| Drought and salt stress are the major environmental stresses affecting human productive life.Global warming has led to a clear upward trend in the duration,frequency and intensity of dry seasons.In addition,about one billion hectares of soil worldwide are affected by salinity.Drought and salt stress not only affect plant growth and nutrient uptake,but also cause changes in the soil microenvironment that seriously threaten ecosystem stability and service functions.Arbuscular mycorrhizal(AM)fungi can promote soil nutrient activation,improve soil microbial community structure,and play an important role in ecosystem nutrient cycling processes.Previous studies on AM fungi under abiotic stress conditions have mostly focused on plant physiology and biochemistry or basic soil physicochemical properties,but less on microbial properties related to soil nutrient cycling.In this study,we used a greenhouse pot experiment to design different drought stress treatments(mild drought: 60% field water holding capacity;severe drought: 30% field water holding capacity)and salt stress treatments(mild salt stress: 50 mmol NaCl;severe salt stress: 100 mmol NaCl)to analyze the effects of inoculation with AM fungi(Glomus mosseae)on plant growth(maize(Zea mays L.),soybean(Glycine max(L.)Merr.),Leymus chinensis and alfalfa(Medicago sativa))and rhizosphere soil physicochemical properties and extracellular enzyme activities under drought and salt stress.To investigate the regulatory effects of AM fungi on plant growth and soil properties in response to drought and salt stress,and to provide scientific basis for AM fungi to improve soil fertility and promote sustainable development of ecosystem in the context of climate change.The main results of this study are as follows:(1)The AM fungi(Glomus mosseae)established good symbiosis with all four plants under drought and salt stress conditions,with root infestation rates ranging from 46.85% to82.33%.Meanwhile,both drought and salt stresses reduced the plant root infestation rate to different degrees.Under drought and salt stress conditions,inoculation with AM fungi increased the biomass of maize,soybean,leymus chinensis and alfalfa(mild and severe drought: 94.80% and 168.97%,4.21% and 12.74%,9.65% and 20.91%,95.00% and223.26%;mild and severe salt stress: 188.24% and 238.53%,(17.98% and 6.57%,18.69%and 15.40%,128.36% and 204.35%).(2)Under drought-free conditions,inoculation with AM fungi increased soybean and alfalfa soil nitrate nitrogen(11.75% and 11.58%),leymus chinensis and alfalfa soil available phosphorus(8.37% and 6.02%),maize soil ammonium nitrogen(18.17%)and total phosphorus(5.00%),and soybean soil organic matter content(9.2%),but decreased maize and soybean soil available phosphorus(13.36% and 8.10%)and maize soil organic matter content(7.75%).Under mild drought conditions,inoculation with AM fungi increased soybean and alfalfa soil total N(11.51% and 11.85%),soybean and leymus chinensis soil nitrate nitrogen(11.00% and 2.18%),and maize soil total phosphorus(5.61%).Under severe drought conditions,inoculation with AM fungi increased maize soil ammonium nitrogen(22.21%)and soybean soil total phosphorus content(5.80%),but decreased soybean and leymus chinensis soil available phosphorus(10.43% and 17.26%)and maize soil organic matter content(5.73%).In addition,inoculation with AM fungi in the absence of salt stress increased ammonium nitrogen(18.17% and 9.13%)in maize rhizosphere soils,nitrate nitrogen(11.75% and 11.58%)in soybean and alfalfa soils,available phosphorus(8.37% and 6.02%)in leymus chinensis and alfalfa soils,total phosphorus(5.00%)in maize soils,organic matter(9.20%)in soybean soils,and alfalfa soil total N content(21.32%);however,it reduced the maize and soybean soil available phosphorus(13.36% and 8.10%)and maize soil organic matter(7.75%).Under mild salt stress conditions,inoculation with AM fungi increased the content of soybean soil organic matter,nitrate nitrogen(6.39%)and available phosphorus(17.19%)and alfalfa soil total nitrogen(20.00%);but decreased the content of leymus chinensis soil total nitrogen(6.88%)and alfalfa soil organic matter(4.61%)and ammonium nitrogen(10.91%)and available phosphorus(11.62%).Under severe salt stress,inoculation with AM fungi increased nitrate nitrogen in maize,soybean and alfalfa soils.(11.86%,12.37% and 9.93%),soybean soil organic matter(5.21%)and alfalfa soil total nitrogen(15.94%),but decreased soybean soil total phosphorus(6.88%)and leymus chinensis soil total nitrogen(8.18%)and ammonium nitrogen content(15.38%)(3)Under drought-free conditions,inoculation with AM fungi increased alfalfa soil cellobiose hydrolase(CBH)(22.28%)and β-1,4-N-acetylaminoglucosidase(NAG)activity(17.17%),but decreased soybean and leymus chinensis soil carbon cycle enzymes(CBH,β-1,4-xylosidase(BX),β-1,4-glucosidase(BG))and leucine aminopeptidase(LAP)as well as alkaline phosphatase(ALP)activities in maize and soybean soils(29.73% and 59.02%);under mild drought conditions,inoculation with AM fungi increased maize soil CBH activity(66.67%)but decreased ALP activity(32.38%)and reduced soybean soil carbon and cycle enzyme activities;under severe drought conditions,inoculation with AM fungi increased maize soil CBH activity(66.67%)but decreased ALP activity(32.38%).inoculation with AM fungi increased maize soil CBH(14.75%)BX(25.69%)and NAG activity(69.01%),alfalfa soil BG(42.81%)and LAP(55.70%)activities and soybean soil carbon cycle enzymes and nitrogen cycle enzymes(NAG、LAP)and phosphorus cycle enzyme(ALP)activities,but decreased leymus chinensis soil CBH(28.98%)and BX activities(21.50%).In addition,inoculation of AM fungi under no salt stress conditions reduced soybean and leymus chinensis rhizosphere soil CBH(48.72% and 24.60%),BX(39.22% and 16.00%),BG(38.84% and 29.77%)and LAP(47.35% and 28.08%)activities and increased alfalfa soil CBH(22.28%)and NAG(17.17%)activities.Under mild salt stress conditions,inoculation of AM fungi reduced the rhizosphere soil carbon,nitrogen and phosphorus cycling enzyme activities in maize and leymus chinensis as well as soybean soil BG(14.84%),LAP(30.55%)and ALP activities(29.73%).Under heavy salt stress conditions,inoculation of AM fungi reduced soil carbon,nitrogen,and phosphorus cycle enzyme activities of maize,soil carbon and nitrogen cycle enzyme activities of alfalfa,and soil nitrogen and phosphorus cycle enzyme activities of leymus chinensis.Under drought stress and salt stress conditions,soil pH and nitrogen and phosphorus content had a greater effect on plant rhizosphere soil enzyme activities.The effect of soil phosphorus content on enzyme activity was weakened by the addition of mycorrhizal agents,and nitrogen content became the main controlling factor.In summary,this study investigated the effects of AM fungi on plant growth,rhizosphere soil nutrients and enzyme activities under drought and salt stress using an indoor pot experiment.The results showed that AM fungi could promote plant growth and affect rhizosphere soil nutrients and enzyme activities under drought and salt stress conditions,and mitigate the adverse effects of stress on ecosystem nutrient cycling to some extent.Meanwhile,the positive effects of AM fungi on plants and soils were related to stress and plant type.This study reveals the important role of AM fungi in plant growth and soil nutrient turnover under environmental stress,extending the study of plant and soil response to AM fungi in multiple contexts.Therefore,the use of inoculum or management strategies to enhance the positive effects of native AM fungi is recommended for better management and utilization of land resources for sustainable ecosystem development. |
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