Response Mechanism Of Soil Seed Banks To Long-term Nitrogen Deposition In Alpine Meadow On The Tibetan Plateau

Atmospheric nitrogen(N)deposition is one of the three major threats to global biodiversity.Due to human activities such as industrial gas emissions,atmospheric nitrogen deposition continues to rise,which has an important impact on the services and functions of terrestrial ecosystems.At present,most studies of nitrogen deposition on terrestrial ecosystems focus on above-ground plant vegetation,while studies on underground potential vegetation-soil seed banks are often neglected.The soil seed bank can maintain species diversity through mechanisms such as storage effects,which is a crucial resource of plant community regeneration,key factors of ecosystem recovery and resistance,and important question in ecological research.However,the mechanism of effects of nitrogen deposition on soil seed bank in alpine meadow on the Tibetan Plateau is still unclear.In this research,we want to reveal the potential role of soil seed bank in plant community regeneration and degraded vegetation restoration in the context of increased nitrogen deposition in the future,and we think our research could contribute to a more comprehensive understanding of the mechanism of nitrogen deposition on the loss of species diversity.We collected 31 common species in the alpine meadow plant community on the eastern Tibetan Plateau,and used the seed burial method to bury the seeds in a long-term nitrogen addition platform(four N addition treatments,including 0g/m~2,5 g/m~2,10 g/m~2and 15 g/m~2)at the"National Field Scientific Observation and Research Station of Gannan Grassland Ecosystem".We used this experiment to simulate the effect of long-term nitrogen deposition on the soil seed bank of alpine meadows.All the buried seeds were retrieved and tested for the seed germination,seed decay,seed vigor,and seed inactivation after burial for 4,16,and 28 months,respecitvely.We used linear mixed models and analysis of variance(One Way ANOVA)to examine the effects of long-term nitrogen deposition on seed germination,seed decay,seed vigor,and seed inactivation index.Meanwhile,we investigated the species richness,abundance,species composition,cover,and aboveground biomass of plant communities,and also determined soil physicochemical factors(including soil p H,total nitrogen,available nitrogen),and obtained the soil microbial composition and structure(saprophytic fungi)data by phospholipid fatty acid biomarkers.Moreover,we used linear regression and structural equation modeling(SEM)to explore the direct effects of long-term nitrogen deposition on soil seed banks and the indirect effects on soil seed banks through soil biotic and abiotic environments.The main results were listed in the follow.(1)At the species level,species responses to long-term nitrogen addition are highly species-specific.In terms of burial time,25 species in seeds germination 22 species in seeds rot,22 species in seeds vigor,and 10 species in seeds inactivation differed significantly among the buried time;In N addition gradient,4 species in seeds germination,1 species in seed rot,and 4 species in seeds inactivation differed significantly among the N addition treatment;Moreover,4 species in seeds germination,2 species in seeds rot,and 5 species in seeds vigor have a significant interaction effect between burial time and nitrogen addition.(2)At the community level,the burial time had a significant effect on the seed germination,seed rot,and seed vigor;At the nitrogen addition gradient,nitrogen addition had a significant effect on seed germination and seed vigor at the community level;There was no interaction effect between time and nitrogen addition on seed germination,seed decay,seed vigor and seed inactivation.After 16 months of seed burial,the seed vigor of species at the community level decreased significantly with increasing nitrogen concentration.(3)Long-term nitrogen addition indirectly decreased seed germination and increased seed rot in soil seed banks by reducing soil p H and saprophytic fungi;Long-term nitrogen addition indirectly increases seed rot and inactivation in soil seed banks by increasing soil total nitrogen(TN)and available nitrogen(AN)content,resulting in a decrease in the number of viable seeds in soil seed banks;Long-term nitrogen addition directly decreased the species richness and abundance of aboveground plant communities,and indirectly decreased aboveground plant community species richness by increasing soil available nitrogen(AN).(4)Large seeds are easier to germinate,and small seeds are more likely to remain in the soil.Large seeds can germinate quickly and increase soil seed bank depletion,while small seeds survive longer and are more persistent than large seeds.Although the soil seed bank can buffer the adverse effects of the climate environment(such as N deposition)to a certain extent,our study indicated that long-term high-concentration nitrogen deposition would reduce the number of viable seeds in soil seed banks of the alpine meadow on the Tibetan Plateau,which may reduce the recovery potential of soil seed banks of alpine meadow after ecosystem disturbance.