| With the implementation of a series of restoration projects such as“Grain for Green”,the ecological environment of the Loess Plateau has been greatly improved.In the present,major strategies such as"Two Mountains Theory","Carbon Peaking and Carbon Neutrality Goals","Beautiful China Initiative"and"Ecological Conservation and High-quality Development of the Yellow River Basin"have been put forward successively.Chinese government is paying more and more attention to the ecological environment of the Loess Plateau,and the ecological functions associated with it have received more attention.Small watershed is the basic unit for comprehensive ecological environment management on the Loess Plateau.It is of great scientific significance and practical value to study the evolution of land use and soil process in small watershed,evaluate the long-term change and trade-off relationship among soil functions,and simulate their changes in different scenarios.Therefore,this paper takes Zhifanggou watershed in loess hilly-gully region as the study area through field sampling by grids,literature data synthesis,model evaluation,and scenario simulation,which focuses on the land use evolution（1938-2020）,soil carbon,nitrogen and water dynamics at depths and driving factors,soil function assessment and scenario simulation.This study aims to reveal the mechanism of the formation and evolution of the small watershed ecosystem services in the loess hilly-gully region,and thus provide a scientific basis for the promotion of ecosystem service.The results are as follows:（1）From 1938 to 2020,the ecological environment and land use structure of Zhifanggou watershed changed significantly,which could be roughly divided into the following four stages:Ecological damage period（1938-1984）,Initial recovery period（1985-1999）,Stable recovery period（2000-2009）and Benign cycle period（2010-present）.From 1938 to 1958,the introduction of national policies drove the significant changes of land use,with the proportion of woodland decreased sharply from 61.9%to 0.1%,and that of cropland increased from 10.0%to 55.9%with a change rate of 23.1%·yr-1.Until 1985,the ecological environment of the small watershed was in a period of continuous destruction.Since 1985,comprehensive management has been carried out in Zhifanggou watershed,and the development of science and technology has become an important driving force of land use change.In the 21st century,with the large-scale development of the“Grain for Green”project,the change of land use structure in Zhifanggou watershed gradually decreased and entered a stable recovery period.National policy and soil and water conservation engineering technology become the key factors to maintain the land use pattern in the small watershed.After 2010,large-scale restoration was completed,and the ecological environment of the small watershed began to enter a virtuous cycle stage.At present,the area of woodland,shrubland,and grassland accounts for 45.0%,6.9%and 38.2%respectively,while the area of cropland and orchard accounts for 5.1%and4.8%respectively.National policy,basically stable industrial structure and economic income are important factors to ensure the stability of ecological environment and land use structure of small watershed at this stage.（2）Ecological restoration of small watersheds significantly increased soil carbon and nitrogen content in both shallow and deep soil layers,the largest increase was found in shrubland（58%and 78%,respectively）and the smallest increase in grasslands（17%and 42%,respectively）.In addition,soil carbon and nitrogen sequestration in the 0-20 cm of different land use types increased with restoration ages,with the highest rate of increase in woodland(0.058 yr-1 for both carbon and nitrogen).In the soil layer below 20 cm,the soil carbon and nitrogen sequestration of orchards showed higher advantages,with the 100-200 cm soil layer increasing 1.1 and 0.9 times more than that of woodland,respectively.Different land use types have long-term soil carbon and nitrogen sequestration potential which is more obvious in woodland and orchard.Furthermore,the coupling interaction of soil carbon and nitrogen sequestration in woodland,shrubland and grassland was higher than that in orchards and cropland in the 0-20 cm layer.Furthermore,the coupling interaction gradually decreased with the soil depth,and was overall lower than that in orchards and cropland finally.From the perspective of increasing soil carbon and nitrogen sequestration,it is recommended to plant planted forests in locations with smaller slopes,while grassland and shrubland restoration may be preferred in locations with larger slopes at the scale of small watersheds.Vegetation allocation in areas with lower initial soil carbon and nitrogen content may have higher soil carbon and nitrogen gains.（3）Compared to sloping cropland,woodland and shrubland had the highest soil moisture decrease（31%to 37%）,especially in soils below 100 cm;grassland showed no significant change in soil moisture,while orchards and cropland exhibited a higher water storage capacity（28%and 31%）.With the increase of restoration ages,the soil moisture in woodland showed a trend of decreasing and then increasing,and the soil layer below 20 cm in shrubland had similar trends,and the turning points occurred between 20 and 35 years.Soil moisture did not change significantly in the grassland,while it increased significantly with ages in the orchard.At small watershed,soil moisture changes were influenced by both topographic factors such as slope,aspects,and vegetation types.The greater the slope,the lower the soil moisture at different depths in woodland and shrubland,while soil moisture in grassland increased with slope but did not change significantly in deeper soils.The higher the initial soil moisture,the greater the soil moisture decrease in woodland,shrubland and grassland,while orchard and cropland were not sensitive to the initial soil moisture.（4）The unit soil functions of different land uses in the watershed were different.The differences in soil carbon storage functions（SCSF）were small between all land use types,but the soil conservation functions（SCF）per unit area of cropland and orchard were lower than those of other land uses by 24.3%to 66.3%;the water regulation functions（WRF）per unit area of woodland,shrubland,grassland and orchard were lower than those of cropland by 75.3%to 97.9%,and nitrogen output by 74.1%to 87.8%.Topographic factors have a greater impact on WRF,water purification（WPF）and SCF.The areas with high fragmentation of landscape pattern and diverse types have relatively higher SCSF and WRF and lower WPF;while areas with more homogeneous landscape types have higher SCF.Among the soil functions in the watershed,the trade-off relationships between WRF-SCSF and WRF-SCF were the strongest,accounting for 53.1%and 59.6%,respectively.The relationships between WRF-WPF and SCSF-SCF were mainly transitional and coordinated development.And the relationships between WPF-SCSF and WPF-SCF were relatively coordinated,accounting for 96.6%and73.2%of the watershed area,respectively.（5）From 1938 to 2020,except the WRF,other soil functions of SCSF,WPF,and SCF in the watershed experienced four stages with land use changes:Ecological destruction period,Initial recovery period,Stable recovery period and Virtuous cycle period.The change trend of WRF was generally opposite,and there was an obvious trade-off relationship with other functions.In the last 10 years,all soil functions have stabilized.The scenario simulation analysis showed that compared with the scenario of status quo,grassland grazing,and woodland thinning increased the overall function level of watershed by 3.6%and 18.2%,respectively,which mainly increased the WRF by 79.1%and 262.2%.At the same time,the two scenarios also significantly improved the trade-off relationships,especially that between the WRF and other soil functions.In conclusion,the comprehensive management of small watershed changed the land use type and significantly increased soil carbon and nitrogen sequestration in the deep profile,but the woodland and shrubland aggravated soil water consumption.Land use type,soil depth,restoration age,topographic factors and initial soil carbon,nitrogen,and water content were the key factors affecting soil carbon,nitrogen,and water dynamics in small watershed.In addition,soil functions in small watershed showed phased changes with land use evolution,and there was an obvious trade-off between water regulation and other soil functions.Thinning and grazing could promote the coordinated development of soil function in small watershed to some extent.Overall,the results can provide a scientific basis for the improvement of ecosystem services in small watershed on the Loess Plateau. |
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