Species Loss Studies In Alpine Meadows Based On Soil Nitrogen And Phosphorus Contents And Their Stoichiometric Ratios

Alpine meadow ecosystem is one of the most important natural ecosystems in the Qinghai-Tibet Plateau.Due to the global changes and the rapid development of human industrialization,the increasing atmospheric nitrogen deposition leads to the increase of nitrogen enrichment in the grassland ecosystem and the loss of species.In addition,the species diversity of grasslands subjected to long-term overgrazing or fencing protection will also decrease,but some studies have suggested that the lower species diversity of nitrogen-enriched grasslands can be promoted by grazing or clipping which consumes dominant competitors in resource utilization.In the face of the increasing threat of atmospheric nitrogen deposition and grassland degradation caused by long-term overgrazing,it is necessary to study the effects of nutrient enrichment and grazing on species loss in alpine meadow.In recent decades,the experimental studies on species loss have focused on testing the mechanisms of species loss caused by nutrient enrichment(such as fertilization),but there is no consensus.At the same time,there are few studies on the effects of interaction between fertilization and grazing(or clipping)on species richness.At present,ecologists propose two kinds of hypotheses to explain the loss of species resulted from nitrogen enrichment,one is the assemblage-level thinning hypothesis based on neutral theory of community assembly,and other is the interspecific competition hypothesis based on niche theory.The counterpart of the species loss hypothesis is the species diversity maintenance hypothesis.Habitat heterogeneity hypothesis holds that the spatial pattern of resource distribution in local communities is discrete patches,each patch has its own unique resource state or resource supply ratio(such as specific nitrogen/phosphorus ratio),and there is no interaction between patches.Species with different resource needs can survive in these independent patches.Therefore,the heterogeneous distribution pattern of habitat resources will lead to high community species diversity.Species loss caused by increasing nitrogen deposition or exogenous nitrogen input should be related to the reduced heterogeneous distribution of soil nutrient resources in local community which makes the resource distribution become a nutrient-rich and homogeneous state,thus reducing the resource niche dimension for multi-species coexistence.As predicted by niche dimension hypothesis,the reduction of resource niche dimensions is associated with a decrease in species diversity.Therefore,if grazing or clipping can reduce soil nutrient enrichment and increase the heterogeneity of resource distribution,it can alleviate or prevent species loss,thus maintaining a higher species diversity of communities under nutrient enrichment.Based on the habitat heterogeneity hypothesis and niche dimension hypothesis,this study first analyzed the responses of some measures,such as aboveground net primary productivity,nitrogen and phosphorus content and their stoichiometry at three levels of soil,plant functional groups and communities,and spatial variability of soil nutrient distribution,to clipping and fertilization and their interactions,through a long term field experiment that simulated grazing(including three clipping levels of non-clipping,moderate clipping and heavy clipping)and atmospheric nitrogen deposition(including two levels of non-fertilizing and fertilizing)in alpine meadow.Secondly,we compared the variability of soil nutrient spatial distribution within and between treatment communities.Finally,the structural equation model was used to test the direct or indirect effects of clipping and fertilization on species richness and its possible paths.The main findings are as follows:(1)Clipping and fertilization significantly increased and decreased species richness,respectively.Because this opposite effect was independent of each other,and the negative effect of fertilization on species richness was much greater than the positive effect of clipping.Species richness decreased significantly at any clipping level after fertilization.Therefore,increasing clipping intensity can only alleviate but not prevent species loss caused by fertilization,and current nitrogen addition levels will lead to continuous species loss in this alpine meadow.(2)Both clipping and fertilization can significantly improve the aboveground net primary productivity of the community.This effect was greater in clipping than in fertilization,and their interaction was not significant,so the aboveground net primary productivity was independently affected by them.In addition,no matter what fertilization level,aboveground net primary productivity was the largest in moderate clipping community.(3)No matter what the clipping intensity,fertilization increased the nitrogen and phosphorus content and the ratio of available nitrogen to available phosphorus in the soil,and the increased soil nutrients relieved the nitrogen limitation of the alpine meadow.Although clipping also significantly increased the nitrate nitrogen content and the ratio of available nitrogen to available phosphorus in soil,the increase was mainly caused by heavy clipping,and the total nitrogen and available phosphorus decreased significantly after heavy clipping.The results showed that moderate clipping could maintain the relative balance of soil N and P stoichiometry,while the strong absorption of available phosphorus by plants caused by heavy clipping resulted in a significant increase in the ratio of available nitrogen to available phosphorus in the community.(4)The nitrogen,phosphorus content and nitrogen/phosphorus ratio at the plant functional group level show a complex response to fertilization and clipping,which depends on both the variation of disturbance factors and their disturbance intensity and the difference of plant functional groups.The nitrogen content of grasses,compositaes and sedges increased significantly after clipping or fertilization,but the increase occurred only after clipping for legumes and only after fertilization for forbs.The phosphorus content of these five plant functional groups increased after clipping and fertilization.Fertilization and clipping also significantly increased the nitrogen and phosphorus contents at the community level.However,both at plant functional group level and community level,the relative increase of nitrogen were smaller than that of phosphorus,which led to a significant decrease of nitrogen/phosphorus ratio.(5)The regression analysis of binary relationship showed that fertilization was more likely to cause negative correlation response of species richness to soil nutrient changes,and mainly occurred between soil available nitrogen,phosphorus content and species richness.Clipping did not completely eliminate this effect because it did not cause a positive correlation between the two,suggesting that clipping increased species richness may not simply change soil nutrient content.Different plant functional groups have different ability to absorb soil nutrients.Forbs and compositaes were more likely to have nutrient supply and demand contradiction,but heavy clipping can not completely relieve this contradiction.However,the contradiction at community level can be relieved by heavy clipping.Under the combined effect of clipping and fertilization,the relationship between aboveground net primary productivity and soil nutrient content was not generally positive as expected,but the aboveground net primary productivity of heavy clipping community was significantly positively correlated with soil available nitrogen content or with available nitrogen/available phosphorus ratio,which may be related to heavy clipping prolonging the period of plant compensatory growth and increasing plant ability to absorb soil available nitrogen.Net primary productivity and species richness were only significantly positively correlated in fertilization-heavy clipped communities,which should be related to clipping and fertilization to increase productivity and clipping to increase species richness.(6)Analysis of structural equation model(SEM)showed that clipping and fertilization had direct positive and negative effects on species richness,respectively,and the negative effect of fertilization(-0.49)was larger than the positive effect of clipping(0.22),and its strength decreased with the increase of clipping intensity.Among the six combinations of experimental treatments,the increase of soil ammonia nitrogen content was associated with the decrease of species richness(-0.14?-0.64),and the strength of this effect was regulated by clipping and was larger in non-fertilizing and fertilization communities on clipping level gradient than in three clipping level communities on fertilization level gradient,therefore,soil ammonia nitrogen content can be used as a general meaure to predict species richness changes in each combination.The increase of soil total phosphorus content was positively correlated with species richness(0.22)and also mediated by clipping.The results further show that the effect of fertilization on reducing species richness and its pathway can be regulated by clipping,but clipping can not prevent species loss caused by fertilization.Clipping can affect species richness through relatively fixed factors and pathways.(7)An analysis of the coefficient of variation(CV)of soil-related nutrient distribution between plots within the treatment community showed that although increasing clipping intensity increased the spatial heterogeneity of nitrogen/phosphorus ratio distribution in soil,fertilization eliminated this heterogeneity and caused the homogenization of soil nutrient distribution.Analysis of the CV between the treatmemt communities showed that the heterogeneity of the spatial distribution of all soil-related nutrient meaures decreased to the lowest at long-term constant clipping or fertilization levels,but the highest spatial heterogeneity occurred in communities combined with different clipping intensity and different fertilization levels,which were consistent with the predictions of habitat heterogeneity hypothesis and niche dimension hypothesis.Hence,changing the utilization intensity and fertilization amount of grassland in time is the key to improve soil nutrient spatial heterogeneity and maintain higher species richness in local grassland community.In conclusion,the results of this study support the prediction of habitat heterogeneity hypothesis and niche dimension hypothesis,indicating that the species loss in alpine meadows caused by nutrient enrichment is related to the soil eutrophication homogenization.The reason why the species diversity of alpine meadow can be maintained at a certain level should be directly related to the spatial heterogeneity of soil nutrient distribution increased by grazing.It is worth emphasizing that if the degree of nutrient enrichment is too high,the effect of grazing or clipping to alleviate species loss will be weakened.Hence,against the background of species loss caused by increasing nitrogen deposition,long-term grazing at constant intensity or fencing protection for grassland is not conducive to maintaining higher species diversity.The grazing intensity should be changed at the right time,and the fencing protection should be removed in due course to improve spatial heterogeneity of soil nutrient distribution and species diversity in grassland.For example,changing grazing intensity with a specific annual cycle or performing patch rotation grazing in the same year.In addition,significant reductions in atmospheric nitrogen emissions and exogenous nitrogen inputs should be most important to mitigate species loss and maintain the sustainable development of the ecosystem.