Mechanisms And Ecosystem Effects Of Dung Beetles On Livestock Dung Decomposition In A Typical Steppe Of Inner Mongolia

In recent years,with the increase of livestock stocking rate driven by the strong human demand for animal products from natural grassland,the deposition of livestock dung has significantly altered the nutrient cycling of grassland ecosystems,that has a series of ecosystem-level effects.It is imperative to understand the regulatory factors in dung decomposition for understanding the nutrient cycling of grazing grassland ecosystem and developing ecologically healthy grassland management regimes.Many studies have been done on the individual and interactive effects of dung quality,climate conditions and soil decomposer communities on dung decomposition,but the effects of soil fauna in the decomposition of large herbivores dung are not well understood.The arthropods such as dung beetles also play an important role in dung decomposition by feeding,crashing and moving the dung pellets,which alters dung physicochemical properties and has cascade effect on the structure of soil micro-food webs under the dung pellets,whereas the role of arthropods(dung beetles)in the process of dung decomposition is relatively less studied.We did an in situ experiment using the’dung and soil’ mesocosm to investigate the separate and combined effects of two functionally contrasting dung beetle species,a dweller Aphodius sordecens and a tunneler Onthophagus gibbulus,and the interaction of precipitation,dung types and soil fauna,on the decomposition processes of the dung from horses and cattle,the two major grazers with different digestive systems in native steppes in northern China.We determined the changes in dung mass,dung C and N contents,microbial respiration from the ’dung and soil’ mesocosm,and the soil micro-food web community composition from the soil beneath the dung pats under five different dung treatments,that is,1-soil only,no dung,no beetle;2-soil with dung,no beetle;3-soil with dung and dwelling beetles;4-soil with dung and tunneling bettles;and5-soil with dung,and dwelling and tunneling bettles.The results showed that:1.There also existed an interactive effect of dung type and dung beetle treatment on dung mass loss,with a significant effect of tunneler beetle species(in T2 & T3)on the mass loss of cattle dung,but not of horse dung.Compared to the dung treatment without beetles(T0),the tunneler beetle(T2)significantly accelerated the cattle dung dry mass loss,while the dweller species(T1)did not,either for cattle or horse dung.However,regardless of dung type,the synergistic promotion of two functionally contrasting beetle species on the release of dung C and N contents were consistent with the synergistic,positive effects of two species on soil microbial respiration and N mineralization.2.The direct effect of precipitation on the dry mass loss of horse dung was stronger than on that of cattle dung,which was related with the relatively higher C/N ratio,larger porosity and lower hardness of the horse dung compared to cattle dung.The presence of soil fauna significantly reduced microbial respiration of dung at day 7 and 15 of the experiment,but precipitation addition increased microbial respiration of mesocosm with horse dung at day1,3 and 7 of the experiment.Dung C and N loss were mainly controlled by soil fauna and dung quality in the early stage of decomposition,whereas they were mainly controlled by precipitation that regulated microbial respiration in the later stage of decomposition.3.The deposition of dung and addition of beetles changed soil physio-chemical properties(organic carbon,available nitrogen and p H)and fungal diversity in top soil layer of 0-5cm,but had no significant effect on bacterial diveristy.In contrast,the addition of dung beetle significantly increased bacterial diversity,but not fungal diversity,in subsoil layer of 5-15 cm.4.The presence of dung beetle species significantly increased the abundance of nematode community in top soil layer of 0-5cm,and the presence of tunneler beetle species also significantly increased the abundance of nematode community in subsoil layer of 5-15 cm.The effects of dung beetles on the abundance of nematodes varied across different trophic guilds,dung beetles promoted the abundance of bacterivores and plant parasites,but decreased that of the fungivores and the effects were more obvious in top soil lay of 0-5cm.5.The structure of micro-food web was more complex in top soil layer of(0-5cm)than subsoil layer(5-15cm),signifying that N mineralization either from the functional feeding guilds or from the energy pathways of the food web was greater in top soil layer of(0-5cm)than subsoil layer(5-15cm).In conclusion,our results indicate that the deposition of animal dung and beetle affects the interactions among soil nematode and micorbes at different trophic levels in the soil micro-food web,which in turn affects the biogeochemical cycle of grassland ecosystem.Our study has deepened the understanding of the decomposition process of livestock dung,especially the roles and mechanisms of dung beetles in the process,and have important implications for the development of an ecologically healthy grassland management system.