Long-term Effects Of Nitrogen And Phosphorus Fertilization On Plant Community And Stability In Alpine Meadow

Low temperature inhibits soil mineralization in the alpine meadow on the Qinghai-Tibet Plateau,which has led to nitrogen and phosphorus becoming the limiting factors for plant growth,despite the large amount of nitrogen and phosphorus stored in the soil.In this study,the long-term changes of soil physicochemical properties,plant community characteristics,different functional groups characteristics and their stability in an alpine meadow of the Northwestern Sichuan under different nitrogen and phosphorus addition gradients（0,5+5,10+10,15+15 g/m²）were investigated by field investigation and laboratory studies.The aims were to explore the effects of exogenous nitrogen and phosphorus additions on plant communities and the maintenance mechanism of its stability in alpine meadows.The results showed that:1.Nitrogen and phosphorus fertilization increased soil TC,TN and TP,but did not cause soil acidification.The pH and SWC after 7 years were significantly higher than those after the first fertilizer application,（P<0.05）,while the R/S after 7 years was significantly lower than that after the first fertilizer application （P<0.05）,and improved with the increase of fertilization gradient.2.Long-term nitrogen and phosphorus fertilization had no significant effect on soil C:N,but resulted in significant changes in soil C:P and N:P（P<0.05）. Soil C:N,C:P and N:P with different fertilization gradients were all lower than the global grassland average（13.7,174.5 and 12.4）.3.Fertilization significantly increased aboveground biomass and coverage of the plant community,but decreased richness,among which the richness of Forbs decreased most significantly.Nitrogen and phosphorus fertilization increased the absolute aboveground biomass of Grass and Sedge and their proportions in the total biomass of the community,but decreased the proportions of Legume and Forbs in the total biomass of the community.4.The important values of different functional groups were Forbs>Grass> Sedge>Legume.At the species level,consecutive nitrogen and phosphorus fertilization stabilized the status of dominant species of Grass（Important value> 5%）and increased the heterogeneity of plant community.5.Fertilization improved the plant community temporal stability,but it decreased with the increase of the fertilization gradient（NP10-NP30）,and community temporal stability was the highest under the low concentration of fertilization gradient(23.33 kg N·hm^-2·a^-1,8 kg P₂O₅·hm^-2·a^-1).At the functional groups level,the temporal stability of Sedge,Legume and Forbs decreased with the increase of fertilization gradient,but the temporal stability of Grass was the highest under NP30.6.Under different fertilization gradients,the biomass of Grass was the key determinant of plant community aboveground biomass,and the richness of Forbs was the key determinant of total richness of plant community.7.Path analysis at community level showed that plant productivity and richness had significant negative effects（P<0.05）.The direct effect of fertilization gradient on soil TC led to the change of plant community coverage,and then affected the temporal stability of plant community.In addition,fertilization gradient indirectly changed plant community temporal stability through affecting R/S.8.Path analysis of different functional groups showed that the richness of Grass and the biomass and richness of Sedge had direct negative effects on the community temporal stability,while the coverage and richness of Forbs had direct positive effects on the community temporal stability.In summary,the alleviation of nitrogen and phosphorus restricted by exogenous N and P addition in alpine meadow which increased plant community coverage and aboveground biomass,and resulted in the decrease of community effective niche dimension and richness.Long-term fertilization also reduced the soil capacity and affected the growth of roots.Furthermore,low fertilization concentration(23.33 kg N·hm^-2·a^-1,8 kg P₂O₅·hm^-2·a^-1)was more conducive to the stability of the plant community.Under low fertilization gradient,the stabilizing properties of Grass as the dominant species with high yield improved the community temporal stability.With the increase of fertilization gradient,the key driver to maintain plant community temporal stability changed from the stability of Grass to the stability of the dominant functional group,the Forbs.Therefore,the compensatory dynamic driven by the competitive interaction and negatively correlated responses between dominant species and dominant functional group is important mechanism for maintaining plant community stability.