| Soil nitrogen(N)cycling plays a major role in structure,function and the response to global change of forest ecosystem.The root of almost tree species forms symbiotic associations of arbuscular(AM)or ectomycorrhizal(ECM)fungi.Mycorrhizal is benefit for trees to obtain soil N and thus regulate soil nitrogen cycling.However,it is still not clear that the effect and mechanism between different mycorrhizal species on soil nitrogen cycling.Hence,we conducted field surveys to establish a natural gradient with increasing ECM tree dominance in a subtropical forest,and explored the effects of tree mycorrhizal type on soil N cycling.Across this mycorrhizal gradient,we quantified total N(TN),dissolved organic nitrogen(DON),microbial biomass N(MBN),ammonia nitrogen(NH4+-N),nitrate nitrogen(NO3--N),net N mineralization rates(Nmin)and net N nitrification rates(Nnit)as well as N-acquiring enzyme activities,litterfall biomass and quality,and soil physicochemical properties such as pH,the abundance of functional genes of soil nitrogen cycling and the composition of microbial community.The main findings are as follow:(1)The soil TN and NH4+-N had no significant change,while the dissolved organic N and nitrate N decreased significantly and the microbial biomass N increased linearly with increasing ECM tree dominance.Nmin and Nnit decreased with the increasing ECM tree dominance in topsoil,and there was a significant positive correlation between Nnit and NO3--N.Soils dominated by AM trees had 1.8-2.3 times larger DON,NO3--N,and net nitrification rate than ECM-dominated stands.What’s more,NO3-/NH4+ was higher than 1 in AM-dominated soil and MBN/(NO3-+NH4+)was significantly higher than that in ECM-dominated soil,which suggesting more sufficient soil available with a rapid N cycling in AM-dominated stands.(2)Annual litterfall biomass had no significantly change while the ratio of litter C:N increased with increasing ECM tree dominance and prominent higher than AM-dominated soil,which indicated that the litter quality of ECM lower and less easily decompose.Our results demonstrated that the increasing ECM tree dominance increased organic carbon(SOC)and soil C:N ratio in the organic horizon but decreased pH and water content.The β-N-acetylglucosaminidase(NAGase)to depleted N directly from soil organic matter,which had significant positive correlation with ECM tree dominance increase in organic horizon.(3)AM-dominated stand exhibited higher abundance of soil functional genes related to soil N transformation,such as amoA,napA and nirK than that in the ECM-dominated stand.Soil bacterial communities were mainly composed acidobacteria,actinobacteria and proteobacteria,but there was no significant difference among different mycorrhizal stands.Ascomycota and basidiomycota were the dominant phyla in soil fungal community,and the abundance in ECM dominant plots was significantly higher than that in AM plots.(4)The relative proportion of ECM trees had an exclusive effect on soil N availability under the condition of accounting for aboveground biomass and diversity,and other soil environmental factors.Path analysis based on structural equation models(SEMs)explained that ECM tree abundance reduced litterfall quality(high C:N ratio),increased soil C:N ratio,reduced pH and water content,and secreted more NAGase to deplete N directly from soil organic matter,which changed soil microbial community structure and inhibited the activity of nitrogen converting microorganisms,and all of these factors decreased the rate of soil nitrogen mineralization and nitrification soil N transformation rate and subsequently available-N content.Given all of that,the abundance of ECM and AM species was strongly influenced soil nitrogen cycling in the topsoil,which provides a scientific basis for the reasonable collocation of mycorrhizal in predicting the consequences of forest composition shifts under global changes and plantation establishment on soil N cycling,and the degradation of vegetation restoration in the engineering practice of different tree species.Additionally,it is of great significance for scientific management and maintenance of forest ecosystem function stability. |
