Effects of long-term nitrogen addition on the fine root morphology and anatomy of four main shrub species in an old-growth forest

Abstract

ABSTRACT Soil nitrogen (N) availability is critical to root function and morphological traits, which reflect strategies for nutrient uptake and survival of plants. However, these factors have rarely been studied in temperate old-growth forests, especially understory shrubs. Therefore, we conducted an 8-year continuous N addition field experiment in Lesser Khingan Mountains northeastern China, including four N treatments. The results showed that under low and medium N treatments, the root diameter of all shrubs generally increased, which was caused by the increase in root stele diameter and cortical thickness, which were mainly regulated by soil NH4 +-N. Low N treatment significantly increased the ratio of stele to root diameter, promoted the development of stele and tracheid characteristics, and thus improved the transport capacity of fine roots. High N treatment decreased the cortical thickness of all shrubs, but increased the total area and number of tracheid, which might have a significant positive impact on the growth of fine roots. These results suggest that N deposition may affect the physiological function and activity of fine roots by affecting their diameter, cortical thickness, and stele diameter. Collectively, plants can adjust their root nutrient absorption strategies to adapt to changes in the nutrient environment.