The Role of Amino Acid Uptake by Mycorrhizal Ericaceous Shrubs in the Nitrogen Economy of Nutrient-Deficient Peat Bogs

以白垩灌木和泥炭藓为主的泥炭地在环缘地区的覆盖面积达4亿公顷。这些生态系统的特点是缓慢的分解和养分循环，以及泥炭（主要是苔藓凋落物）的积累。营养型（雨养）泥炭沼泽是所有生态系统中最缺乏营养的生态系统之一，因为泥炭在周围景观上方积累，因此，这些系统只从大气来源获得营养输入。因此，一般认为疏养泥炭沼泽受氮(N)的强烈限制。我们的野外和实验室的净氮矿化率远低于矿化泥炭地，甚至低于北极冻土带。此外，从大气输入的无机源（主要是沉积，固氮作用可以忽略不计）和氮矿化中，只有大约1/4的年氮素需求可以得到满足。虽然无机氮在泥炭沼泽中供应不足，但有机氮以部分分解的泥炭形式大量存在，少量以溶解有机氮（DON）形式存在。在有机土壤中，一些DON以“游离”氨基酸的形式出现，以前认为这些氨基酸不能被植物吸收。最近的实验室调查和为数不多的实地研究表明，ericaceous菌根植物（甚至一些北极的非菌根莎草）可以吸收氨基酸。这个项目的目标是确定菌根灌丛对氨基酸（也许是更复杂的有机氮形式）的吸收是否可以解释植物年吸收需求中明显的氮缺陷。如果有机氮的吸收是富有机质泥炭沼泽中埃里克森植物氮营养的主要形式，那么它将从根本上改变这些系统以及其他埃里克森主导的生态系统（如石楠荒原和营养贫乏的橡树林）对氮的限制和氮循环的看法。本项目主要研究在富有机质的沼泽生态系统中，灰杉属植物通过灰杉类菌根吸收氨基酸N和含氮糖可能导致的分解-矿化循环短路。这个项目的目标是：1)确定灰藓类沼泽植物是否在田间条件下直接或通过菌根吸收氨基酸和氨基糖；2)表征土壤中“游离”氨基酸和氨基糖的水平，并确定溶解有机氮的释放，特别是相对容易吸收的根和叶；3)确定灰藓类沼泽植物是否需要菌根来吸收氨基酸。该项目计划包括：对菌根菌属植物对同位素标记的甘氨酸（氨基酸）和葡萄糖胺（含氮糖）的吸收进行实地试验，对凋落物进行实地和实验室培养以测量DON的释放，对菌根依赖性进行实地和实验室研究，以及利用标记了15n的分解根和叶进行N周转率研究。

Johnson 9527907 Peatlands, dominated by ericaceous shrubs and Sphagnum moss, cover 400 million ha in the circumboreal region. These ecosystems are characterized by slow decomposition and nutrient cycling, and accumulation of peat (primarily Sphagnum moss litter). Ombrotrophic (rain-fed) peat bogs represent one of the most nutrient-deficient of all ecosystems because peat accumulates above the surrounding landscapes and, therefore, these systems receive nutrient inputs form only atmospheric sources. Consequently, ombrotrophic peat bogs are thought to be strongly limited by nitrogen (N). Our field and laboratory net N mineralization rates from ombrotrophic peat bogs are much lower than minertrophic peatlands and even lower than arctic tundra. Furthermore, it appears that only about 1/4 of the annual N requirement of ericaceous shrubs and Sphagnum moss can be met by inorganic sources from atmospheric inputs (mostly deposition, N fixation is negligible) and N mineralization. Although inorganic N is in short supply in peat bogs, organic N is present in large quantities as partially decomposed peat and, in lesser amounts, as dissolved organic nitrogen (DON). In organic soils, some DON occurs as 'free' amino acids, previously considered as unavailable for plant uptake. Recent laboratory investigations and the few field studies that have be conducted indicate that ericaceous mycorrhizal plants (and even several arctic non-mycorrhizal sedges) can take up amino acids. The goal of this project is to determine if uptake of amino acids (and perhaps more complex organic N forms) by mycorrhizal ericaceous bog shrubs can account for the apparent N deficit in the plants' annual uptake requirements. If the uptake of organic N is a primary form of N nutrition for ericaceuous plants in organic-rich peat bogs, it would fundamentally alter ideas about N limitation and N cyclinging, these systems and perhaps in other Ericace-dominated ecosystems, e.g. heathlands and nutrient-poor oak forests. This proje ct is focused on the possible short-circuiting of the decomposition-mineralization cycle by uptake of amino acid N and nitrogenous sugars by ericaceous plants via ericoid mycorrhizas in an organic-rich bog ecosystem. The objectives of this project are to 1) determine if ericaceous bog plants take up amino acids and amino sugars under field conditions, either directly or via mycorrhizas, 2) characterize the levels of 'free' amino acids and amino sugars in the soil, and determine the release of dissolved organic N, especially from relatively liable roots and leaves, and 3) determine if ericacous bog plants require mycorrhizas for their amino acids uptake. The project plan includes: a field test of uptake of isotopically-labelled glycine (amino acid) and glucose amine (nitrogenous sugar) by mycorrhizal ericaceous plants, field and laboratory incubations of litter to measure release of DON, field and laboratory studies of mycorrhizal dependency, and N turn-over studies using 15N-labelled decomposing roots and leaves.