Effects Of Different Agronomic Practices On Arbuscular Mycorrhizal Fungal Communities In A Rainfed Agroecosystem

Arbuscular mycorrhizal（AM）fungi can form mutualistic symbiosis with the roots of most terrestrial plants.It is well known that AM fungi can help their host plants to absorb mineral elements,such as phosphorus（P）and nitrogen（N）,and can enhance the tolerance of host plants to against drought and pathogens.Almost all crop species are AM plants,and rich evidence shows that the growth of crop plants and agroecosystem sustainability are highly dependent on both the species and functional diversity of AM fungi.Since AM fungi are beneficial to crop growth and soil health,their potential in ecological intensification agriculture has been increasingly appreciated.Previous studies have shown that AM fungi are sensitive to various agronomic disturbance.However,whether conventional intensive agronomic practices,such as soil tillage and chemical fertilizers,would damage the symbiotic relationship between AM fungi and crops,whether sustainable agronomic practices,like no-tillage,organic manure,and straw return,could facilitate to maintain AM fungal activity and diversity,are still largely unclear.These knowledge gaps make it difficult to manage and harness mycorrhizal symbiosis for sustainable crop production.This study was conducted in a typical rainfed dryland agroecosystem on the Loess Plateau,including two independent experiments:the first one was a long-term（14-16yr）interactive treatments with two tillage（conventional tillage or no-tillage）and six fertilization（single or combined mineral nitrogen,phosphorus and/or cow manure addition）;the second one was a 5yr of field treatments with straw return（with or without straw return）and reduced application of N fertilizer(common N application rate[100%N application;225 kg N ha^-1 yr^-1],80%N application,60%N application,and no N application).In the first experiment,we used high-throughput sequencing to characterize AM fungal communities of wheat roots for three consecutive years,and measured the mycorrhizal functioning of each experimental plot using a mycorrhizal inoculation experiment in a greenhouse.In the second experiment,we investigated the effects of straw return and N fertilizer reduction on the AM fungal community in maize roots.The main findings are presented below.（1）A total of 77 AM fungal VTs were identified from wheat roots that were collected from the tillage and fertilization plots for three consecutive years.Of which,VT193（related to Claroideoglomus lamellosum）and VT65（Funneliformis caledonium）were the two most dominant VTs.Conventional tillage decreased VT richness by about 34%compared to no-tillage,combined fertilizers（mineral nitrogen,phosphorus and/or cow manure）caused to decline over 45%compared to the unfertilized control,and continuous monoculture reduced VT richness only under tilled condition.Conventional tillage,fertilization and continuous monoculture strongly affected AM fungal community,and shifted community composition towards dominance of VT193 and VT65.The variations in VT richness and community composition of AM fungi were tightly associated with the changes in soil fertility,and the VT richness declined dramatically with the increasing of soil integrated fertility index.The effects of AM fungal communities on wheat biomass（i.e.,mycorrhizal functioning or phenotype）,as measured by mycorrhizal growth responses（MGRs）,were generally negative across all treatments;however,no-tillage treatment could alleviate the negative mycorrhizal effects,and even produced positive mycorrhizal effects under low fertility condition.AM fungal VT richness correlated positively with MGR,indicating that the high diversity of AM fungi could promote the growth of wheat.Wheat yield increased significantly with the increase of soil integrated fertility,but there was no significant difference between no-tillage and conventional tillage.These results indicate that tillage,fertilization,and continuous monoculture would significantly affect the structure and function of AM fungal community,and that no-tillage as well as low N and P fertilizer input could maintain the species diversity and symbiotic function of AM fungal community.Additionally,these results also suggest that no-tillage might be an effective practice in synchronously maintaining mycorrhizal diversity and crop yield in the rainfed agroecosystem of the Loess Plateau.（2）A total of 40 AM fungal VTs were identified from maize roots,of which the two most dominant VTs were VT113（Rhizophagus fasciculatus）and VT156（Dominikia gansuensis）.Root length colonization and VT richness of AM fungi were sensitive to N fertilization but not to straw return,and both were gradually increased with the decreasing of N fertilizer rate.Similarly,the community composition of AM fungal was affected by N application rate but not by straw return,and the community variation could be well explained by soil available N and P concentrations.Under straw return treatments,60%,80%and full N fertilization produced similar maize yields,indicating that straw return could replace at least 40%of inorganic N fertilizer.These results suggest that straw return with N fertilizer reduction may be a promising practice to synchronously maintain mycorrhizal symbiosis and crop productivity.In summary,intensive agronomic practices,such as tillage,high level fertilization and continuous monoculture,would exert negative effects on mycorrhizal symbiosis,while sustainable agronomic practices,such as no-tillage and straw return,would be beneficial to maintain the activity and diversity of AM fungi.This study highlights the importance of sustainable agronomic practices in maintaining mycorrhizal symbiosis in agroecosystems,and our research findings can provide theoretical guidance and technical supports for the ecological intensification of dryland farmland.