| Degraded grasslands are often accompanied by the invasion and spread of poisonous plants,in addition to the substantial reduction of productivity,vegetation coverage and plant diversity.It is generally believed that the widespread distribution of poisonous plants will have a negative impact on the function of grassland ecosystem.Some poisonous plants can directly or indirectly change the interspecific relationship of plants by secreting allelochemicals,and then affect the growth of their neighboring plants.However,it is rare to detect allelopathic effects of poisonous plants on coexisting grasses in the field.Soil microbes can mediate plant allelopathy in a variety of ways,and then change the positive or negative effects of poisonous plants on neighboring plants and plant communities.However,little is known about the effects and underlying mechanisms of poisonous plants on the growth of neighboring plants,rhizosphere microorganisms,and soil physicochemical properties of poisonous plants.Previous studies have shown that the poisonous plants can increase local soil nutrients concentration,which may improve the growth environment of neighboring plants,and then alter the plant diversity of grassland communities.In addition,soil heterogeneity is the basic factor affecting the spatial distribution and plant-soil interaction.Soil heterogeneity is also closely related to the invasion and spread of poisonous plants in grasslands.However,the effects and mechanisms of increased coverage or density of poisonous plants on plant community characteristics,soil nutrient concentration and spatial heterogeneity are currently uncertain.Stellela chamaejasme is a perennial herb with woody taproots,belonging to the Thymelaeaceae family.It is a common poisonous plant to livestock in gasslands of the north of China.In severely degraded grassland,S.chamaejasme can become the dominant species.In this study,greenhouse experiments were conducted to investigate the effects of different concentrations of leachate from S.chamaejasme root on the growth characteristics,rhizosphere soil physical and chemical properties,rhizosphere arbuscular mycorrhizal fungi(AM fungi)and bacterial community diversity of Leymus chinensis,to reveal the allelopathic mechanism of S.chamaejasme.At the same time,we explored the response of plant diversity,plant functional group abundance,and soil nutrient elements concentration to the process of S.chamaejasme dispersion(the increase of the coverage ladder of S.chamaejasme)in degraded grassland,by comparing the differences of plant community characteristics and soil nutrient element concentration in plant communities with different S.chamaejasme coverage in degraded Stipa Baicalensis steppe of eastern Inner Mongolia.In addition,this study analyzed the spatial heterogeneity of soil nutrients,and the changes in the spatial correlation between soil nutrients and the plant functional groups abundance by using geostatistical methods in the plant communities with and without S.chamaejasme to explore the ecological mechanisms by which S.chamaejasme affects spatial heterogeneity of soil nutrients in grassland.The main results and conclusions of this study are as follows:The leachate from S.chamaejasme root significantly inhibited α-diversity of AM fungi,and had no significant effect on β-diversity.The leachate had no significant effect on the bacterial α-diversity,but had a positive significant effect on the β-diversity.The leachate of S.chamaejasme root significantly increased the bacterial community similarity.The relative abundance of different dominant bacteria phylum had different responses to the leachate from S.chamaejasme root.At the same time,the addition of the leachate from S.chamaejasme root significantly decreased soil pH,available phosphorus(AP)and soluble organic carbon(DOC)contents,but increased soil nitrate nitrogen,nitrogen: phosphorus ratios,and microbial biomass carbon content.Soil AP was an important factor affecting AM fungi.The changes of carbon in the aboveground part of L.chinensis(Shoot C)and DOC content in the soil caused by the addition of leachate were important ways that S.chamaejasme affects the bacterial community similarity.The leachate from S.chamaejasme root had no significant effect on the number of ramets,height,number of buds,rhizomes length,aboveground and underground biomass of L.chinensis.However,there was a tendency to increase the number of ramets,buds,aboveground and underground biomass and to reduce the length of rhizomes.The root leachate decreased Shoot C content,but increased nitrogen content in the aboveground part of L.chinensis.In addition,the decrease in soil AP content caused by the root leachate may be the main reason for affecting L.chinensis growth.At the same time,root leachate treatment suppressed the mutualism relationship between L.chinensis and AM fungi and bacterial community.Therefore,the effect of S.chamaejasme on nutrient content of L.chinensis may be the result of the reduction of soil AP concentration and the weakening of the relationship between L.chinensis and its rhizosphere microorganisms。In degraded S.Baicalensis grassland,the existence of S.chamaejasme significantly increased the α-diversity and β-diversity of plants community.S.chamaejasme mainly directly or indirectly affects the α-diversity of plant communities in three ways:(1)S.chamaejasme increased directly the plant richness,(2)S.chamaejasme increased Forbs abundance,and(3)The increase of the concentrations of soil copper(Cu)caused by S.chamaejasme.Cu was the main driving factor affecting the changes of dominant plants characteristics.In addition,soil nitrogen and phosphorus played an important role in theβ-diversity of plants community in the patches with the presence of S.chamaejasme.In conclusion,the presence of S.chamaejasme had a positive effect on plant species diversity.The concentrations of potassium(K),sodium(Na)and Cu were increased in the communities with the presence of S.chamaejasme compared with those without S.chamaejasme,but calcium(Ca),phosphorus(P),iron(Fe)and manganese(Mn)concentrations were decreased compared with those without S.chamaejasme,while the nitrogen(N),magnesium(Mg)and zinc(Zn)concentrations were not significantly affected.The spatial heterogeneity of soil K,Ca,Mg,Fe,Mn,Cu,and Zn was significantly increased in the communities with the presence of S.chamaejasme compared with the communities without S.chamaejasme,but the spatial heterogeneity of soil phosphorus(P)and sodium(Na)was decreased.Soil Ca,Mg,Fe,Mn,Cu,and Zn in the plant community with S.chamaejasme had strong spatial heterogeneity and an obvious patchy distribution pattern.The shoots density of S.chamaejasme was an important factor affecting the spatial heterogeneity of N,P,Fe,Mn,Cu and Zn in the community with S.chamaejasme.There were more spatial correlations between soil nutrients and plant functional group abundance in plant communities with S.chamaejasme compared with plant communities without S.chamaejasme.In a wrod,the spatial coupling relationship between soil nutrient elements and plant functional groups was closer due to S.chamaejasme diffusion in degraded grassland.The changes in soil nutrient content and plant community structure may be attributed to the alteration in soil nutrient content and plant community structure caused by S.chamaejasme.In conclusion,this study showed that S.chamaejasme affected carbon and nitrogen distribution and rhizosphere microorganisms of L.chinensis.S.chamaejasme affected L.chinensis growth by changing soil nutrient concentrations and weaking the positive relationship between L.chinensis and rhizosphere AM fungi and bacterial community.The high coverage of S.chamaejasme suppressed dominant plants and promoted forbs growth,improved the plant diversity and in degraded grassland.At same time,the high coverage of S.chamaejasme increased the spatial heterogeneity of soil nutrient elements and enhanced the spatial coupling relationship between plants and soil nutrient elements.This study emphasizes the positive role of poisonous plants in maintaining plant diversity,spatial heterogeneity of soil nutrients,and ecological functions of degraded grassland.This study provides a new theoretical basis for objectively understanding the ecological functions of grassland poisonous plants,especially the maintenance species diversity and soil spatial heterogeneity,which plays an important reference in the development of prevention and control strategies for poisonous plants. |
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