|Other Abstract||In recent years, many studies have shown that in arctic tundra, boreal forests, alpine meadows and other ecosystems that are stronglynitrogen 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 nitrogenforms, 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 wasused as the research object. The concentration, diffusion fluxand composition of amino acids in the three types of grasslandwere determined. The 15N natural abundance (&delta15N) of the three plants and their soils were quantified. Under the in situconditions in the field, the 13C15N dual-labeled isotope tracing technique was used to studythe absorption characteristicsof soil amino acids by the three plants and soil microorganisms.The influenceof litter on soil amino acidswas clarified, and the feedback mechanismof nitrogen between plants and soilwas discussed. The purposes of the study were to clarify the strategies of alpine meadowplantsto absorb and utilize soil organic nitrogen, and the mechanism by which Kobresia dominates in nitrogen-limited alpine meadows, in order to provide scientific basisfor maintaining the stability and sustainable development of the alpine meadow ecosystem.The main results are as follows:
(1)The contentsof exchangeable amino acids in the soil of K.graminifolia, P.viviparum, and E.nutansgrassland were0.2~2.6, 0.2~1.5, and 0.1~1.8 &mug 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.graminifoliagrassland was significantly higher than that of E.nutansgrassland during the peak season of plant growth.
(2)The diffusion fluxes of free amino acids in the three grassland soils were 22.6~39.9 nmol cm-2h-1, which were 1.2~2.3 times and 5.9~12.5 times of the diffusion fluxes of NH4+and NO3-, respectively. The free amino acidswith the highest diffusion fluxes wereGlu, Gly, Ala, Ser, and Lys. The diffusion flux of free amino acids in the soil of K.graminifoliagrasslandwas significantly higher than that of P.viviparumgrassland during the peak season of plant growth.
(3)The &delta15N valuesof K. graminifolia, P.viviparum, and E. nutanswere -2.2&permil~1.8&permil, 0.5&permil~1.8&permil, and 0.3&permil~2.4&permil, respectively.The&delta15N values of their soilswere4.6&permil~5.0&permil, which was significantly higher than that of plants.The &delta15Nvalue of K. graminifoliawas significantly lower than that of P.viviparumand E.nutans.
(4) In the grassland with K. graminifoliaas the dominant speciesandP. viviparumand E. nutansas the main associated species, the three plants can absorb the 13C15N dual-labeled amino acids in the soil, and their percentage of complete absorption of various amino acids were 0~76.8%. The absorption rates of 15N in amino acids bythe three plants were 0.07~0.9, 0.05~0.4, and 0.02~0.5 &mug g-1 h-1, respectively, which were close to theirabsorption rate of NH4+. The absorption rate of amino acids by the three plants was significantly affected by the season, soil depth, and organic nitrogenform.
(5) When the three plants competedwith soil microorganisms for amino acids, there was niche differentiation in time, space, and organic nitrogenform. Plants weremore competitive in August than in September and at a soil depth of 3 cm than at 7 cm. K. graminifoliaand E. nutanshadstronger competitiveness againstGly 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 acidsin the soil.|