The Regulation Mechanism Of Arbuscular Mycorrhizal Fungi On Carbon And Nitrogen Turnover Under Drought Stress In Songnen Grassland

The frequency and intensity of extreme weather events,especially droughts,have been increasing caused by global warming and changes in atmospheric circulation,which threaten the stability and service functions of ecosystems.Grassland ecosystems cover about a quarter of Earth’s land area and play a pivotal role in the processes of carbon（C）and nitrogen（N）cycles in terrestrial ecosystems.Increased drought intensity not only alters plant nutrient uptake and soil nutrient balance,but also changes the structure of plant and soil microbial communities,which significantly affects ecosystem biogeochemical cycling processes.Arbuscular mycorrhizal fungi（AMF）,one of the major ubiquitous important components of the soil microorganisms,can form mutualistic associations with more than 72%of terrestrial plant species.AMF plays an important role in the nutrient cycling process of the ecosystem by helping host plants to take up mineral nutrients such as nitrogen,phosphorus and water,as well as obtaining carbon fixed by photosynthesis for their growth and development.However,it is not clear whether AMF can accelerate or slow down the carbon and nitrogen turnover process in ecosystems under drought conditions.Clarifying the patterns of AMF regulation of C and N cycling processes in response to drought stress in grassland ecosystems,and evaluating the potential of AMF effect on the coupling of C and N coupling under different drought intensities will not only be critical for understanding the mechanisms by which soil microorganisms affect nutrient cyclings,but also provide a theoretical basis for adaptive management of grassland ecosystems under climate change conditions.In this study,we established an interactive experiment with simulated different drought intensity and AMF treatment in Songnen grassland northeastern China.The drought intensity consisted of control（natural precipitation）,light drought（reduce growing season precipitation by 30%）,moderate drought（reduce growing season precipitation by 50%）,and heavy drought（reduce growing season precipitation by70%）.The AMF treatment consisted of AMF suppression and AMF non-suppression.The effects of AMF on three important C and N turnover processes in the ecosystem were studied including entry into plants（plant assimilation）,release from plant residues into the soil（litter decomposition）,and emission into the atmosphere（greenhouse gas emissions）.The aim was to reveal the potential role of the AMF effect on C and N cycling processes in the soil-plant-atmosphere continuum（SPAC）in grassland ecosystems under global climate change.The main results of this study are as follows:（1）We investigated the effects of AMF on the resistance and potential mechanisms of perennial C₃grass（Leymus chinensis）and C₄grass（Hemarthria altissima）to drought stress.The results found that AMF mitigated the adverse effects of drought stress on plant growth by enhancing the photosynthetic C sequestration function of plant leaves and the utilization of soil N.Firstly,the increase of drought intensity meanly reduced aboveground biomass of L.chinensis and H.altissima by21%and 12%under light drought,26%and 22%under moderate drought,and 27%and 61%under heavy drought,respectively.However,AMF increased biomass of L.chinensis and H.altissima by an average of 58%and 37%under light and moderate drought conditions,respectively.Secondly,AMF improved the photosynthetic characteristics and N uptake capacity under drought conditions.Under light and moderate drought conditions,AMF significantly increased the photosynthetic assimilation rate of CO₂in the leaves of both plants,while increasing the stomatal conductance and water use efficiency of L.chinensis leaves for alleviating drought-induced stomatal limitation and water limitation.In addition,AMF significantly increased the total N content of the aboveground parts of L.chinensis and significantly increased the superoxide dismutase（SOD）activity under light and moderate drought conditions,and reduced the extent of malondialdehyde（MDA）in plant cells by an average of 32%.In contrast,AMF had no impact on the N content and antioxidant enzyme activity of H.altissima.In addition,this study found that the contribution of AMF to the resistance of the two plants（C₃vs C₄）to drought stress was different,under the light and moderate drought conditions,the contribution of AMF on drought resistance of the C₃plant was higher than C₄plant.These results imply that AMF may regulate the plant community structure by affecting the drought resistance of different plant functional groups in grassland ecosystems under future climate change.（2）Here,we used stable isotope dual-labeling(¹³C and ¹⁵N)of plant litter to investigate the effects of AMF on the dynamics of C and N cyclings along the soil-plant-atmosphere continuum under drought in a temperate grassland.The results showed that drought and AMF had significant effects on the decomposition of L.chinensis litter.Drought meanly reduced the decomposition rate of L.chinensis litter by 31.8%,while AMF increased the decomposition rate of litter by 31.7%.In addition,the presence of AMF resulted in more litter-derived C being released into the soil through decomposition and significantly increased the utilization of litter-derived C by soil microorganisms compared to the absence of AMF.This result suggests that AMF may facilitate the long-term sequestration of litter carbon in the soil by promoting the release of plant litter-derived carbon and the fixation of C by microorganisms.AMF contributed to the release of litter-derived N during the decomposition of litter,and also significantly promoted the resorption of litter-derived N by plants,thereby reducing the litter-derived N into the soil and ultimately reducing the emission of litter-derived N from the soil.Our results further revealed that AMF not only leads to changes in the C/N ratio of the litter itself before and after decomposition,but also leads to a significant increase in the litter-derived C/N ratio of soil and microbial biomass.These results suggest that AMF plays a crucial role in regulating the nutrient cycling function during the decomposition of plant litter under drought conditions in Songnen grassland ecosystems.（3）The effects of AMF on soil CO₂and N₂O emissions during the peak growing season were investigated under different drought conditions in grassland ecosystems.The results showed that drought and AMF play important roles in regulating soil CO₂and N₂O gases emissions.Increased drought intensity significantly suppressed soil CO₂and N₂O emissions,and the presence of AMF significantly reduced N₂O emissions but increased CO₂emissions.AMF altered soil bacteria community structure,reduced the amount of soil available N.AMF significantly increased the activities ofα-1,4-glucosidase（αG）andβ-1,4-xylosidase（βX）.In addition,increased drought intensity significantly inhibited the rate of net N nitrification.Our results highlight that there are three key mechanisms by which AMF inhibits soil N₂O emissions:（i）the reduction of N₂O-producing substrate concentrations,AMF significantly reduced the amount of available N in the soil under drought conditions,especially nitrate N;（ii）the inhibition of N₂O production pathways,the presence of AMF reduced the rate of net N nitrification in the soil,while AMF reduced the production of key N₂O genes nir K and nir S;（iii）the depletion of N₂O products,AMF significantly increased the abundance of the key N₂O-consuming gene nos Z.In summary,AMF is an important driving factor for regulating C and N turnover under drought in grassland ecosystems,especially under light and moderate drought conditions.AMF mitigated the negative impacts of drought on C and N turnover by enhancing plant resistance to drought stress and altering soil C and N cycling processes in the studied grassland ecosystem.Our results suggested that AMF would mitigate the response of nutrient turnover to climate change by accelerating the rate of nitrogen and carbon turnover in grassland ecosystems.And it would affect the plant community composition and ecosystem stability via altering the ability of different functional groups of plants to resist drought stress in future global change.This study highlight that AMF plays a vital role in the cyclings of C and N in the soil-plant-atmosphere continuum under global climate changes,the protection of soil microorganisms should be considered for ecosystem stability and grassland sustainability under future global change.