Effects Of Nitrogen Deposition On Macrofauna In The North-south Transition Zone Of China

Atmospheric nitrogen deposition is one of the important issues of global change.The increase of nitrogen deposition will have an important impact on terrestrial ecosystems,especially forest ecosystems.As important decomposers and consumers in forest ecosystems,soil macrofauna play an important role in litter decomposition,nutrient turnover,microbial community regulation,and maintenance of ecosystem structure and function.However,there are few studies on the effects of nitrogen deposition on forest ecosystem macrofauna,and there are no consistent conclusions.In addition,most experiments simulating nitrogen deposition in forest ecosystems are based on traditional understory simulations,which ignore forest canopy processes(e.g.,nitrogen sorption,retention,and biotransformation).To investigate the effects of more realistic atmospheric nitrogen deposition on macrofauna,we chose two sampling methods,the trap method and the sample method,for a one-year sampling period(October 2020 to September 2021)based on the longterm field control platform of canopy simulated nitrogen deposition at the Henan Dabieshan National Field Observation and Research Station of Forest Ecosystem.The aim was to investigate the effects of different N application methods(forest canopy N application: CAN and understory N application: UAN)and N application concentrations(0,25,50 kg N ha-1yr-1)on macrofauna.The results of the study were as follows:The soil fauna was significantly affected by the time effect.The composition of the dominant groups of soil fauna was similar among months,mainly dominated by the predatory Staphylinidae,Araneae and omnivorous Formicidae.The abundance of supergene soil macrofauna(trap method)fluctuated more with the month,while the abundance of litter layer soil macrofauna(sample method)fluctuated less with the month.This is due to the fact that the study area is located in the transition zone between the northern subtropical and warm temperate zones,and there are obvious seasonal changes in temperature and precipitation,which can cause habitat changes and thus affect the community composition of surface macrofauna.The small fluctuation of soil fauna in the litter layer is due to the abundant litter resources in the transition zone,and the feeding activities of soil fauna are not limited by the substrate resources.For the nitrogen application method,we found that UAN had a positive effect on soil fauna compared with CAN,and there were seasonal differences in the effects of nitrogen application methods on soil fauna.Specifically,in autumn,the abundance and number of macrofauna in the litter layer increased under the UAN treatment compared to CAN..In winter,the dominance of surface soil macrofauna decreased under the UAN treatment compared to CAN;this was due to the different response strategies of soil fauna to N deposition due to changes in food resources and species habitat caused by seasonal changes.In autumn,the amount of litter increased significantly,which provided abundant food resources for soil fauna.In contrast,in winter,temperature became the main limiting factor for soil faunal diversity,with lower temperatures leading to lower soil faunal diversity.For the applied N concentration,high understory N had a positive effect on the superficial soil macrofauna community and showed seasonal differences.Specifically,in winter,the number of superficial soil macrofauna groups,H’ diversity index and evenness index were significantly increased in the understory high N treatment(UAN50,50 kg N ha-1yr-1)compared to CK(control,0 kg N ha-1yr-1).The number of individuals and the number of groups of litter in the apoplastic layer of the forest canopy high N treatment(CAN50,50 kg N ha-1yr-1)was significantly lower in autumn compared to CK.To further explore the effect of nitrogen deposition on the trophic level of soil fauna,we classified the soil fauna according to their feeding habits.The results showed that the mode of nitrogen application had significant effects on the functional groups of soil fauna,and the effects were different under different seasons and sampling methods.Specifically,compared with CAN,the saprophagous richness of litter layer under UAN treatment increased in summer,while the predator richness increased significantly in autumn.Compared with the control,high nitrogen application significantly reduced the abundance of predators and omnivores.This is due to the fact that soil faunal functional groups with different habitat preferences and feeding preferences adopt different survival strategies in the face of environmental changes and,therefore,respond differently to different N application methods and N application concentrations.In addition,nitrogen deposition indirectly affects soil faunal communities by altering soil physicochemical properties.The results of redundancy analysis(RDA)showed that soil p H was the main driver of macrofaunal community changes.Soils in the study area were acidic,and soil acidification resulted in reduced soil fauna,weakened metabolism and reduced growth and reproductive capacity,thus p H was positively correlated with soil fauna.The results of structural equation modeling indicated that nitrogen deposition directly affected soil fauna of different feeding habits.CAN had a significant positive effect on predator abundance due to the high N application to the forest canopy significantly increased the amount of apoplankton,which provided abundant substrate resources for detritus eaters and led to an increase in predators through a bottom-up cascade effect;UAN had a significant negative effect on phytophagy due to the spraying of the forest understory ammonium nitrate solution,residuals to low shrub and herb leaves,and phytophagous feeding on the leaves had a direct toxic effect on soil fauna,while a significant positive effect on omnivore abundance was due to the increase in some nitrogen-loving microorganisms caused by understory nitrogen application,which provided a food source for some omnivores,thus leading to an increase in omnivore abundance.N deposition also had an indirect effect on soil fauna by changing environmental factors such as soil microbial carbon and total nitrogen content,with total soil nitrogen having a significant positive effect on the abundance of plant feeders,and CAN soil nitrate having a significant positive and negative effect on the abundance of omnivores and plant feeders,respectively.It can be seen that nitrogen deposition not only affects soil fauna indirectly through soil physicochemical properties,but also directly.The sample method is more efficient than the trap method in obtaining macrofauna,and the two sampling methods have complementary effects.The study area has more vegetation,which is more suitable for obtaining macrofauna by the sample method,while both methods alone cannot fully reflect the species composition of macrofauna in the study area,so it is suggested that the combination of the two methods can obtain a more comprehensive species composition of soil fauna in the study area.