| In order to prevent serious soil erosion and improve the regional ecological environment,a large-scale project of returning farmland to forests(grass)has been carried out in the Loess Plateau in China since 1999.In the process of vegetation restoration,a large amount of artificial vegetation is the main vegetation type.Among them,alfalfa(Medicago sativa)is widely cultivated as a high-quality forage due to its advantages such as drought resistance,barren tolerance,soil conservation,and high yield.At the same time,it has a large biomass,is a perennial forage,and has characteristics such as strong evapotranspiration and high water consumption.After continuous planting for many years,a large amount of soil moisture will be excessively consumed,causing the ecological environment to deteriorate.On the other hand,with the rapid development of urbanization in the Loess Plateau,the large-scale migration of rural populations to cities and towns has led to the abandonment of a large number of alfalfa grasslands.In this paper,the grassland communities at different succession stages in the arid field deforestation process are taken as the research object.Through the combination of field surveys and indoor analysis,the above-ground and underground biomass,litter biomass,soil moisture,and soil organic matter of the communities at different succession stages are studied.Carbon,soil total nitrogen,and other indicators were studied.The characteristics of community biomass,functional group composition,and changes in underground biomass distribution during the restoration succession of abandoned land were analyzed.The changes of soil moisture during the restoration succession of abandoned land were analyzed.,Revealing the trade-off relationship between the five main ecological functions of species diversity,above-ground carbon storage,soil water storage,soil organic carbon storage and soil total nitrogen storage in the process of natural succession of vegetation restoration grassland,so as to provide scientific basis for vegetation restoration on the Loess Plateau.(1)Through the study of the above-ground and underground biomass,litter biomass,and root-shoot ratio of four grassland communities during the alfalfa field abandonment process,it was found that the above-ground biomass,underground biomass,and litter biomass of different grassland communities are significantly different(P<0.05).Among them,the above-ground biomass of alfalfa as the dominant species community was significantly higher than the other three community types(P<0.05),and the above-ground biomass of the three community types of Medicago sativa+Leymus secalinus community,Leymus secalinus community and Stipa bungeana community Significantly(P>0.05).In the composition of functional groups,the proportion of legumes in the alfalfa community was significantly higher than that of other communities(P<0.05);The proportions of grasses and weeds showed an upward trend,and the percentages of grasses in Leymus secalinus community and Stipa bungeana community were significantly higher than that in the other two communities.It is shown that with the restoration of grassland communities,grasses and weeds are the main body of community biomass.The underground biomass of four grassland communities decreased with the increase of soil depth.Among them,the underground biomass of 0～20 cm soil layer is the largest,accounting for more than 50% of the total underground biomass of all kinds of grassland.The results showed that the root shoot ratio of Alfalfa community was the highest(3.71).In the process of grassland community restoration,the ratio of root to crown decreased gradually.(2)Dynamically monitor the soil moisture of the alfalfa(Medicago sativa)community,Medicago sativa+Leymus secalinus community,Leymus secalinus community and Stipa bungeana community in the growing season of 2016～2018.It was found that during the succession of abandonment,soil moisture increased first and then decreased with the extension of community recovery time.The significant difference in soil moisture between the drought years and the water years(P<0.05)indicates that the relationship between precipitation and soil moisture is closely related.In the process of abandonment,the soil moisture from 0 to 0.4 m was greatly affected by precipitation,and there was no significant difference in soil moisture among communities.It shows that the soil layer can replenish the soil moisture of different communities through precipitation.Precipitation has a certain effect on soil moisture at a depth of 0.4～1 m,and there are significant differences in soil moisture between different grassland communities(P<0.05);there is no significant change in soil moisture over time at a depth of 1～1.8 m,indicating that precipitation has a significant effect on deep soil moisture.No effective supplement was made,and the soil moisture content was relatively stable.The soil moisture in the 2～5 m deep layers of different grassland communities showed an increasing trend with the increase of soil layer depth.At the same time,the soil moisture of grassland communities below 2 m at the same depth were significantly different(P<0.05).The above research shows that the deep soil moisture is restored to a certain extent by the abandonment process.(3)In the process of natural restoration of abandoned alfalfa land,the changes of five ecosystem functions including species diversity,aboveground carbon storage,soil water storage,soil organic carbon storage and soil total nitrogen storage,as well as the trade-off and synergy between ecosystem functions.The results showed that: the relative benefits of five ecosystem functions were significantly different with the process of natural restoration of abandoned alfalfa land;the relative benefits of species diversity increased significantly with the community succession;the relative benefits of aboveground carbon storage of grassland community decreased first and then increased;the relative benefits of soil water storage increased first and then decreased with the community restoration However,the relative benefits of soil organic carbon and total nitrogen decreased first and then increased.In the balance of grassland ecosystem function,the balance between SOC and STN is the lowest,indicating that there is a stable synergy between the two ecosystem functions;the balance between species diversity and SOC,as well as between species diversity and STN is medium;the highest balance is related to the above ground carbon and soil water reserves Ecological function index.When the natural recovery of Alfalfa wasteland to Leymus secalinus community,the trade-off between the functions of Leymus secalinus community is the smallest(the most balanced),and the multi-function of Stipa bungeana community can be restored. |
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