Soil Carbon Sequestration Mediated By Arbuscular Mycorrhiza Under Stimulated Nitrogen Deposition

Arbuscular mycorrhizal?AM?fungi,which form associations with roots of⁸0%of land plant species,obtain carbon?C?from their host plants in return for mineral nutrients.AM fungi can significantly improve plant nutrition and growth,and they are an important node which connecting above-and below-ground ecosystems.AM fungi influence the biogeochemical cycle and many other ecosystem process.AM fungi are not only an important part of soil C pool,but also can affect organic matter degradation and soil C sequestration through affecting the activities of other soil microorganisms.However,how do AM fungi mediate the C sequestration process of soil,and if soil C sequestration mediated by AM fungi is affected by global factors such as N deposition remains unknown,which seriously hinders our further understanding of the ecological functions of AM fungi.To answer the questions above,a long term fertilization(0,5,10 and 15 g N m^-2 yr^-1)district was designed in an alpine meadow ecosystem on the Qinghai-Tibet Plateau to stimulate N deposition.The corresponding abbreviations are N0,N5,N10 and N15.Two types of mycelium growth bags were embedded?with or without plant litter;the pore diameter is 38.5?m;the plant roots cannot enter the meshbag,but the mycelium of AM fungi can enter?.Therefore,we carried out the study of soil C sequestration mediated by AM fungi.The main research results are as follows:1.In the soil outside the bag,N addition increased soil organic C content,but had no significant effect on AM fungal C content(N0:1.54 mg C g^-1 soil;N15:1.47 mg C g^-1 soil).The contribution rate of AM fungi C to soil organic C pool gradually decreased from 6.4%of N0 to 5.4%of N15 with the increase of N fertilizer concentration?N effect:P=0.02?.N addition significantly increased the total microbial PLFA content in soil?P=0.015?and bacterial PLFA content?P=0.009?,but had no effect on the total fungal PLFA and AM fungi PLFA content?both P?0.2?,resulting in significantly lower fungal and bacterial PLFA ratio under N treatment.2.In the two kinds of bags with?Res?and without?CK?litters,N significantly reduced the activity of disaccharide hydrolase?both P?0.001?and increased the activity of neutral pHospHatase?both P?0.001?,but had no significant effect on the activity of?-glucosidase?both P?0.09?.There was no significant effect of N application on the degradation rate of litters?effect of N application:P=0.15?.There was no significant effect of N fertilizer on the C content of AM fungi?both P?0.3?and the PLFA content of AM fungi?both P?0.08?,showing that N addition had no significant effect on carbon sequestration directly mediated by AM fungi.At the same time,N addition to the meshbag had no significant effect on total microbes and the total bacteria PLFA content,but bacteria and saprophytic fungi PLFA content have different performance?significantly increase the content of Ck group bacteria PLFA bag saprophytic fungi:P=0.01,the Res group had no significant effect?.Besides,N addition had no significant effect on the amount of net carbon sequestration?both P?0.3?,showing that N addition had no significant effect on carbon sequestration indirectly mediated by AM fungi.3.The enzyme activity,PLFA content of total microorganisms,AM fungi C content and the contribution rate of AM fungi to total net C sequestration in the bags in Res group were significantly higher than those in Ck group,but the annual net C sequestration amount was significantly lower than that in Ck group?bag-type effect:P<0.001?.The results showed that in the alpine meadow ecosystem,the N deposition had no significant effect on the direct or indirect C sequestration mediated by AM fungi.Litter addition increased the direct contribution rate of AM fungi to the amount of soil net C sequestration,but reduced the amount of annual net C sequestration.This study not only revealed the influence of N deposition on the C sequestration mediated by AM fungi and its regulatory mechanism,but also provided a theoretical basis for us to scientifically evaluate and predict the ecological impact of human activities and global changes such as burning fossil fuels,artificial fertilization and atmospHeric nutrient deposition.