Fine root production, mortality, and turnover in response to simulated nitrogen deposition in the subtropical Abies georgei (Orr) forest

Abstract

Increased nitrogen deposition has important effects on below-ground ecological processes. Fine roots are the most active part of the root system in terms of physiological activity and the main organs for nutrient and water uptake by plants. However, there is still a limited understanding of how nitrogen deposition affects the fine root dynamics in subtropical Abies georgei (Orr) forests. Consequently, a three-year field experiment was conducted to quantify the effects of three forms of nitrogen sources ((NH₄)₂SO₄, NaNO₃, and NH₄NO₃) at four levels (0, 5, 15, and 30 kg N·ha^-1·yr^-1) on the fine root dynamics in Abies georgei forests using a randomized block-group experimental design and minirhizotron technique. The first year of nitrogen addition did not affect the first-class fine roots (FR₁, 0 < diameter < 0.5 mm) and second-class fine roots (FR₂, 0.5 < diameter < 1.0 mm). The next two years of nitrogen addition significantly increased the production, mortality, and turnover of FR₁ and FR₂; the three year of nitrogen addition did not affect the dynamics of the third- class fine roots (FR₃, 1.0 < diameter < 1.5 mm) and fourth- class fine roots (FR₄,1.5 < diameter < 2.0 mm). Nitrogen addition positively affected the dynamics of FR₁, FR_2, FR₃ and FR₄ by positively affecting the carbon, nitrogen, and phosphorus contents of fine roots and indirectly affecting the soil pH. Increased carbon allocation to FR₁ and FR₂ may represent a phosphorus acquisition strategy when nitrogen is not the limiting factor. The nitrogen addition forms and levels affected the fine root dynamics in the following orde: NH₄NO₃ > (NH₄)₂SO₄ > NaNO₃ and high nitrogen > medium nitrogen > low nitrogen. The results suggest that the different-diameter fine root dynamics respond differently to different nitrogen addition forms and levels, and linking the different-diameter fine roots to nitrogen deposition is crucial.

Keywords: Fine root diameter; Fine root dynamics; Minirhizotron; Nitrogen addition form; Subtropical forests.