Until recently, a concept concerning nitrogen availability in ecosystems has been absolutely accepted in ecologists, i.e., plants can use only inorganic nitrogen (NH₄⁺, NO₃^-) 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 (NH₄⁺, NO3-). 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, NO3-), 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.