Lowland tropical forests with chronic nitrogen (N) deposition and/or abundant N-fixing organisms are commonly rich in N relative to other nutrients. The tropical N richness introduces a paradoxical relationship in which many tropical forests sustain high rates of asymbiotic N fixation despite the soil N richness and the higher energy cost of N fixation than of soil N uptake. However, the mechanism underlying this phenomenon remains unclear. Dr. Mianhai Zheng and his colleagues, under the guidance of Prof. Jiangming Mo, Ph. D. supervisor of College of Resources and Environment, UCAS, conducted a chronic N-addition experiment in a N-saturated tropical forest in southern China and measured the N fixation rates, carbon (C), N, and phosphorus (P) concentrations, and stoichiometry in different substrates (soil, forest floor, mosses, and canopy leaves).
They found that total N fixation rates were high (10.35？12.43 kg N ha-1 yr-1) in this N-saturated forest because of the high substrate C:N and N:P stoichiometry (which explained 13-52% of the variation in N fixation). Atmospheric N deposition (34？50 kg N ha-1 yr-1) failed to down-regulate asymbiotic N fixation in this forest. They further found the insensitivity of N fixation in all the tested substrates to low N addition (50 kg N ha-1 yr-1); however, medium and high N addition (100？150 kg N ha-1 yr-1) stimulated the moss and foliar N fixation because of the increases in substrate C:N stoichiometry (which explained 30-34% of the variation in N fixation). These results emphasize the importance of substrate (particularly mosses and foliage) stoichiometry as a driver of asymbiotic N fixation and sustained N richness in lowland tropical forests.
The above findings have been published online in Ecology, with the linked paper below: https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1002/ecy.2416
Figure 1. Substrate stoichiometry controls asymbiotic nitrogen fixation
Source: South China Botanical Garden