Effects And Mechanisms Of Earthworm And Arbuscular Mycorrhizal Fungi Interactions On Nitrogen Cycling In The Wheat Field After Rice Cultivation Under Different Straw Managements

Nitrogen（N）availability is usually the limiting factor for the productivity of terrestrial ecosystems,N contained in straw is one of the important sources in agroecosystems,and the N cycling process driven by straw addition plays an important role in improving soil fertility and crop yield.However,the effects of management methods and soil organisms on the N cycling process driven by straw returning are still not well explored.Earthworms and arbuscular mycorrhizal fungi（AMF）are two groups of important soil organisms,both of which play an important role in organic matter mineralization and nitrogen cycling.The effects of earthworms and AMF interactions on N cycling under different straw managements are still unclear.This study was based on t he long-term straw returning experiment（since 2008）,aimed at the rice-wheat rotation system in the middle and lower reaches of the Yangtze River agricultural district,set up three straw return methods ditch buried straw return（DBSR）,rotary tillage straw return（RTSR）and no tillage with no straw（NTNS）.The effects of earthworms and AMF interactions on N cycling through a combination of field micro-plots and greenhouse pot experiments.First,a micro-plot experiment was set up to study the effect of earthworm-AMF interaction on soil N transformation and N₂O emission under different straw returning methods.Two earthworm ecotypes were observed in the field,a factorial microcosm experiment by simulating of three straw returning methods（NTNS,RTSR and DBSR）was conducted to test the effects of AMF-earthworm interaction on soil N transformation by including two earthworm species with different feeding behaviors（Eisenia foetida,epigeic,EP and Metaphire guillemi,endogeic,EN）under different straw returning methods.A microcosm experiments by simulating of three straw returning methods（NTNS,RTSR and DBSR）was conducted to test the effects of AMF on N₂O emission and plant N assimilation with different textures.Based on the long-term straw returning experiment,a micro-plot experiment was set up to study the effect of earthworms on m ycorrhiza-mediated nitrogen uptake under different straw returning methods.Finally,a microcosm experiment was conducted to test the effects of AMF-earthworm interaction on wheat ¹⁵N uptake from rice straw under different straw managements.This study will provide theoretical evidence for N₂O emissions under straw return in farmlands from the perspective of soil biological interaction.The main results are as follows:1.Effects of earthworm-mycorrhizal interaction on soil N transformation and N2O emissions under different straw managementsEarthworm promoted the cumulative N₂O emission under RTSR and DBSR treatments,but promoted the cumulative N₂O consumption under the NTNS treatment.Earthworm decreased nitrification and denitrification under NTNS treatment;however significantly increased nitrification and denitrification under RTSR treatment;but only affect denitrification under DBSR treatment.AMF decreased N₂O emission under NTNS treatment,but increased the N₂O emission under RTSR and DBSR treatment.Earthworm-AMF interaction increased the cumulative N₂O consumption in NTNS treatment,but decreased the cumulative N₂O consumption under DBSR treatment,and had no effect under RTSR treatment.The earthworm-mycorrhizal interaction significantly increased the denitrification at jointing and booting stage under RTSR and DBSR treatments.2.Effects of earthworm ecotype and AMF interactions on soil N transformation under different straw managementsAMF significantly increased aboveground plant N uptake under NTNS,RTSR and DBSR treatments.AMF significantly decreased NH₄⁺-N,but increased NO₃^--N under three straw management treatments.Under NTNS treatment,AMF significantly increased the potential ammonium oxidation,but significantly decreased potential ammonium oxidation under RTSR and DBSR treatments.Under RTSR and DBSR treatments,AMF significantly decreased potential N2 O reductase activity but exerted no effect on it for NTNS treatment.Earthworms significantly affected soil NH4+-N concentration,but the impacts were shown variable with their feeding behavior under different straw management.Interactions between earthworms with different feeding types and straw management could affect AMF-mediated nitrification and denitrification in soils.Earthworms promoted mycorrhizal 15 N uptake under RTSR treatment,but reduced it under DBSR treatment.However,the interaction between AMF and endogeic earthworms promoted total plant N uptake under DBSR treatment.3.Effects of AMF on wheat nitrogen uptake and N2 O emission under different straw managements and soil textureAMF has positive effect on plant N uptake in both sandy and clay soil;however,this effect is greater in sandy than clay soil.We also observed distinct roles of AMF on soil N availability in different soil textures.AMF significantly affect soil NH4+ concentration,but not affected soil NO3-concentration in sandy soils.However,AMF increased the availability of NO3-but decreased NH4+ in clay soils.Under NTNS and RTSR treatments,AMF suppressed N2 O emission in the sandy soil,but did not affect N2 O emission in the clay soil.However,under DBSR treatments,AMF promoted N2 O emission in the sandy soil,but promoted N2 O consume in the clay soil.Compared with NTNS treatment,RTSR treatment increased mycorrhizal dependency of plant growth,while DBSR treatment decreased mycorrhizal dependency of plant growth.The effects of AMF on N2 O emissions vary with wheat growth stages.These findings highlight that the effect of AMF on N2 O emission and N translocation depend on soil textures and straw return methods.4.Effects of earthworms on AMF community and N uptake under different straw managementsCompared with NTNS treatment,both of DBSR and RTSR treatments significantly changed AMF community composition and enhanced the mycorrhiza-mediated plant N uptake through altering some dominant AMF taxa.Shift of dominant AMF species by straw return provide the greatest contribution to the variation of AMF communities.The effect of earthworm activity on AMF community composition and mycorrhiza-mediated N uptake strongly depended on the straw management regimes.While earthworms increased AMF dominance（+32.9%）and mycorrhizal nitrogen uptake（+2.05-fold）under NTNS treatments,they decreased AMF dominance（-30.4% and-41.9% respectively）and mycorrhizal nitrogen uptake（-37.3% and-34.3% respectively）under both DBSR and RTSR treatments in comparison with no earthworm addition.The results suggest that straw management and its interaction with earthworms can affect mycorrhiza-mediated plant nitrogen uptake,possibly through altering some dominant AMF taxa.5.Effects of earthworm-AMF interactions on N mineralization from straw and N uptake by cropsAMF significantly increased wheat N uptake under RTSR（47.29%）and DBSR（3.67%）treatments.However,AMF significantly decreased wheat 15 N uptake from rice straw under RTSR treatment（-19.28%）,but had no s ignificant effect under DBSR treatment.The effect of earthworm on wheat N uptake depended on the straw returning method.Under RTSR treatment,earthworm increased wheat N uptake,but had no significant effect under DBSR treatment.However,earthworm significantly decreased the wheat uptake of 15 N from straw under RTSR treatment,but increased the wheat uptake of 15 N from straw under DBSR treatment.Under both of RTSR and DBSR treatments,interaction between AMF and earthworm significantly increased wheat 15 N uptake from straw.Overall,the earthworm-mycorrhizal interaction significantly increased nitrification and denitrification.However,earthworm-mycorrhizal interaction mainly affects N2 O emission by affecting the last step of the denitrification process.Under RTSR treatment,earthworms increased mycorrhizal N uptake;while in DBSR treatment,mycorrhizal N uptake was decreased.We further found that RTSR treatment increased mycorrhizal dependency of plant growth,while DBSR treatment decreased mycorrhizal dependency of plant growth.Straw return methods and its interaction with earthworms can affect mycorrhiza-mediated plant N uptake through altering some dominant AMF taxa.Compared with DBSR treatment,earthworms decreased mycorrhiza-mediated plant N uptake under RTSR treatments.Therefore,earthworm-AMF interaction had better effect on N emission reduction and wheat yield increase under RTSR treatment.