Responses Of Plant Productivity And Soil Nutrients Of Alpine Meadows To The Disturbance By Plateau Pika

The population density change of small rodent herbivores will inevitably influence simultaneously grassland plant communities and soil properties.Plateau pika(Ochotona curzoniae)is a small endemic and common rodent herbivore in the Qinghai-Tibetan Plateau(QTP),and creates extensive disturbance on alpine meadow,and these disturbances can influence plant productivity and soil nutrients,thereby further affecting the structure,function and sustainability of alpine meadow ecosystems.Therefore,examining the effect of the disturbance by plateau pika on plant productivity and soil nutrients of alpine meadows is regarded as international hot in land process of QTP.Although many previous studies,which examined the effects of plateau pika disturbances on plant productivity and soil nutrients of alpine meadows,provide fruitful information for understanding the role of plateau pika in alpine meadow ecosystem,sampling methods in these studies have major defects,consisting of two reasons.Most of previous studies did not design the undisturbed plot as reference;even if some studies had reference plot,these studies ignored the bare soil patches within disturbed plot.In addition,many studies placed plots in different alpine meadow type.Furthermore,most of previous studies only sampled in one site,and this could not find the general pattern concerning the effect of the disturbance by plateau pika on plant productivity and soil nutrients because results from one site could not exclude the influence of climate and topography.Therefore,this study examined the effects of plateau pika disturbance on plant productivity and soil nutrients in alpine meadows at quadrat and plot scales by combing multi-point sampling and positioning monitoring methods,constricting the disturbed plots and the undisturbed plots in same alpine meadow type.The results of this study will find general pattern about the effect of plateau pika disturbance on grassland plant productivity and soil nutrients,clarify the simultaneous change process of plateau pika population,plant productivity and soil nutrients with the increasing years,and provide the evidence that leaf stoichiometry and leaf traits can be used to explain why plant community performs differently as the disturbance intensity of plateau pika increases.Such,this study will improve the method of the impact of small rodent herbivores on grassland,and provide scientific support for plateau pika management in the Qinghai-Tibet Plateau.The main results are as follows:1.The plot scale is better than quadrat scale to evaluate the effect of plateau pika disturbance on plant productivity and soil nutrient stocksAt the quadrat scale,plateau pika disturbance had no impacts on plant productivity of community,grass,legume and forb,while sedge productivity was 21%lower when the disturbance by plateau pika was present.At the plot scale,considering the bare soil patches with lack of plants,plateau pika disturbance had no significant effect on the productivity of grass,legume and forb,while decreased plant community productivity and sedge productivity,which were 14%and 29%lower than undisturbed plots,respectively.Although the inorganic carbon,total phosphorus,total potassium,and available potassium stocks were not significantly different between the vegetated soil and bare soil within the disturbed plot,the organic carbon,total carbon,and total nitrogen stock of bare soil was significantly lower than it of vegetated soil,and the nitrate nitrogen,ammonium nitrogen,inorganic nitrogen,available phosphorus stock,and ANR of bare soil were all higher than those of vegetated soil.The heterogeneity of soil nutrient stocks within disturbed plots indicated that neither nutrient stock of bare soil nor vegetated soil can reflect the soil nutrients of a disturbed plot.Therefore,measurement at plot scale should be used to evaluate the effect of plateau pika disturbance on plant productivity and soil nutrient stock.2.The disturbance by plateau pika altered the relationship between plant productivity and plant diversity from positive correlation to hump-shaped correlationThe plant species richness was higher in the disturbed,although the plant productivity did not differ between disturbed and undisturbed plot.The plant species richness increased positively with plant productivity in the absence of plateau pika.In the presence of plateau pika,the plant species richness peaked at intermediate plant productivity levels.Therefore,a small rodent herbivore could also be an important biotic factor that influences the relationship between plant species richness and plant productivity by inducing an increase in plant species richness.3.There were general patterns concerning the effects of plateau pika disturbance on plant community productivity,soil inorganic carbon stock,ammonium nitrogen,inorganic nitrogen,nitrate nitrogen,total potassium and available potassium stocks,and ANR.When the data from five sites was analysed together,or the data from each site was analysed separately,the disturbance by plateau pika reduced the productivity of plant communities and the inorganic carbon stock,increased soil ammonium nitrogen and inorganic nitrogen stock,and ANR(P<0.05),while had no significant effects on soil nitrate nitrogen,total potassium,and available potassium stock.Therefore,effects of plateau pika disturbance on plant community and these soil nutrient stocks have common patterns.However,the responses of soil organic carbon,total carbon,total nitrogen,total phosphorus,and available phosphorus to the disturbance by plateau pika were site-independent.4.The moderate disturbance intensity of plateau pika increased grass productivity,soil carbon,nitrogen stocks at plot scale,and plant species richnessWith the increase of disturbance intensity of plateau pika,the productivity of plant community and sedge plant productivity decreased,while the productivity of grasses increased and then decreased.The species richness of plant community firstly increased and then decreased with the increase of the disturbance intensity of plateau pika,and it was the highest at the moderate disturbance intensities.Soil nitrate nitrogen,soil total potassium,and soil available potassium stocks had no obvious relationship with the intensity of plateau pika disturbance.However,with increase of disturbance intensity,soil inorganic carbon stock showed a gradual decreasing trend,and soil organic carbon,total carbon,total nitrogen,ammonium nitrogen,inorganic nitrogen,total phosphorus and available phosphorus stock all showed hump-shaped process.The results suggest that moderate disturbance intensities of plateau pika can improve the grazing quality of alpine meadows by increasing the productivity of grass plants at the plot scale,improve soil quality,increase soil carbon sequestration capacity,and improve soil nitrogen utilization by increasing soil total carbon,organic carbon,total nitrogen,ammonium nitrogen,total phosphorus and available phosphorus stocks.5.Inter-annual variations of plant productivity and plant species richness were different under different initial disturbance levels of plateau pikaAt light initial disturbance level,the active burrow entrance density of plateau pika decreased from 139/hm~2 in 2013 to 53/hm~2 in 2018,and the productivity of plant communities increased,while the species diversity began to decrease in 2017.At moderate initial disturbance level,the active burrow entrance density of plateau pika decreased to 256/hm~2 in 6 years,and the productivity of plant community gradually increased from 2016(P<0.05),the species richness decreased significantly in 2018(P<0.05).At heavy initial disturbance level,the active burrow entrance density of plateau pika firstly increased and then decreased,while it was still as high as 443/hm~2in 2018;the productivity of the plant community increased significantly in 2016(P<0.05).The species diversity was not significantly different among 6 years.These results suggest that the active burrow entrance density of plateau pika,plant productivity and species richness had different trend at different initial disturbance levels with the year after excluding large herbivores.6.Clarified the inter-annual variations of soil nutrient stocks under different initial disturbance levelsAt light initial disturbance level,all soil nutrient stock at plot scale did not change significantly with increasing years.At moderate initial disturbance level,the soil inorganic carbon and total carbon stocks firstly increased and then decreased.Soil ammonium nitrogen,nitrate nitrogen,and inorganic nitrogen total phosphorus,total potassium and available potassium stocks all decreased with increasing years(P<0.05),while soil available phosphorus stock did not differ among 6 years.At heavy initial disturbance level,soil inorganic carbon,total carbon,total phosphorus,and available phosphorus and total potassium stocks did not change significantly with increasing years,while soil ammonium nitrogen,nitrate nitrogen,and inorganic nitrogen stocks decreased with the increasing years(P<0.05).Available potassium stock decreased significantly from 2013 to 2015(P<0.05).The results indicate that the soil organic carbon and total nitrogen stock and ANR at plot scale were all not different among 6 years under different initial disturbance levels of plateau pika,while other main soil nutrient stocks had obviously inter-annual variations after excluding large herbivores(P<0.05)7.The leaf traits and stoichiometry can be used to explained the strategies how plant community responded to the disturbance by plateau pikaThe specific leaf area,dry matter content,and leaf nitrogen content of Kobresia pygmaea were smaller in the disturbed plots than in the undisturbed plot,and these leaf traits together with leaf index and leaf nitrogen-phosphorus ratio of the K.pygmaea all showed a gradual decrease as the plateau pika disturbance increased.As for Elymus nutans,plateau pika disturbance increased its specific leaf area,leaf dry matter content,leaf nitrogen content and leaf phosphorus.With increase of disturbance intensity,the leaf index,specific leaf area,leaf dry matter content,leaf nitrogen content,and leaf nitrogen-to-phosphorus ratio of E.nutans show downward parabolas.Plateau pika disturbance also increased the specific leaf area,leaf nitrogen content,and leaf nitrogen-to-phosphorus ratio of Anemone rivularis,and its leaf index,specific leaf area,leaf dry matter content,leaf nitrogen content,and leaf phosphorus content were positively correlated to disturbance intensity.These results indicated that K.pygmaea,E.nutansand A.rivularisvar grew well at relatively low,moderate,and high disturbance intensity conditions,respectively.Our findings suggest that leaf traits and stoichiometry are effective tools to explain how small burrowing herbivore disturbances often lead to the improvement or deterioration of alpine meadows under different disturbance intensities.