植物对有机氮源的利用及其在自然生态系统中的意义

近来大量实验研究表明,许多植物能够在不经矿化的情况下直接吸收、利用环境介质中的生物有机氮,尤其氨基酸类。而且,有些植物利用氨基酸的效率可以与矿质氮源(NH4 、NO3)相当或更高。自然界植物赖以生存的土壤生境中同时存在多种有机氮和矿质氮养分,这是导致植物(至少部分植物)进化产生利用各种不同氮源能力的环境驱动力。土壤中的游离氨基酸尽管含量不高,但其周转快、通量大,理论上可远大于植物的氮需求。尽管植物在与土壤微生物的有机氮源竞争中处于根本性劣势,但土壤中氨基酸的巨大潜在通量和植物相对于微生物的生命周期仍可使植物在长期竞争中获取数量可观的氮。基于植物根对氨基酸的吸收能力、土壤中游离氨基酸库的大小和通量、植物与土壤微生物对氨基酸氮源的竞争以及有关的原位实验结果,近来许多研究者都认为植物有机氮营养在多种生态系统中是重要或潜在重要的。尤其是在一些极地、高山、亚高山、北方针叶林或泰加林生态系统中,由于低温等因素限制有机氮矿化,土壤氨基酸浓度常超过矿质氮(NH4 、NO3-)浓度,氨基酸可能代表着植物的一个主要氮源。认识到现实生态系统中植物对有机氮源利用的重要性意味着传统的矿质营养观念的更新,这将在很大程度上改变人们对某些重要生态过程的理解,并导致对若干生态学中心问题的再认识。研究以森林生态系统为例,阐述了我国开展该领域研究的科学意义和基本框架。

Organic nitrogen use by plants and its significance in some natural ecosystems

Until recently,a concept concerning nitrogen availability in ecosystems has been absolutely accepted in ecologists,i.e.,plants can use only inorganic nitrogen(NH 4,NO-3)which is released in excess of microbial requirements.As a result,the traditional "measure" of soil nitrogen availability to plants is mainly based on the pool size or net flux(net mineralization rate)of inorganic nitrogen.Over the last decade,however,the direct use of organic nitrogen by plants and its potential importance in some ecosystems has been highlighted,and the traditional concept on nitrogen availability has been challenged in ecological studies.A series of studies have demonstrated that many plant roots not only absorb inorganic nitrogen,but directly absorb bio-organic nitrogen,mainly dissociated amino acids,from the culture medium or bulk soil.Moreover,the efficiencies for some plants to use amino acids are comparable to,or even higher than,that of mineral nitrogen(NH 4,NO-3).Organic and inorganic nitrogen coexist in primeval habitats(e.g.,natural soils),which might drive the evolution of plants(at least in some plants)to use diverse nitrogen sources.Notwithstanding the concentrations of dissociated amino acids are generally lower in most soils,their concentrations exceed the requirement for plant nitrogen use due to their high turnover rate and large flux in soil.Plant roots commonly compete nitrogen source with soil microorganisms,but they fundamentally in an inferior position.However,the potentially large flux of amino acid nitrogen in soil makes plant to gain larger magnitude nitrogen even if they only explore a small fraction of soil where the bio-organic nitrogen turnover occurred.Based on laboratory experiments of amino acid uptake by plants,amino acid concentrations and fluxes in soil,plant-microbe competition,and isotope 15N in situ,most researchers recognized that organic nitrogen play important roles in many ecosystems for plants,especially in such ecosystems as arctic tundra,alpine,subalpine forest,and boreal forest,where organic nitrogen mineralization is constrained by low temperature,amino acid concentrations commonly exceed mineral nitrogen(NH 4,NO-3),and represent a main nitrogen source for plants.Recognizing the importance of organic nitrogen use by plants in these ecosystems implied that traditional idea of mineral nutrition should be renewed,and this would significantly improve our understanding of many important ecological processes.In China,there are existing divers ecosystems,such as cold-and mid-temperate forests,subalpine forests,alpine meadow and tundra etc.Soils in these natural ecosystems are typically characterized by low temperature,high organic material,high organic nitrogen,and low mineralization rate,implying the potential supply of soil organic nitrogen is important to ecosystem nitrogen cycling.In this paper,the author discussed the organic nitrogen use by plants and emphasized four questions as following:(1)to reevaluate nitrogen availability in forest ecosystems and integrate assessable criteria;(2)to gain a new understanding of nitrogen cycling in forest ecosystems and to modify its models established during the last two decades;(3)to develop applied techniques for nitrogen nutrition management in forest ecosystem,e.g.,belowground factor regulation which control organic nitrogen supply,ectomycorrhize effectiveness,and species selection in specific nitrogen sources etc;(4)to assess impacts of increased ratio of inorganic/organic nitrogen on forest succession and restoration under the background of widespread disturbances and global change.Finally,the author proposed an integrated conceptual framework to evaluate organic nitrogen availability in soil,species and ecosystem levels.