| Grazing exclusion has been widely applied for restoration management of degraded grassland on the Loess Plateau,due to its efficiency and economics.Grazing exclusion initiated self-recovery process of degraded grassland by excluding human disturbances,such as grazing and mowing.The coverage and productivity of grassland increased,and soil structure,nutrient availability and microbial activities were improved after grazing exclusion.Besides,grazing exclusion induced fluctuations at plant community structure and species diversity.As two big components of grassland ecosystem,plant,soil and interactions between them drive vegetation succession during the restoration process.Therefore,studies with focus on plant-soil feedbacks in vegetation succession after grazing exclusion would provide important implications in theory and practice.Since vegetation pattern has changed during 35 years' succession process after grazing exclusion in Yunwu Mountain on the Loess Plateau,we chose the Stipa community where replacements of the dominant species occured as the study object,and analyzed root traits of grassland along a chronosequence after grazing exclusion,the decomposition traits of litters of the dominant Stipa species,the responses of Stipa plants to changes in soil properties,and the plant-soil feedback indexes based on field and indoor experiments.The main results were as follows.(1)During the succession process of grassland with long-term grazing exclusion,root biomass and morphological traits showed increases of 49.65% and 94.80%,respectively.After grazing exclusion,soil moisture and nutrient contents increased,while species richness of root community suffered a spcecies loss of 37.11%.Different plant functional groups differed in their proportions in root community and their responses to long-term grazing exclusion.Proportions of grasses roots in grassland root community increased from 41.65% to 81.22%,where perennial rhizome grasses gained a tenfold increase,which was mainly due to the higher specific root length and specific root surface area of grasses and the enhancement effects from soil carbon and nitrogen resources.In contrast,proportions of forbs roots continuously declined and forbs roots were strongly influenced by soil phosphorus.(2)With time increasing(0,5,9,22 and 30 years)after grazing exclusion,root biomass and morphological traits of Stipa plants had been significantly increased by 87.29% and 119.73%,respectively.Meanwhile,grazing exclusion induced shifts in species composition of Stipa roots.The proportion of Stipa bungeana reached the highest value at 77.12% in grazed grassland and decreased to 13.42% in grassland with 22 years' grazing exclusion and vanished in grassland after 30 years' grazing exclusion.The proportion of Stipa grandis started with 22.88% in grazed grassland and gradually increased and owned the highest value at 86.58% in grassland after 22 years' grazing exclusion,then suffred a rapid decrease.Stipa przewalskyi appeared in grassland after 30 years' grazing exclusion and owned a higher proportion at 77.22%.(3)The two years' decomposition study in field showed leaf litters and root litters of three Stipa species suffered average organic matter losses at 36.07% and 32.00%,respectively.The decomposition rate of leaf litter of S.grandis was highest,and root litters of S.przewalskyi decomposed fastest,compared with litters of other Stipa species.Dynamics of element concentrations during the decomposition process were similar between three Stipa species,and were affected by litter type.In detail,carbon concentration of leaf litter decreased from 452.24 g·kg-1 to 440.21 g·kg-1,while root litter increased from 420.73 g·kg-1 to 435.09 g·kg-1 with fluctuations.Nitrogen concentration of litters generally decreased,which made C/N ratio generally increase with fluctuations.Phosphorus concentration of leaf litters increased from 0.29 g·kg-1 to 0.58 g·kg-1,while that of root litters declined from 0.30 g·kg-1 to 0.26 g·kg-1.Carbon,nitrogen and phosphorus were generally released from leaf litters and root litters in the decomposition process,with releasements at 23.78%-39.28%,37.20%-60.38% and 21.30%-52.28%,respectively.The decomposition rate of litters was significantly positively correlated with its initial nitrogen concentration,and was significantly negatively correlated with its initial C/N ratio.(4)There are significant interspecific differences in plant traits between three Stipa species,with higher specific leaf area,leaf nutrient concentrations,specific root length and specific root surface area in S.bungeana,with higher plant height,the ratio of root to shoot,and root morphological traits in S.grandis,and with moderate values of indexes described above and higher leaf dry matter concent and tiller numbers in S.przewalskyi.The biomass of S.grandis was higher than that of other Stipa species,considering the positive correlations between plant biomass and plant height,and root lengh.(5)Water supply increased biomass and root morphological traits of three Stipa species by 94.91% and 55.63%,respectively.While nutrient supply had limited influence on plant biomass and root morphological traits of three Stipa species.The soil inoculums from early succession stage improved specific root length and root surface area of three Stipa species by 11.90% on average,while the soil inoculums from middle and late succession stages improved biomass of three Stipa species by an average of 51.30% and decreased root/shoot ratio of three Stipa species by an average of 22.72%.Compared with single growth mode,three Stipa species gained an avergae increment of 11.75% in specific root length and specific root surface area in mixed growth mode.The competition ability of three Stipa species was ranked as S.bungeana>S.grandis>S.przewalskyi.Furthermore,the influences of water and nutrient supply on competiton ability of three Stipa species were strongly dependent on soil microenvironment.In detail,water and nutrient supply increased competition ability of S.bungeana in soils at early succession stage,and increased competition ability of S.grandis in soils at middle succession stage,and increased competition ability of S.przewalskyi in soils at late succession stage.(6)There were significant interspecific differences in plant-soil feedbacks(PSFs)between three Stipa species.S.grandis exerted significant positive PSFs to conspecific plants,and had mutual positive PSFs with grasses.While S.bungeana and S.przewalskyi exerted negative PSFs to their conspecific plants,and they showed positive PSFs to grasses,receiving negative PSFs in return.S.bungeana showed significant positive PSFs to S.grandis and received negative PSFs in return.S.przewalskyi had mutual positive PSFs with S.bungeana and S.grandis.The interspecific differences of three Stipa species in PSFs pattern with conspecfic and heterospecific plants may play an important role in the replacement of S.bungeana with S.grandis during grassland succession after long-term grazing exclusion. |
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