Study On The Mechanism Of Species Loss In Alpine Meadows Based On The Disturbance Of Mowing And Fertilization

With the rapid development of human industrialization,the atmospheric nitrogen deposition is increasing,and the nitrogen enrichment rate of the ecosystem is also increasing.Numerous studies have shown that nutrient enrichment,especially nitrogen enrichment,generally leads to species loss.At the same time,long-term enclosure also reduces species diversity,while herbivores or grazing can maintain species diversity through consuming competitive competitors.In recent decades,many studies have focused on the verification of the effects of nutrient enrichment(or fertilization)and grazing on species richness(SR),but the current researches on this content have not yet reached a consensus,and there is little research about the influence mechanisms of the interaction of grazing and fertilization on SR.Recently,there are mainly two hypotheses to explain species loss caused by fertilization.The assemblage-level thinning hypothesis assumes that the species mortality is equal,the nutrient enrichment causes the increasement of individual size of all species and the above-ground biomass of the community,increases the competition,and leads to the random death of small individuals,and then the total density of individuals is decreased,rare species will be lost preferentially.However,the interspecific competitive exclusion hypothesis,especially the light competition hypothesis believes that nutrient enrichment increases light competition with relieving soil nutrient restrictions,and that interspecific competition is transformed from underground to aboveground,variation coefficient of plant height is increased,tall species competes for light resources and excludes low species.This study takes alpine meadow as the research object to explore the influence mechanisms of long-term clipping,fertilization and their interaction on SR based on three major contents including the assemblage-level thinning hypothesis,light competition hypothesis and the effect of soil nutrient in the 11-year field experiment which artificially simulated grazing and fertilization.The results of this study are as follows:(1)In the three levels of clippingtreatment communities,fertilization all brought about a significant decrease in SR,but the percentage of SR decreased by fertilization in heavy clipping community was the smallest;clipping increased SR both in the two fertilization treatment communities,but the percentage of SR increased by heavy clipping in fertilization community was much greater than the percentage of SR increased by moderate clipping in fertilization community.Fertilization decreased SR,while clipping increased SR,for which moderate or heavy clipping could keep SR at the highest level in nofertilization community,followed by the combination of no clipping and no fertilization treatment,and SR was lower after fertilization whether clipping or not.(2)Fertilization led to the enrichment of soil available nitrogen(AN)and available phosphorus(AP)with the increasement of total nitrogen and total phosphorus content of community and the increasement of aboveground biomass of community;while long-term clipping significantly reduced soil organic matter(OM)content and significantly reduced soil AP content in fertilization community,but clipping did not significantly affect total nitrogen andtotal phosphorus content of community.(3)The responses of density and individual size of species to fertilization varied with different species,and fertilization significantly increased the density of those dominant and sub-dominant species in the fertilization community and the average density of all species,but had no significant effect on the total species density of the community.After fertilization,the individual biomass of some species was increased,some was decreased,and some did not change significantly,so the average individual size of species did not change significantly.The responses of density and individual size of species to clipping varied with different species,clipping significantly increased average density of species,whereas it did not significantly affect community total density and average individual size.(4)Fertilization significantly increased the average plant height,variation coefficient of plant height(CVH)and light attenuation rate(LAR)in community,but significantly reduced the community bottom light intensity(BLI).However,clipping causes a significant increase in BLI.The increase of BLI after clipping in the unfertilization community was significantly higher than that in the fertilization community.Therefore,fertilization significantly increased the intensity of light competition,while clipping significantly reduced the intensity of light competition,and the enhancing effect of fertilization on light competition was greater than the weakening effect of clipping on light competition.(5)Regression analysis showed that SR was negatively correlated with the soil total nitrogen(TN)content and soil OM content of unfertilization community and the soil AP content of the fertilization community,and SR was also negatively correlated with CVH and average species density of fertilization community.Clipping increased SR,but did not lead to the significant changes of community total density and average individual size,which indicated that clipping maintained higher SRmainly through weakening soil nutrient enrichment andintensity of light competition instead of by weakening thinning effect.Therefore,ALTH is not supported in our research.(6)Regardless of the clipping treatment community,fertilization mainly resulted in species loss by increasing soil available nutrient contentsand enhancing light competition,and fertilization did not bring about the increasement of community total density and average individual size.In addition,the negative correlation between the average density of species and SR and the non-negative correlation between the initial density of species and the species loss probability did not support the assemblage-level thinning hypothesis(ALTH).Among the three clipping communities,soil AN,AP,LAR,and CVH were significantly negatively correlated with SR,while BLI was significantly positively correlated with SR.Moreover,the effects of BLI and LAR on SR in heavy community(HC)were weaker than those in no clipping(NC)and moderate clipping(MC)communities,which indicatedthat clipping especially HC attenuatedspecies loss caused by the increase of light competition after fertilization and that the main mechanism of species loss caused by nutrient enrichment was the exclusion effect of light competition.(7)The above results are further supported by the structural equation model(SEM)analysis.No matter in which clipping treatment community,fertilization all increased light competition by significantly increasing CVH,and then SR was declined.At the same time,fertilization also had a significant direct negative effect on SR.In no fertilization(NF)community,soil OM and CVH had significant negative effects on SR,but clipping did not significantly affect soil OM and CVH actually,so their relationships with SR were independent of the clipping disturbance.Clipping also had a significant direct positive effect on SR.In the fertilization-clipping combination(CG)community,fertilization only reduced SR by significantly increasing CVH,and clipping only maintains SR by significantly reducing soil OM,and clipping and fertilization had significant direct positive and negative effects on SR,respectively.In summary,the results of this study support the light competition hypothesis(LCH)rather than the assemblage-level thinning hypothesis(ALTH),that is,the species loss of alpine meadow caused by nutrient enrichments is deterministic,not random.Since clipping and fertilization have direct positive and negative effects on species richness,respectively,appropriate clipping or grazing schemecan be taken to decrease or prevent the species loss of alpine meadow caused by nutrient enrichments.In addition to the direct impacts on species richness,clipping and fertilization have indirect effects,but the influence pathways are different.Clipping mainly reduces the negative effects of nutrient enrichment on species richness by reducing soil organic matter content to maintain SR,which may be related to the reduction of soil organic matter;Fertilization increased the competitive exclusion effect ofspeciesto lightby increasing CVH,which is consistent with the prediction of the asymmetric light competition hypothesis.This study shows that it is necessary to study the mechanisms of species loss and its regular changes in alpine meadow in the futureunder the background of increasing atmospheric nitrogen deposition.As a method to restore degraded grassland and improve grassland productivity,fertilizer will also cause species loss.Our study emphasizes that appropriate grazing measures should also be taken to mitigate or prevent species loss due to fertilization,which is of great significance for maintaining the function and sustainable development of alpine meadow ecosystems.