| Arbuscular mycorrhizal(AM)fungi can form symbiotic associations with?72%of terrestrial plant species and exert profound influences on soil carbon cycling.AMF provide plants with soil nutrients and water,in return for plant carbohydrates,forming a great amount of extra-radical mycorrhizal hyphae(EMH)and glomalin in the soil.However,these carbon also release from soil through respiration.In addition,AMF can influence litter decomposition process which may subsequently alter soil C sequestration.Glassalnd ecosysytems play an important role in the terrestrial ecosystems,and grazing is an important way of interference of grassland soil sequestration.However,it is still poorly understood that how grazing regulates the effect of AMF on soil C sequestration.In this study,observatory and simulated grazing approaches were used to address the following three contens in temperate grasslands:(1)the effect of grazing on AMF abundance;(2)the effect of simulated foraging on AMF abundance and hyphal contribution to soil CO2 efflux;(3)the effect of simulated foraging on mycorrhizal contribution to litter decomposition.The observatory experiment was conducted in along-term(10 year)grazing experiment with grazing intensity(0-9.0 sheep ha-1)and topography treatments(flat or sloped).Wemeasured plant species richness,aboveground plant productivity,soil nutrient contents,edaphic properties,and AM hyphal length density(HLD),to reveal the mechanisms and pathways of grazing effects on AMFabundance.We found that HLD linearly declined with increasing grazing intensity in sloped areas,but HLD increased at low grazing intensity,peaked at the moderate grazing intensity(4.5 sheep ha-1)and decreased at high grazing intensity in flat areas.Structural equation modeling analysis indicates grazing intensity reduces HLD by altering soil nutrient dynamics in sloped areas,but non-linearly influences HLD through plant community and edaphic changes in flat areas.Our findings highlight topography influences the effects of grazing intensity on AMF abundancein temperate grasslands.We conducted a field experiment using in-growth mesocosms to partition soil CO2 efflux from roots,AM mycelia and free-living soil microbes.Defoliation and plant functional group loss were utilized to simulate defoliation by herbivores and changes in plant functional group,respectively.We investigated the effects of defoliation on CO2 efflux partitions of C3 grasses,C4 grasses,forbs and diverse plant community.We found that the contributions of roots,AM mycelia,and free-living soil microbes to total soil respiration in diverse grasslands are 30.96 ±7.88%,8.65 ±9.90%and 60.39 ±5.55%in 2017,respectively.AM mycelia contributions to soil respiration were significantly greater(32 ± 8%)in C3 grasses-dominated grassland plots than any other plant functional group.Plant functional groups differed in their ability to produce EMH,indicated by hyphal production efficiency(hyphal length divided by aboveground biomass production).C3 grasses had the lowest hyphal production efficiency,whereas C4 grasses and diverse plant community were intermediate and forbs had the greatest efficiency.In addition,mowing did not influence AM mycelial respiration but increase EMH length.We conclude that C3 grasses tended to have the lower hyphal production efficiency and higher AM mycelial respiration,which may contribute to soil carbon loss in C3 grasses-dominated grasslands.Our study suggests that C4 grasses and forbs tend to be more efficient in hyphae production and have lower AM mycelial respiration,which indicate a potential carbon sequestration by AMF.Thus,changes in plant community composition by grazing may alter AM hyphal respiration and hyphal production,and consequently regulate mycorrhizal contribution to soil C sequestration.To test the effect of plant functional group and mowing on mycorrhizal contribution to litter decomposition,we used nylon meshes that were allow or exclude AMF hyphe.Our results show that the presence of AMF facilitated the activites of P-glucosidase and polyphenol oxidase in order to accelerate the degradation of litter.In addition,litter decomposition rate was higher in C3grasses,and the lowest was in the C4 grasses.There is no influence of plant functional group on mycorrhizal contribution to litter decomposition.Mowing has no effect on litter decomposition probablly because it is in early stages of disturbance.Our results indicate that the presence of AMF facilitated the the degradation of litter,and changes in plant community composition may alter the rate of litter decomposition.In summary,topography influences the effects of grazing intensity on AMF abundance in temperate grasslands.Shifts in plant community composition by grazing can alter AM hyphal respiration and hyphal production,and thus regulate mycorrhizal contribution to soil C sequestration.The presence of AMF facilitated the activites of enzymeand accelerate the degradation of litter.The presence of AMF facilitated the the degradation of litter,and changes in plant community composition may alter the rate of litter decomposition. |
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