Uptake Of Soil Organic Nitrogen By Plant Species In Alpine Meadow

In recent years,many studies have shown that in arctic tundra,boreal forests,alpine meadows and other ecosystems that are strongly nitrogen limited,many plants have the ability to directly absorb small molecules of organic nitrogen,especially amino acids,from the soil.However,systematic studies on how seasons,soil depth,organic nitrogen forms,and soil microorganisms affect the absorption of amino acids by plants are very limited.Therefore,in this study,the alpine meadow dominated by Kobresia graminifolia,Polygonum viviparum,and Elymus nutans was used as the research object.The concentration,diffusion flux and composition of amino acids in the three types of grassland were determined.The ¹⁵N natural abundance(?¹⁵N)of the three plants and their soils were quantified.Under the in situ conditions in the field,the¹³C¹⁵N dual-labeled isotope tracing technique was used to study the absorption characteristics of soil amino acids by the three plants and soil microorganisms.The influence of litter on soil amino acids was clarified,and the feedback mechanism of nitrogen between plants and soil was discussed.The purposes of the study were to clarify the strategies of alpine meadow plants to absorb and utilize soil organic nitrogen,and the mechanism by which Kobresia dominates in nitrogen-limited alpine meadows,in order to provide scientific basis for maintaining the stability and sustainable development of the alpine meadow ecosystem.The main results are as follows:?1?The contents of exchangeable amino acids in the soil of K.graminifolia,P.viviparum,and E.nutans grassland were 0.2².6,0.2¹.5,and 0.1¹.8?g N g^-1.The size of exchangeable amino acid pools was similar to that of the inorganic nitrogen pools.The five dominant exchangeable amino acids were lysine?Lys?,glutamic acid?Glu?,leucine?Leu?,alanine?Ala?,and serine?Ser?.The concentration of exchangeable amino acids in the soil of K.graminifolia grassland was significantly higher than that of E.nutans grassland during the peak season of plant growth.?2?The diffusion fluxes of free amino acids in the three grassland soils were22.6³9.9 nmol cm^-2h^-1,which were 1.2².3 times and 5.9¹2.5 times of the diffusion fluxes of NH₄⁺and NO₃^-,respectively.The free amino acids with the highest diffusion fluxes were Glu,Gly,Ala,Ser,and Lys.The diffusion flux of free amino acids in the soil of K.graminifolia grassland was significantly higher than that of P.viviparum grassland during the peak season of plant growth.?3?The?¹⁵N values of K.graminifolia,P.viviparum,and E.nutans were-2.2‰¹.8‰,0.5‰¹.8‰,and 0.3‰².4‰,respectively.The?¹⁵N values of their soils were 4.6‰⁵.0‰,which was significantly higher than that of plants.The?¹⁵N value of K.graminifolia was significantly lower than that of P.viviparum and E.nutans.?4?In the grassland with K.graminifolia as the dominant species and P.viviparum and E.nutans as the main associated species,the three plants can absorb the¹³C¹⁵N dual-labeled amino acids in the soil,and their percentage of complete absorption of various amino acids were 0⁷6.8%.The absorption rates of ¹⁵N in amino acids by the three plants were 0.07⁰.9,0.05⁰.4,and 0.02⁰.5?g g^-11 h^-1,respectively,which were close to their absorption rate of NH₄⁺.The absorption rate of amino acids by the three plants was significantly affected by the season,soil depth,and organic nitrogen form.?5?When the three plants competed with soil microorganisms for amino acids,there was niche differentiation in time,space,and organic nitrogen form.Plants were more competitive in August than in September and at a soil depth of 3 cm than at 7cm.K.graminifolia and E.nutans had stronger competitiveness against Gly than other amino acids.The ability of the three plants to compete with soil microorganisms for amino acids was characterized by K.graminifolia>P.viviparum>E.nutans.?6?After the litter of the three plants returned to the soil,they significantly promoted soil microbial activity and soil enzyme activity related to the nitrogen cycling,inhibited net nitrogen mineralization,and increased the content of exchangeable amino acids in the soil.