Adaptation Mechanism Of Alpine Meadow Plant Community And Stability To Long-term Nitrogen Addition

Nitrogen(N),as a major limiting element in terrestrial ecosystems,plays an important role in plant growth,development and reproduction.To explore the effects of exogenous N addition on plant community and soil environment in alpine meadow.This research has been based on the nitrogen addition experiment platform established in 2012 by the Southwest Minzu University in the Qinghai-Tibet Plateau Ecological Protection and Animal Husbandry High-tech Innovation Practice Research and Development Base,the changes of community structure(including species diversity and dominance),key functions(including aboveground productivity,stability and aboveground-underground interconnection)and soil environment in alpine meadow under different N addition gradients(Including four N elements added gradients 0,10,20 and 30 g/m~2,namely CK,N10,N20 and N30)were observed for nine consecutive years(2012-2020)by field investigation and indoor analysis,the aim was to clarify the response mechanism of soil physical and chemical properties,characteristics of plant communities and different functional groups,and community stability in alpine meadow under long-term exogenous N addition.The results show that:1.The important value(mean value)of plant function groups was Forbs>Grass> Sedge>Legume.Long-term exogenous N addition increased the dominance of Grass(important value>5%),and the dominance of Grass was the highest under the N30 gradient(14.00 g·N/m~2·a).2.Long-term exogenous N addition significantly decreased the species richness of the plant community(P<0.05),especially the species richness of Forbs.At low N concentration(4.67 g·N/m~2·a),the number of community species(24species)decreased by 0.62%on average,while at medium-high N concentration(9.33 g·N/m~2·a-14.00 g·N/m~2·a),the number of community species(21 species;19 species)decreased by 12.04%to 20.52%.At the same time,the inhibition effect of N addition on the species richness of Legume was the most obvious,and the species number of Legume decreased by the largest amount of 40.91%.3.N addition significantly increased the aboveground biomass of the plant community(P<0.05),which increased by 11.27%,29.75%and 26.22%with the increase of fertilization gradient(N10-N30)respectively,and reached the highest value under the N20 gradient.In addition,medium-high N addition increased the biomass of Grass and Sedge,but decreased the biomass of Legume and Forbs.4.Exogenous N addition improved the temporal stability of the community.With the increase of fertilization gradient,the temporal stability of the community showed a"V"shape,which was consistent with the temporal stability of Sedge, Legumes and Forbs.Under the condition of high N concentration(14.00 g·N/m~2·a),the temporal stability of Grass was the key factor to determine the temporal stability of the community.5.N addition increased soil TC and TN contents to some extent,but significantly decreased soil SWC and pH(P<0.05).With soil depth,the contents of TC and TN in soil were significantly decreased(P<0.05).6.Under long-term N addition,soil C/N varied in the range of 8.4-11.8,C/P varied in the range of 37.9-95.4,and N/P varied in the range of 3.4-9.0,which were all lower than the global average level of grassland(13.7,174.5 and 12.4).7.The path analysis at the community level that aboveground biomass and species richness had direct negative effects on temporal stability of the community. Soil pH and SWC not only directly affected community temporal stability,but also indirectly affected community temporal stability by changing aboveground biomass and community richness.8.The path analysis at the functional groups level showed that the richness of Grass,the biomass of Sedge and Legume had a direct positive effect on the community temporal stability,while the biomass of Grass had a direct negative effect on the community temporal stability.9.Exogenous N addition directly changed soil N content and pH,leading to changes in soil ecological stoichiometry(C/N,C/P,N/P),promoted C-N-P mineralization,improved soil fertility,and increased aboveground biomass of plants and soil capacity.In conclusion,long-term exogenous N addition alleviated nitrogen limitation in alpine meadow,increased aboveground biomass and temporal stability of the community,and decreased species richness.In addition,higher fertilization concentration(14.00 g·N/m~2·a)was more conducive to the temporal stability of the plant community.With the increase of fertilization gradient(N10-N20),the synchronic changes of three functional groups(especially the temporal stability)driven by the rapid growth of Grass resulted in the decrease of community temporal stability.Under high N concentration,the main driving force of community temporal stability was the complementary effect between the productivity of dominant species(Grass)and subdominant species(Sedge).Therefore,the competition among different functional groups and the complementary effect between dominant and subdominant species were important mechanisms affecting community temporal stability.