Seasonal Dynamics Of Amino Acid Contents And Diffusive Fluxes In Soils Of Three Grassland Types In Alpine Meadow

In the past 30 years,a large number of studies have shown that many plants in various ecosystems have the ability to directly absorb small organic nitrogen molecules,especially amino acids,in the soil.If amino acids can constitute one of the important nitrogen sources for plants,they need to have a high content or flux(the amount of amino acids passing through a unit area per unit time,nmol N cm^-2h^-1)in the soil.The amino acid flux ranther than its content can better reflect its turnover in the soil over a period of time,and can reflect the actual diffusive rate of soil available nitrogen to the root surface.At present,there are few studies on soil amino acid diffusive fluxes in the world,and the factors influencing amino acid fluxes have not been elucidated.Therefore,in this study,the alpine meadow dominated by the non-mycorrhizal Kobresia graminifolia,the ectomycorrhizal Polygonum viviparum and the endomycorrhizal Elymus nutans were used as the research objects.Exploring The seasonal variation of amino acid content and fluxes in different grassland types were explored to reveal the influencing factors of soil amino acid content and diffusive fluxes.The results of this study will provide a theoretical basis for elucidating the mechanism of plant uptake of soil organic nitrogen,and provide a guarantee for maintaining the stability and species diversity of the alpine meadow ecosystem on the Qinghai-Tibet Plateau.The main findings are as follow:（1）During the growing season,the contents of free amino acids and extractable amino acids in the three grassland types measured by water and potassium chloride extraction methods were 0.02～0.25μg N g^-1 and 0.57～4.67μg N g^-1,only accounting for 0.18%～13.66%and 8.98%～29.40%of the content of total soil available nitrogen,respectively.There was a very significant difference between the amino acid content and ammonium nitrogen content obtained by water and potassium chloride extraction methods.（2）During the plant growth season,the diffusive fluxes of free amino acids in the soil of K.graminifolia,P.viviparum and E.nutans grassland types measured by microdialysis were 2.31～62.09 nmol cm^-2h^-1、3.22～9.43 nmol cm^-2h^-1 and 2.11～17.06 nmol cm^-2h^-1,accounting for 8.71%～82.73%、15.54%～47.41%and 8.18%～58.61%of the total diffusive flux of soil available nitrogen,respectively.In the late growing season,the diffusive fluxes of free amino acid were significantly higher than that of inorganic nitrogen in the three grassland types,and the diffusive fluxes of free amino acid in the soil of the K.graminifolia grassland type was significantly higher than that in the other two grassland types.The composition of free amino acid pools in the three grassland types was similar,including 22 protein amino acids and 3non-protein amino acids.The free amino acids with the highest diffusive fluxes were Ala,Gly and Ser.Overall,the diffusive fluxes of nonpolar amino acids and polar uncharged amino acids were significantly higher than that of other types of amino acids,while the diffusive flux of aromatic amino acids was significantly lower than that of other types of amino acids.Amino acid diffusive fluxes in the three grassland types were significantly affected by species and season,respectively,and their interactions.（3）The microbial biomass nitrogen in the three grassland types was higher in regreening stage of the plants.The soil microbial biomass C:N ratio showed a trend of first increasing and then decreasing with the growth season.In the early growing season,the C:N ratios of microbial biomass in the soils of K.graminifolia and E.nutans grassland types were significantly higher than that in the soil of P.viviparum grassland type.Among the three grasslans types,the soil protease activity in the early growing season was significantly higher than that in other seasons,while the soil protease activity in the middle growing season was significantly lower than that in other seasons.During the whole growing season,the potential protease activity in the K.graminifolia grassland type was significantly higher than that in the P.viviparum and E.nutans grassland types.In summary,species,seasons and other factors may affect soil amino acid content and diffusive flux by comprehensively affecting the types of amino acid transporters in plant roots and mycorrhizal surfaces,the composition of rhizosphere exudates and litter,the number and community structure of soil microorganisms and soil protease activity.