Effects Of Nitrogen Deposition On Soil Collembola Community In A Poplar Plantation

Increasing atmospheric nitrogen deposition already has caused a serious threat to global biodiversity and ecosystem function.Studies have shown that global anthropogenic reactive nitrogen production has increased to 187 Tg in 2005.Nitrogen deposition is likely to increase further in the coming decades,and excessive atmospheric nitrogen deposition may have significant impacts on ecological and biological processes of forest ecosystems.Most previous studies have focused on the effects of nitrogen deposition on above-ground processes in ecosystems,while less on below-ground processes,especially the biology and ecology of soil fauna,although the vast majority of deposited nitrogen ends up in soil ecosystems.Soil faunal diversity is an important component of ecosystem biodiversity,and nitrogen deposition may have an impact on the composition and structure of soil faunal communities.Soil fauna usually has been paid little attention due to the abundance of species,their small size and the difficulty of research.It is found that elasmobranchs（Collembola）,commonly known as springtails,are small microscopic arthropods that are widely distributed in soil.Soil springtails are very sensitive to environmental changes and play an important role in regulating soil ecosystem functions in plantation forests,and have been used as important indicators of environmental changes.This study was conducted in a poplar plantation that has been subjected to simulated nitrogen deposition for 10 years to understand the effect of nitrogen deposition on the community structure of soil springtails.A control（N₀）and other three treatments with different N concentrations i.e.,5g N/（m²·a）（N₅）,15g N/（m²·a）(N₁₅),30g N/（m²·a）(N₃₀),respectively,were used to simulate atmospheric N deposition in the sample plots established in May 2012.The dynamics of soil springtail communities in the litter layer and soil layer（0～10 cm,≥10～25 cm,≥25～40 cm）of the experimental sample plots treated with different N deposition levels were investigated in different seasons,respectively.The effects of N addition treatments on the number of individuals,groups（genera）andα-diversity index of soil springtails and the correlation between different groups（genera）of springtails and eight soil environmental factors（including water content,organic carbon,carbon to nitrogen ratio,total nitrogen,nitrate nitrogen,ammonium nitrogen,soluble organic carbon and p H）were analyzed.The results showed that:1.A total of 1,446 individuals of soil springtails were captured in this study,belonging to 4orders,7 families and 18 genera,respectively.Among them,the dominant groups were Isotoma,Toocerus,Entomobrya and Isotomiella,accounting for 66.87%of the total number of individuals.Rare taxa are Proisotoma,Folsomena,Onychiurus,Arrhopalites,Sinella and Pseudoachorutes,which account for 2.14%of the total number of individuals.2.In the poplar plantation sample plots,compared with the control（N₀）,the composition,group（genus）number andαdiversity index of soil springtails under different nitrogen deposition levels showed different degrees of change.At different N deposition treatment levels,the number of soil springtail individuals increased significantly at N₁₅ concentration,but decreased at N₃₀ concentration,and even below control.The diversity of soil springtails had the highest Shannon-Wiener diversity index and Simpson dominance at the concentration of N₁₅ treatment,while the highest Pielou evenness index was found at N₃₀ treatment concentration.It is evident that N deposition significantly affects the community structure of soil springtails.Moderate N addition increases the number of individuals of soil springtails,while too high N addition leads to a decrease in the number of springtail individuals and there is a threshold effect.3.In different soil levels,the number of soil springtail individuals in the apoplankton layer was significantly higher than that in the soil layer.Thus,it can be seen that soil springtails have the phenomenon of surface aggregation and prefer to live near the surface.In different seasons,the number of individuals of soil springtails showed that October（autumn）>July（summer）>April（spring）>January（winter）,showing some seasonal differences.It can be seen that the cause of this phenomenon should be related to temperature and soil water content.Futhurmore,RDA analysis showed that p H,nitrate nitrogen and carbon to nitrogen ratio were the main environmental factors affecting the distribution of soil springtails.In conclusion,nitrogen deposition treatment mainly affected the community composition,distribution and diversity characteristics of soil springtails by changing soil environmental factors,and showed some seasonal differences;moderate nitrogen addition could increase the number and diversity of springtail individuals,while excessive nitrogen addition led to the decrease of springtail individuals.