Driving Mechanisms Of Plant-Soil Feedbacks On Grassland Community Succession Of Loess Hilly Region

Studying the plant-soil feedback of species in different stages of succession is crucial for revealing the driving mechanism of plant community succession.However,the soil legacy effects on plant growth in the process of plant-soil feedbacks and the direction,strength and stability of plant-soil feedbacks were fragmentary.In this study,we selected three dominated species(Setaria viridis,Stipa bungeana,and Bothriochloa ischaemum)and three sub-dominated species(Artemisia sacrorum,Achillea capillaris,and Artemisia giraldii)on the Loess Plateau as the objects.Through three years of plant-soil feedback experiment,we focused on analyzing the characteristics of growth of plants,soils and enzymes changes in different succession stages,and the mechanism of grassland community succession driven by plant-soil feedback,as well as the response of plant-soil-microbe stoichiometry and nutrient limitations to plant-soil feedbacks.The main findings are as follows:(1)Soil microbial activity significantly affects the plant growth effects.Specifically,the aboveground biomass of the early-stage(early-)species in the sterilized soil in the three succession stages accounted for 21.2%,20.0% and 18.1% of that in the non-sterilized soil,respectively.The aboveground biomass of the species in the middle-stage(mid-)of the sterilized soil in the three succession stages accounted for 40.8%,26.2% and 38.0% of that in the non-sterilized soil,respectively.The aboveground biomass of the late-stage(late-)species in the sterilized soil in the three successional stages accounted for 54.7%,21.1% and 45.5% of that in the non-sterilized soil,respectively.At the same time,the plant-soil feedback effect still occurred in the sterilized soil,because the plant biomass in the early-sterilized soil was overall higher than that in the mid-and late-sterilized soils.This indicated that soil enzyme activity and soil nutrient contents play an important role in plant growth.(2)Soils at different successional stages significantly affected the plant-soil feedback effects of early-,mid-and late-species.Early-species had higher plant biomass in early-soil(first growth cycle)or soil disturbed by early-species(second growth cycle).Mid-species had higher biomass in early-soil or in soil disturbed by mid-species.Late-species had higher biomass in late-soils or soils disturbed by latespecies.At the same time,soil legacy effects(nutrient content and enzyme activity)significantly affected plant growth effects,because soil legacy effects produced by early-species in the previous succession stage promoted the growth of plants in adjacent stages(mid-species),but inhibited the growth of intergenerational(latespecies)plants.Additionally,the soil legacy effect produced by late-species inhibited the growth of early-and mid-species.(3)Plant-soil feedbacks were significantly affected by plant growth period and their temporal effects varied greatly among different species.The aboveground biomass of early-species gradually decreased with the plant growth cycle,while the aboveground biomass of late-species increased first and then decreased,and the aboveground biomass of mid-species was not affected by the plant growth cycle.At the same time,the aboveground biomass and plant height of early-and late-species were overall positively correlated with soil nutrient content and enzyme activity,while the aboveground biomass and plant height of mid-species had overall no significant correlation with soil nutrient contents and enzyme activity.In addition,the plant growth cycle overall restricted the growth of the three sub-dominated species,which was mainly attributed to the reduction of soil nutrient content and microbial biomass.(4)Plant-soil feedbacks significantly affected plant-soil-microbe stoichiometric characteristics,but the effects varied by plant species and soil types.The plant growth cycle mainly affected the plant C:N of the early-and late-species.It showed that the plant C:N in early-soil first increased and then decreased with the plant growth cycle.Soil C:P and N:P of the three succession stages overall increased in the third growth cycle,while soil enzyme C:N,vector angle and vector length overall decreased in the third growth cycle,and the vector angle was all less than 45°.Meanwhile,plant,soil and enzyme stoichiometry showed overall insignificant correlations among the three dominated species.(5)The direction,strength and stability of plant-soil feedbacks varied greatly among different species.On the succession sequence,the plant-soil feedback for earlyspecies was positive,with feedback coefficients ranging from 0.02 to 1.14.Plant-soil feedbacks for mid-and late-species were negative and positive,with feedback coefficients ranging from-0.09 to-0.12 and 0.03 to 0.31,respectively.In the temporal sequence,the plant-soil feedbacks of early-species and mid-species were negative and neutral,respectively,while the plant-soil feedbacks of late-species changed from positive to negative.Meanwhile,the plant-soil feedbacks of early-and late-species showed weaker stability,while the mid-species had stronger stability.In addition,plant C:N,C:P and N:P of early-and later-species and enzymatic C:N,C:P and N:P of three species exhibited absolute steady state.Besides,there was no significant correlation between C:N:P stoichiometric ratios and plant biomass.The above results indicated that the plant-soil feedback of early-species was negative and weakly stable,while the plant-soil feedback of mid-species had strong stability.The plant-soil feedback of the late-species changed from positive to negative and had weaker stability.The positive plant-soil feedback effect of late-species indicated that late-species had obvious advantages in herbaceous communities,but this advantage gradually weakened with time.This study emphasizes the importance of plant-soil feedback in promoting the succession of herbaceous communities,and there is no significant correlation between plant-soil-microbe stoichiometry and plant-soil feedback.Our results enhance the understanding of the driving mechanisms of plantsoil feedback on community succession,and provide a scientific basis for vegetation restoration and stability on the Loess Plateau.