Influence Of Arbuscular Mycorrhizal Fungi On Ecostoichiometry Under Warming And Nitrogen Addition In Songnen Meadow Ecosystem

Arbuscular mycorrhizal fungi(AMF)is ubiquitous in terrestrial ecosystems,which can increase plant nutrients uptake(eg.nitrogen,phosphorus)and affect ecosystem nutrient cycles.However,the species composition of AMF also could be influenced by global change.Up to now,the effects of AMF on soil-plant system stoichiometries under global change remain unclear.To test the effects of AMF on ecosystem stoichiometries under global change,a three-year field experiment was conducted in Songnen meadow northeastern China including warming,N addition and AMF suppression.The carbon(C):nitrogen(N):phosphorus(P)stoichiometries of plant,soil,soil microbial biomass,litter and soil hydrolase were studied.The main results are as follows:(1)N addition increased Leymus chinensis,Puccinellia tenuiflora,Phragmites australis N concentration and N:P ratio,decreased plant C:N ratio.The influence of N addition on plant C:N:P stoichiometry was stronger than warming.N addition could increase the requirement or limitation of P and converted soil N limitation to P limitation,resulting in decoupling of N and P under warming and N addition.Significant interactive effect of warming × N addition × benomyl on three species C concentrations was observed in 2015.AMF increased L.chinensis,P.tenuiflora,C:N ratios,promoted L.chinensis,P.tenuiflora P absorption and decreased the L.chinensis,P.tenuiflora N:P ratios.The contribution of AMF to P uptake might be greater than N uptake in the studied Songnen meadow.Our results indicate that AMF might alleviate P limitations by reducing plant N:P ratios.The negative influence of warming and N addition on plant nutrient limitation might be canceled through the positive effects of AMF on plant P uptake,suggesting that global change might increase the dependence of plants on their mycorrhizal symbionts.(2)N addition did not affect soil stoichiometry.Soils showed high stoichiometric homeostasis under warming and N addition However,there were significant main effects of N addition on soil available N and P concentrations.AMF decreased soil available N concentrations under all treatments and AMF increased soil available P concentrations under N addition and warming plus N addition in 2017.These results suggest that AMF might play a key role in regulating soil P fertility and soil nutrient transformations,improving soil nutrient stability andaccelerating nutrient transfer under warming and N addition.(3)Warming had no effect on the residual masses of Leymus chinensis and Phragmites australis litter,but decreased the litter C:P and N:P ratio of L.chinensis.N addition decreased the residual mass of L.chinensis and P.australis litter,increased the N concentration of L.chinensis litter and the N,P concentrations of P.australis litter,decreased the C:N ratios of L.chinensis and P.australis litter and the C concentration and C:P of P.australis litter.AMF promoted the decomposition of litter,meanwhile,AMF decreased the litter C,N concentration and C:N ratios,C:P ratios and N:P ratios of L.chinensis and P.australis,AMF increased the litter P concentration of L.chinensis and P.australis.The results showed that AMF promoted N mineralization.During the decomposition of litter,the fixation and mineralization of P were carried out simultaneously.The mycelium of AMF absorbed P from the decomposed litter and soil,then transferred the P to the host plant.Therefore,the limitation of soil nutrients on plant growth can be alleviated.AMF can accelerate litter decomposition and shorten litter C,N and P cycle,which is of great significance for the absorption and utilization of plant C,N and P and storage of nutrients in Songnen grassland.(4)The results showed that Songnen meadow steppe was C and P colimited.Significant interactive effects of warming,N addition and AMF on the vector L and vector A of the soil hydrolytic enzymes were observed.AMF decreased the vector L of the soil hydrolytic enzymes under control and warming + N addition.Meanwhile,AMF decreased the vector A of the soil hydrolytic enzymes under warming + N addition,suggeting that AMF might alleviate the microbial biomass C and P limitations under warming and N addition.Significant main effects of year on the soil microbial biomass.Significant main effects of warming on the soil microbial biomass C and N in 2017.Significant main effects of N addition on the soil microbial biomass C in 2015 and soil microbial biomass P in 2016.AMF decreased the soil microbial biomass C under warming and N addition in 2015;under N addition in 2016;under warming in 2017.AMF increased the soil microbial biomass P under control and warming+N addition in 2016;under control in 2017.AM fungi increased litter decomposition rate,they inhibited the development of soil microbial community.Meanwhile,AMF decreased microbial biomass C:P ratios under N addition and warming+N addition in 2016;under control in 2017,indicating AMF have great potential to release phosphorus in the process of microbial mineralization and transformation.These results showed that AM fungicould alleviate soil microbial P limitation by reducing microbial biomass C:P ratio.Warming increased ?-1,4-glucosidase,(?G)and C enzyme activities(?G+?-1,4-glucosidase(?G)+?-1,4-xylosidase(?X)+Cellobiohydrolase(CBH)).N addition decreased ?G,and ?X,C enzyme activities,as well as leucine aminopeptidase(LAP)activity and N enzyme activity(?-1,4-N-acetylglucosaminidase(NAG)+LAP)).There were significant main effects of benomyl and interactive effects of N addition and AMF on ?G,?G,?X and C enzyme activity.AMF decreased ?G,?G,?X and C enzyme activities under control and warming,increased ?G,?G,?X and C enzyme activities under N addition,but it did not reach a significant level.Warming and N addition had no impact on C:N,C:P and N:P soil hydrolase ratios.There were significant main effects of AMF and interactive effects of N addition and AMF on C:P enzyme ratios.Our results showed that the effects of AMF on the soil hydrolase stoichiometries depend on warming and N treatments.In summary,under warming and N addition,AM fungi had important effects on the carbon,nitrogen and phosphorus stoichiometry of plant individuals,soil,litter,soil microbes and soil hydrolytic enzymes activities.The negative influence of warming and N addition on plant productivity might be canceled through the positive effects of AMF on plant P uptake,suggesting that global change might increase the dependence of plants on their mycorrhizal symbionts.AMF might play a key role in regulating soil P fertility and soil nutrient transformations,improving soil nutrient stability and accelerating nutrient transfer,promoting the decomposition of litter,accelerating litter decomposition and shorten litter C,N and P cycle,and alleviating the microbial biomass C and P limitations.This study further demonstrates the role of AM fungi in nutrient cycling from the perspective of the plant-soil system as a whole,make clear the regulation of AM fungi on the turnover and allocation of C,N and P in grassland ecosystem in the context of global change,and predicts the future of AM fungi on ecosystems under global change.The results will help us to elucidate the regulation mechanism that AM fungi influence grassland ecosystem C cycling,N cycling and P cycling under global change condition,and evaluate the potential of AM fungi on ecosystem C cycling,N cycling and P cycling in future.