Ecological Processes Of Above- And Below-ground During Grassland Natural Restoration In Semi-arid Region

Natural grassland is the main vegetation type on the Loess Plateau.The stability and sustainability of ecological function in natural grassland are of great significance to regional climate regulation,soil and water conservation and biodiversity conservation.The multifunctional maintenance of the natural grassland requires a better understanding of its mechanism from different perspectives.To explore the underlying mechanism of the above-and below-ground ecological processes and their interactions during natural grassland restoration can further understand grassland ecological functions.In this study,we investigated the typical grassland in the semi-arid area of the Loess Plateau.The data from field investigation,sampling and multiple statistical analysis methods were used to determine the effects of species composition and functional groups on productivity and species diversity during natural restoration of grassland communities.The influences of species diversity on soil organic carbon and soil water infiltration capacity were discussed.The trade-off of four ecosystem servicescommunity productivity,species diversity,soil organic carbon storage,and soil water storage were analyzed in the process of grassland natural restoration.The trade-off relationships between four ecological services were established to determine the optimal time for restoration of natural grasslands in semi-arid area.The results could provide a scientific basis for the restoration and management of grassland vegetation on the Loess Plateau.The main conclusions of this study were showed as follows:(1)The productivity and species diversity remained relatively high level throughout the restoration process,while the composition of the grassland community changed significantly during the succession of typical grasslands in the semi-arid region of the Loess Plateau.The species composition was influenced by species of different heights at different restoration stages.With the increase of restoration time,the height of main species influencing the community composition is also increasing.The grass functional group was the dominant functional group of grassland on the Loess Plateau,and it was the main driving force for the maintenance of natural grassland productivity.The maintenance of species diversity was dominant by forb functional group in the early stage of grassland restoration,but with the increase of the restoration time,the community changed from being dominated by grass and forb functional groups to being dominated only by the grass functional group.The dominant functional group completed the transition from forb functional group to grasses functional group during the whole recovery process.(2)Grazing exclusion significantly promoted the accumulation of surface litter in grassland community,and the increase of litter effectively inhibited soil moisture evaporation and increased soil water holding capacity,hence there was a significant positive correlation between litter biomass and soil water content.Species diversity increased soil organic carbon input by promoting above-ground biomass,changing community structure,surface litter and root systems,thus effectively increased soil water retention.Species diversity could account for 49% of the variation in soil organic carbon sotrage.(3)Grazing exclusion significantly promoted the accumulation of root on the surface of 0-20 cm soil layer.In the process of natural grassland restoration,the species diversity has a significant correlation with the average weight diameter of soil aggregate,the total soil porosity and soil water infiltration.Species diversity promoted the increase of soil water stability aggregates,improved soil structure,and enhanced the ability of the soil particles to resist water dispersion by affecting the distribution of below-ground root system and soil organic carbon content.At the same time,diversity increased the soil total porosity and improved the infiltration capacity,which was beneficial to soil reservoirs replenishment.Species diversity can explain the variation of soil infiltration capacity by 26%.(4)The trade-off relationship among the ecological services during the restoration process of natural grassland showed different changes in each plot.In general,the tradeoff relationship between productivity and diversity was relatively balanced in grazing grassland.The relative benefits of different ecosystem services were showed the alternating changes of diversity-productivity-diversity with the increase of restoration time.The relative benefits of productivity,diversity and soil organic carbon storage was increased and the relative benefits of soil water storage was increased significantly by grazing exclusion.When the grassland is naturally restored for 16 years,the trade-off values of community productivity-species diversity,soil water storage-soil carbon storage is minimized,the community production function and ecological function reached a trade-off point.Our results suggested that the restoration of natural grassland on the Loess Plateau should be moderately utilized in the 16 th year.