Effects Of Fertilization And Enclosure On Species Diversity In An Alpine Meadow Community

Fertilization and enclosure are two widely used management practices in grazing grassland communities,which are important factors influencing the structure and function of grassland ecosystems.The mechanisms underlying the response of species diversity to fertilization and enclosure is one of the important issues in ecological research.In this study,we conducted the experiment of enclosure,fertilization(addition of nitrogen,phosphorus,and potassium),and fertilization+enclosure treatments in grassland communities on the eastern Qinghai-Tibet Plateau for three consecutive years.We investigated plant productivity(above-ground productivity,functional group productivity,and litter quantity),species richness,soil nutrient content,and plant functional traits,and aimed to assess the underlying mechanisms of species diversity changes among treatments.The main results are presented below:(1)Fertilization significantly increased above-ground productivity,and enclosure significantly suppressed the increase in above-ground productivity caused by fertilization.Specifically,at the functional group level,fertilization resulted in a significant increase in grass productivity and decrease in sedge and forb productivity,while enclosure slowed down the increase in grass productivity and the decrease in sedge and forb productivity caused by fertilization.At the species level,compared to the control treatment where Kobresia setschwanensis was the dominant species in the community,and Elymus nutans and Poa pophagorum were transformed into the dominant species in fertilization and fertilization+enclosure treatment.(2)Both fertilization and enclosure significantly reduced species richness,and their interaction exacerbated the decline in species richness.The loss of species caused by fertilization was mainly due to the increased limitation of light resources caused by the increase in above-ground productivity,while at the same time it was also due to the decrease in niche dimensions caused by soil nutrient enrichment.Species loss due to enclosure resulted only from an increase in litter quantity and was not related to above-ground light resources or below-ground soil nutrients.Species loss caused by fertilization+enclosure was due to the combined effects of above-ground light resources,litter and soil nutrients.(3)Plant height and specific leaf area(SLA)increased significantly in the fertilization treatment.SLA decreased significantly in the enclosure treatment.Both chlorophyll content and height increased and leaf dry matter content(LDMC)decreased in the fertilization+enclosure treatment.In addition,species richness decreased with increasing chlorophyll content and plant height.The significant correlation between LDMC and SLA with species richness was not found.In conclusion,long-term winter grazing in the alpine meadow on the eastern Qinghai-Tibet Plateau played a positive role in maintaining species diversity and species coexistence,and different grassland management measures had a synergistic effect on species loss through different competition mechanisms,indicating that the ecological factors regulating species diversity had a complex ecological relationship.In addition,after fertilization,plant functional traits shifted towards traits associated with faster growth rates and higher resource utilization rates(higher chlorophyll content or plant height).Species with higher resource utilization rates could outcompete other species with lower resource utilization rates,resulting in species loss.Therefore,these results suggested that the differential response of plant functional traits to different environments can predict changes in species diversity.The study also provided an important scientific basis for the conservation and management of species diversity in alpine meadows in the future.