Effects Of Fertilization On Soil Fauna Community: A Field Study And A Meta-analysis

Soil fauna is critical for maintaining ecosystem function,and the communities of soil fauna could be significantly affected by human activities such as fertilizer application.However,our knowledge of how soil fauna community composition responds to fertilization remains limited,and the debate over the relative contribution of habitat changes and food resources to the response of soil fauna is ongoing.Here,I used a field experiment to explore the effects of nitrogen?N?addition on the soil fauna community,its dominant controlling factors as well as a meta-analysis of fertilization types on soil fauna to identify global response patterns and regulating factors.In the first study?the field experiment,Chapter 2?,I simulated N deposition(0,50,100,150,and 300 kg N ha^-1 year^-1)to explore the effects of N addition on the total and the phytophagous soil fauna along the soil profile?0-10,10-25,and 25-40 cm?in poplar plantations?Populus deltoids?on the east coast of China.Ammonium nitrate?NH₄NO₃?was dissolved in water and sprayed evenly under the canopy with a backpack sprayer to simulate N deposition.Our results showed that N addition either significantly increased or decreased the density?D?of both the total and the phytophagous soil fauna(D_total and D_p)at low or high N addition rates,respectively,indicating the existence of threshold effects over the range of N addition.However,N addition had no significant impacts on the number of groups?G?and diversity?H?of either the total or the phytophagous soil fauna(G_total,G_p and H_total,H_p).With increasing soil depth,D_total,D_p,G_total,and G_p largely decreased,showing that the soil fauna has a propensity to aggregate at the soil surface.H_total and H_p did not significantly vary along the soil profile.Importantly,the threshold effects of N addition on D_total and D_p increased from 50 and 100 to 150 kg N ha^-1 year^-1 along the soil profile.Fine root biomass was the dominant factor mediating variations in D_total and D_p.Our results suggested that N addition may drive changes in soil fauna community composition by altering belowground food resources in poplar plantations.In the second study?the meta-analysis study,Chapter 3?,I synthesized 896 observations based on 35 published studies to explore the global patterns of soil fauna community in response to fertilization.We found that fertilization generally increased soil fauna abundance at the global scale.The responses of soil fauna abundance to fertilization varied among different fertilizer types,along the soil depth,and among ecosystem types and climate zones:?1?the response of soil fauna abundance to organic fertilizer application was higher than that to mineral fertilizers;?2?the response of soil fauna abundance in topsoil was lower than that in subsoil;?3?soil fauna abundance in artificial ecosystems like farmland and plantation was more sensitive to fertilization than that in natural ecosystems?forests and grasslands?;and?4?the response of soil fauna abundance to fertilization was higher in tropics followed by temperate and then cold climate zones.In addition,the interactions of fertilizer types with ecosystem types and climate zones all had substantial impacts on the response of soil fauna abundance to fertilization.However,fertilization showed no significant impacts on the group number and diversity of soil fauna community.Regression analysis was performed,showing that changes in food resources were critical controls on the response of soil fauna to fertilization,including soil nutrient?e.g.,soil available N?and soil microbes.Our results suggested that fertilization may alter soil fauna community at the global scale through variations in food resources.In general,both of my field study and meta-analysis consistently showed that fertilization could significantly increase soil fauna abundance but had little impacts on the group number and diversity of soil fauna.Variation in food resources is found to be the important factors driving responses of soil fauna community to fertilization.Our findings may contribute to modelling soil fauna response patterns to fertilization globally and assessing appropriate fertilization management practices among different ecosystems worldwide.Further comprehensive analysis is still of necessity to uncover whether responses of soil fauna community to fertilization and their controlling factors have commonalities among ecosystems,aimed at improving the development of the global models.