Effects Of Nitrogen Addition On Soil Fauna In A Temperate Grassland Of Inner Mongolia

Nitrogen（N）addition to terrestrial ecosystems has potential effects on plant and soil biota and relative processes,because plant growth in most ecosystems is restricted by a lack of nitrogen.However,the response of soil biota after nitrogen addition has not been well-studied,because it composes more complex groups and lineages.In this study,we examined how N addition impact soil biota at a temperate grassland community,Inner Mongolia.We established a field experiment consisting a continuous gradients of N fertilization addition（25,50,and 100 kg N ha-1）.We also tested the relationship between soil fauna and environmental factors.The results are as followed:（1）We collected total 15527 soil faunas,belonging to 23 orders,4 classes and 3 phyla.Acridoidea is the dominant groups of ground-dwelling arthropods,accounting for 24.56%of the total of arthropod abundance.The dominant groups of soil mesofauna were Isotomidae and Carabodidae,accounting for 44.38%and 14.83%,respectively.Aphelenchoides and Longidorus were most dominant groups in litter,accounting for 26.30%and 14.45%,respectively.In soil layer（0～5cm）,the dominant groups of soil nematodes were Acrobeles and Longidorus,respectively accounting for 26.28%and 16.49%.（2）The effects of nitrogen addition on three types of soil fauna varied among the different biota.We did not detected significant changes in diversity and total abundance of ground-dwelling arthropod community irrespective of the N levels,but only significantly for low nitrogen addition,where arthropod biomass was 84.06%higher than in the control plot.There was minimal changes in other groups（i.e.,predatory,omnivorous,and saprophagous faunas）to N addition manipulations,but only significantly higher herbivorous biomass for low and high level nitrogen plots than in the control plot.Regression analysis showed that the abundance and biomass of phytophagous arthropods were determined by plant leaf nitrogen content,but saprophagous arthropods were closely related to soil organic carbon.The total abundance of soil mesofauna increased 75.82%for low-nitrogen manipulation,and 76.65%for medium-nitrogen treatments than in the control plot.The diversity of soil mesofauna（i.e.,DG index）was significantly decreased under high nitrogen addition treatments.Soil mesofauna responsed to nitrogen addition varied among the functional groups.The abundance of predatory and saprophagous faunas decreased by 69.05%and 54.41%,respectively,under high nitrogen,but the phytoplasmic and omnivorous functional groups showed minimal response to N additions.RDA analysis detected significantly negative correlations between the abundance of Mesostigmata nymphs、Laelapidae、Hypogastruridae with nitrogen gradients,while the abundance of Isotomidae、Entomobryidae were the indirectly influence by the plant leaf C,N content.In litter layer,the total abundance of soil nematode was significantly increased 171.72%for high-nitrogen manipulation than in the control plot（P<0.05）.The diversity of soil nematode was significantly decreased under high nitrogen addition treatments（P<0.05）.In comparison,the abundance and diversity of nematode community in the soil did not significantly change at all levells of nitrogen（P>0.05）.The structural equation model（SEM）showed that nitrogen addition had a directly effect on the soil nematode community in litter layer（r=0.72）,while the soil nematode community on soil layer was mainly indirectly affected by microbial biomass.In conclusion,the addition of appropriate nitrogen could improve the abundance of soil fauna,while the addition of high nitrogen(10gN·m^-2·yr^-1)has the negative effect on the diversity of soil fauna.Nevertheless,this study found that the effects of nitrogen addition on three types of soil fauna varied among the different biota,which also reflected the complexity of soil fauna community.The abundance or diversity of different groups was affected by not only food resources,but also the control of predators.We also found that it may have a more pronounced effect on the soil fauna groups which were regulated strongly by plant resources than other groups under the increased addition of N.