Symbiotic N fixation is sufficient to support net aboveground biomass accumulation in a humid tropical forest

E N J Brookshire,Bryce Currey,Carlton Roberts,Duncan N L Menge,Michael P Oatham,Nina Wurzburger

doi:10.1038/s41598-019-43962-5

Abstract

Symbiotic nitrogen (N) fixation has been shown to support carbon storage in young regenerating tropical forests, but N-fixing trees can also be strong competitors with non-fixing trees, making it unclear which mechanism drives long term patterns in biomass accretion. Many tropical forests have excess N, but factors such as rising atmospheric CO2 or selective cutting practices might induce additional N demand. Here we combine decades of stem inventory data, in-situ measures of symbiotic N fixation, and simulations of N demand to evaluate demographic and biogeochemical controls on biomass dynamics in legume-rich lowland forests of Trinidad. We document sustained net biomass accumulation and high rates of N fixation in these forests, regardless of the timing of selective timber harvests, including an old growth stand. The biomass accumulation was explained by growth of non-fixing trees, not N-fixing trees, but the total amount of symbiotic N fixation was sufficient to account for most of net above ground N demands, suggesting that N-fixers could contribute to the long-term C sink in these forests via fertilizing non-fixers.

Highlights

Humid tropical forests are linchpins in the global carbon (C) cycle, removing more atmospheric CO2 annually than any other terrestrial biome[1]
Non-fixing trees contributed a larger portion of above ground biomass (AGB) than did N-fixing trees (P = 0.049; using tree “type” (i.e., N-fixer vs. non-fixer) as a fixed effect), and AGB increased over time for non-fixers only
N-fixers did not contribute directly to aboveground C accumulation via their own biomass, their indirect facilitative effects could be substantial given that we documented high levels of nodulation and biological N fixation (BNF)

Summary

Introduction

Humid tropical forests are linchpins in the global carbon (C) cycle, removing more atmospheric CO2 annually than any other terrestrial biome[1]. Net biomass accumulation in mature tropical forests requires large quantities of nitrogen (N) and other nutrients. In many tropical regions, otherwise “mature” forests that have never been cleared or converted to pasture/agriculture are subject to selective cutting practices Such small-scale disturbances can represent a large net source of CO216 and result in vigorous regrowth[17]. BNF in mature tropical forests globally[12,25,28,29,30,31,32,33,34], making generalizations difficult Most of these studies support the idea that BNF is lower in mature than aggrading forests, they raise the critical question as to whether BNF is sufficient to sustain net biomass growth. Understanding the extent to which BNF can support this net growth, and for how long, is critical for predicting the future of the tropical C sink

Methods

Results

Conclusion