Effects Of Simulated Nitrogen Addition And Precipitation Enhancement On Earthworm Community And The Related Factors In Jigongshan Mountains

Nitrogen deposition and changes in precipitation pattern are important components in global change.As an"ecosystem engineer",earthworms exert profound impacts on the key aboveground and belowground ecological processes.The responses and feedbacks of earthworm community to global changes are at the leading edge of studies in ecology and global change fields.It was an important starting point for systematic study on earthworm and global change to examine the effects of nitrogen deposition and precipitation pattern on the characteristics of earthworm community.The Jigongshan Mountains,which located in the north-south climate transition zone in Xinyang,Henan Province,possess characteristics of both the temperate and subtropical regions and thus may be relatively sensitive to climate change.The simulated nitrogen deposition and preciptation experimental platform?NPEP?was established in 2012 in Jigongshan Mountains.Earthworm community,their potential food sources and the living environmental conditions?i.e.,physico-chemical traits and microbial community?were investigated in May 2018.Given that the trophic level of earthworms may partly reflect the potential ecological functions of earthworms,this study could obtain a comprehensive understanding of the sensitivity of earthworm community to nitrogen deposition and precipitation enhancement in this north-south climate transition zone of China.Here,in this study,the responses of both the structure of earthworm community?species composition,abundance and biomass?and the trophic level(¹³C and ¹⁵N natural abundances)of dominant earthworm species to nitrogen deposition approach?canopy vs understory,CN vs UN?and nitrogen concentration(25 and 50 N ha^-1yr^-1)were explored using the NPEP after 5years of treatment.Meanwhile,the main effects and interactive effects of canopy nitrogen deposition?CN25?and canopy precipitation?+30%background rainfall,CW?on the characteristics of earthworm community were explored.The field survey showed that,although the earthworm community composition will theoretically possess both the features of Palaearctic and Oriental realms,the typical Oriental group of pheretimoids dominated in the Jigongshan Mountains.Whereas,the typical Palaearctic group of lumbricids only occurred occasionally.As for the understory-and canopy-simulated nitrogen deposition experiment,we found that:1)The effects of canopy nitrogen deposition and traditional understory nitrogen deposition on earthworm community structure were not fully consistent.The treatment of UN50 reduced earthworm abundance and biomass significanty,but the treatments of UN25and both the CN25 and CN50 did not.Therefore,the potential negative effects of nitrogen deposition with high nitrogen concentration on earthworm community may be overestimated by the traditional approach of understory-simulated nitrogen deposition.2)Both the UN50and CN50 increased the trophic levels of the two representative earthworm species?the epigeic Amynthas sp.1 and endogeic Amynthas sp.2?,but the nitrogen deposition approach did not exert any significantly effects.3)The canopy nitrogen deposition significantly increased the concentrations of TN and NO₃^--N in soils,and reduced pH,the concentrations of TP and AP in soils;while,the understory nitrogen deposition significantly reduced the litter Ca concentration,soil pH,and soil TC,TP and NH⁴⁺-N concentrations.4)Both the the nitrogen deposition approach and nitrogen concentration did not significant affect the biomasses of the major soil microbal groups.5)The RDA results indicated that there were close connections among earthworm community structure,physico-chemical traits in litter and soils,and the soil microbial community structure.For instance,litter TN concentration and the biomasses of major soil microbial groups were negatively correlated with the community size of earthworm;while,soil pH and DOC concentration were positively correlated with the community size of earthworm.Hence,although soil microbes and earthworm community were closely related,the the afformentioned effects of nitrogen deposition on earthworm community charateristics may be attributed to the changes in the physico-chemical traits of litter and/or soils,rather than to the changes in microbial biomass.As for the canopy-simulated nitrogen deposition and precipitation enhancement experiment,we found that:1)both the canopy nitrogen deposition and precipitation enhancement did not significantly affect earthworm abundance and biomass;but the combination of canopy nitrogen deposition and precipitation enhancement significantly reduced the abundance of the earthworms and the biomass of the epigeic species of Amynthas sp.1.2)Both the canopy nitrogen deposition and precipitation enhancement did not significantly affect the trophic level of Amynthas sp.1;but precipitation enhancement tended to increase the trophic level of Amynthas sp.2?P=0.063?.3)The canopy nitrogen deposition significantly decreased litter TP concentration,soil pH,and soil Ca and TP concentrations.Precipitation enhancement has significantly increased concentrations of litter TP and soil TP,but significantly decreased soil NH⁴⁺-N concentration.While,the combination of canopy nitrogen deposition and precipitation enhancement significantly reduced the concentrations of litter TC,TN and Ca.4)Both the canopy nitrogen deposition and precipitation enhancement did not significantly affect the biomasses of major soil microbial groups;but precipiation enhancement significantly increased the bacterial stress index for systems with canopy nitrogen deposition.5)The RDA results indicated that,although the soil microbial community and the physico-chemical traits of litter may exert influences on earthworm community,94.7%of the variation of earthworm community structure could be explained by the soil physico-chemical traits.Note that liter N/P,soil DOC concentration was positively related with the community size of earthworm,while soil AP and soil bacterial stress index were negative related with the community size of earthworm.Overall,under the combination influences of the canopy nitrogen deposition and precipiation enhancement,both the earthworm community and the soil bacterial activity was inhibited.The underlying mechnisms were not still clear,but should be related with the substantially increased availability of nitrogen and phosphorous.