The Role of Soil Organic Nitrogen in Forest Plant Nutrition

Nitrogen (N), an essential element for plant growth, is also a key limiting nutrient in most terrestrial ecosystems. Organic N mainly consisting of proteins, peptides, amino acids, and heterocyclic-N molecules, accounts for most of the N pools in forest soils. In the traditional paradigm, mineralization has been assumed as a prerequisite for the utilization of organic N by plants. However, in most forest ecosystems, the rate of soil N mineralization is often insufficient to meet the annual plant N demand. Hence, soil organic N has been proposed to be an important component of plant N supply. More recent studies point to the direct acquisition of organic N compounds such as amino acids, urea, short peptides, and proteins. Moreover, certain plant-microbe interactions (e.g., symbiotic microbial association) have been found to facilitate plant N acquisition even from mineral-associated or more complex organic N sources. For example, our Guest Editor team found that tree species associated with ectomycorrhizal fungi could acquire N from the protein-tannin complex, a recalcitrant N form bound in soil organic matter (SOM). These findings paved the way for a new paradigm on plant N nutrition that microbial N mineralization was no longer the gatekeepers of plant N availability.

Although it has been increasingly recognized that organic N plays a potential role in plant nutrition over the last decades, there are still large knowledge gaps regarding the underlying mechanisms of organic N uptake. In addition, methodological limitations in characterizing high molecular mass organic N in situ hinder the accurate quantification of the relative importance of organic N for plant N nutrition. Furthermore, the spatial and temporal heterogeneity of biotic and abiotic factors such as temperature, moisture, litter quality, soil chemistry, and microbial community add more complexity to assess the quantitative contribution of organic N to plant N budgets. Nevertheless, elucidating the dynamics of soil organic N supply and how plants effectively obtain organic N helps better understand the structure and functioning of forest ecosystems.

In this Research Topic, we focus on the roles of soil organic N (ranging from monomers to macromolecules or mineral-associated forms) in plant nutrient supply and their ecological implications for forest ecosystem function and the stabilization of SOM, and invite papers investigating plant-microbe interactions on soil N cycling and plant nutrient acquisition strategy under global change scenarios (e.g., climate or land-use changes). We particularly encourage submissions that highlight new approaches to overcome perennial challenges in studying plant acquisition of soil organic N. Different types of contributions are welcome: critical reviews, mini-reviews, opinion, perspective articles and original research articles spanning from laboratory and field empirical experiments to model-based studies.