rain（10～25mm）;Among plant measures,shrub forest had the best performance in reducing runoff and erosion for short-term precipitation event（shorter than 24 h）,but under the long-duration（72 h）precipitation,natural grassland performed the best to prevent the occurrence and development of runoff and soil erosion.Compared with agricultural land,all the studied plant measures had an average erosion reduction rate of 73%,which was about twice the runoff reduction rate（37%）.The factors driving runoff were maximum 30-minute precipitation intensity,rainfall duration,precipitation,antecedent soil moisture,and slope,while runoff,maximum 30-min precipitation intensity,antecedent soil moisture,and slope were the erosion drivers.（2）The hydrological evolution law of the ecosystem in the Zuli River basin and the coupling effect of driving factors on runoff and soil erosion（P<0.01）.The runoff and erosion showed a significant downward trend in the basin,with an average annual runoff and soil erosion of 0.91×10⁴m³.km^-2 and 0.33×10⁴ t.km^-2,respectively.The mutation year was 2000,corresponding to the first main cycle of 20a and 5a,respectively.Compared with the baseline period（1960—2000）,both runoff and soil erosion decreased during the change period（2001—2020）,and the reduction rates were 41.03%and 73.23%,respectively,and the erosion reduction rate was higher than that of runoff reduction.The main driving factors of runoff in the whole time domain are precipitation and vegetation coverage,while runoff,vegetation coverage and precipitation are the key factors driving erosion.（3）The spatial and temporal scale evolution characteristics and transfer status of land use in the Zuli River basin.The Zuli River basin constitutes a composite ecosystem mainly composed of grassland and cultivated land,with grassland being the largest and accounting for more than 50%of the area,followed by cultivated land and forest land.The area of forest land and construction land has increased significantly due to the comprehensive management of soil and water conservation in small watersheds,the return of farmland to forest and the expansion of urban areas;Due to the implementation of the cultivated land protection law,the decrease of cultivated land was not obvious,but the grassland showed a tendency of degradation;cultivated land,construction land and water area are obviously distributed along both sides of the river valley.The most obvious land use transfer is cultivated land←→grassland,grassland←→forest land,cultivated land→forest land and construction land.The implementation of new urbanization and the project of returning farmland to forest and grassland in the basin has been effective,and the transfer will definitely have an impact on the service function of the ecosystem.（4）Evaluation,correlation analysis and spatial heterogeneity of ecosystem services in the Zuli River basin.The runoff service was weakened from 1980 to 2020,while soil conservation service and carbon storage service were on the rise;The runoff service was strongest in 1980—2000 with 8.23mm,but the soil conservation service and carbon storage service were both strongest in 2010—2020with 107.07×10⁶ t and 90.28×10⁶ t,respectively.The spatial pattern of ecosystem services is a clustering pattern（Moran’s I index is higher than 0.6）:runoff services>carbon storage services>soil conservation services;The spatial clustering of runoff services is high in the south and low in the north,while soil conservation services and elevation,carbon storage services and land use divergence patterns are more consistent,showing clustering in the east,south-central and along the river valley.（5）The effect of ecosystem services with land use and topography,respectively.Grassland runoff yield,soil conservation service and carbon storage service were the highest among different land uses at different times,followed by cropland and forest land;meanwhile,soil conservation rate and carbon density were the highest in forest land and grassland.The runoff service,soil conservation service and carbon storage service all increased first and then decreased with the increase of slope,and the ecosystem service was the beat on the slopes of 8～15°and 15～25°.With the elevation of altitude,the runoff service increased,while the soil conservation service and carbon storage service increased first and then decreased,and the ecosystem services was the strongest in the areas of 1500～2000 m and 2000～2500 m above sea level.（6）Trade-off/synergistic effects and driving mechanisms of ecosystem services.Runoff and soil conservation services,soil conservation and carbon storage services are synergistic on time scale,but runoff and carbon storage services are trade-off effects.However,the spatial pattern of runoff and soil conservation service,soil conservation and carbon storage service is dominated by synergistic effect,accompanied by trade-off effect;runoff and carbon storage service are mainly trade-off effects with synergistic effects,and the spatial differentiation of trade-off/synergistic effects is obvious.Geodetector-based precipitation measurements jointly drive runoff services,soil conservation services,and carbon storage services,correlating the different ecosystem services.The interaction of ecosystem services was significantly stronger than that of single factor.Precipitation and vegetation coverage synergistically drove runoff and carbon storage services,slope synergistically drove soil conservation services,and rainfall tradeoff drove soil conservation services.In summary,this study carried out the coupled effect mechanism of climate,underlying surface and human activities combined to drive runoff and soil erosion from ecosystem service formation mechanism.Moreover,the driving mechanism is closely related to the ecosystem services of runoff,soil conservation and carbon storage.Using the Anjiagou watershed runoff plots as the base point for the study of the intrinsic formation mechanism of ecosystem services,it was found that the main factors that jointly drive runoff and soil erosion are maximum 30-minute precipitation intensity,antecedent soil moisture,runoff drivers increase precipitation,rainfall duration,and there is a coupling effect of runoff and soil erosion.To further evaluate the ecosystem service trade-off/synergistic effects,it is also necessary to study the driving mechanism of runoff and soil erosion in the Zuli River basin,so as to provide validation,theoretical and basic data support for ecosystem service model evaluation.In the full-time domain precipitation and vegetation coverage are the main driving factors of runoff,while runoff,vegetation coverage and precipitation are the main driving of soil erosion.Ecosystems with weakened runoff services,enhanced soil conservation services and carbon storage services,precipitation and vegetation coverage synergistically drive runoff and carbon storage services,slope synergistically drives soil conservation services,and precipitation trade-offs drive soil conservation services.The driving forces of different ecosystem services are different,but they are all driven by precipitation,and different ecosystem services are associated through precipitation.Through rational configuration and optimization of plant landscape patterns,synergistic effects can be effectively enhanced to ensure high-quality development and maintenance of multifunctional ecosystem services in the Zuli River basin.