Effects of elevated N availability on anatomy,morphology and mycorrhizal colonization of fine roots: A review

: Increase of nitrogen (N) availability can greatly affect the structure and function of forest ecosystems. Fine root is critical to belowground ecological processes, thus its response to elevated N availability has become a focus of ecological researches. This article reviewed the trend and under-lying mechanism of fine root response to elevated N availability, including root anatomy, morphology and mycorrhizal colonization. We found that cross-sectional areas of root stele and xylem and the diameter, number and wall thickness of xylem vessel (tracheid) all increased, however, cortex thickness, the number of cortical cell layer, diameter of cortical cell and number of passage cell in exodermis decreased with higher N availability. The response of root anatomy was closely related to plant hormones. In addition, mycorrhiza colonization rate decreased after N fertilization, while specific root length (root length per unit root mass) generally decreased in arbuscular mycorrhizal species, while increased in ectomycorrhizal species. By contrast, the root diameter and tissue density varied widely among species under higher N status. These findings based on individual roots and species provided deeper understanding of carbon and nutrient cycles in terrestrial ecosystems. In addition, we discussed some knowledge gaps and proposed several research outlooks for guiding future researches.