Responses Of Microfood Web Within Hotspots Of Earthworm To Nitrogen Input Gradient

Soil biota determine soil and plant health by delivering multiple ecosystem functions.This multifunctionality are driven by the biotic interactions of soil food web,which is mainly constructed by the intense interactions between soil fauna and microorganisms.As a typical ecosystem engineer in soil,earthworms provide multifunctionality which is mainly controlled by indirect effects of soil microbiota including microorganisms and microfauna.Previous studies have shown that earthworm established many hotsposts like gut,drilosphere and cast by the combinations of feeding,burrowing and casting activities.And the hotsposts which support unique abiotic condition inevitably affect soil microbiota.So far,studies of earthworm-mediated soil microbiota have neglected soil protists.And,the influences of earthworm on microbiota are also mediated by the agricultural activities.Currently,agricultural soil is now experiencing the risk of fast degradation under intensive managements,particularly the excess chemical fertilizer.Although it is well-known that earthworm could modulate hotspots microbiota with providing unique gut-associated conditions,the functional role of earthworm gut in manipulating the microbiota,specially under the pressure of surrounding environment changes remains unknown.The present study investigated soil bacteria and protist assemblages and other abiotic properties in the bulk soil,cast as well as gut content of earthworm across four N input gradients(0,120,240,360 kg N ha^-1)and sampling time was the 3rd and 4th year after nitrogen input.The main results are as followed:（1）Hotspot types established by earthworm activities and sampling time have a significant effect on the diversity of soil protist（p<0.001）.Regardless of nitrogen input gradients,OTUs of soil protist in the intestinal contents was significantly lower than that in soil and cast,with the 20%-60%proportion.In the third year of the study,nitrogen input had no significant effect on diversity in the intestinal contents.However,OTUs in the intestinal contents first increased and then decreased,and Shannon diversity decreased across N input gradients in the next year.Correlation analysis showed that,regardless of the sampling time,the protist diversity was negatively related to soil water content and positively to soil pH.（2）Hotspot types established by earthworm activities have a significant effect on the relative abundances of the four dominant functional groups（p<0.001）,the sampling time also significantly affect bacterivores of soil protist.Regardless of nitrogen input gradients,the relative abundance of bacterivores protist in the intestinal contents is higher than that in soil and cast.In the third year of the study,the relative abundance of bacterivores in the intestinal contents under N360 was 12%higher than N0 whereas the reverse in the fourth year.Correlation analysis showed that,regardless of the sampling time,the relative abundance of dominant functional groups were negatively related to soil nitrate nitrogen,and positively related to soil ammonium nitrogen.（3）Regardless of the amount of nitrogen input and the sampling time,the complexity of the protist and bacteria network in the intestinal contents is lower than that of soil and cast.In the third year of the study,the network complexity in bulk soil and cast decreased with increasing N input,and that in the intestinal contents was the highest under N120.In the fourth year of the study,average degree decreased and average path length increased across N input gradients consequently to having lowest network complexity under N360 whereas highest in the intestinal content.Correlation analysis shows that the network topological properties were related to soil abiotic properties as well as the diversity and community structure of soil protist.（4）Nitrogen input affected soil protist diversity and community structure,as well as network complexity between protest and bacteria by indirectly affecting soil water content,pH,mineral nitrogen and DOC content In addition,network properties are also restricted by soil protist diversity and community structure.In the third year of the study,nitrogen input will indirectly reduce the content of mineral nitrogen and DOC and directly increase the diversity in the intestinal contents.At the same time,nitrogen input indirectly increases the complexity by increasing the soil water content of the intestinal contents,and the dominant functional groups have a significant effect.In the fourth year of the study,nitrogen input indirectly reduced the complexity of the protist and bacterial ecological network by reducing the content of DOC,and the network properties were also affected by the positive effect of water content.In general,the study shows that the amount of nitrogen input are strongly buffered by hostspots which formed by the intestinal processes of earthworms.By measuring the diversity and community structure of soil microorganisms,and analyzing the relationship between them and soil physicochemical and biological properties,it is helpful to understand the effect mechanism of intestinal processes of earthworms on the complexity of microbiological networks or biological interactions under nitrogen input gradient.