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

Large abundances and high fixation rates of N-fixing trees prove theoretically sufficient to support 30 years of biomass accumulation.

Member of the Trinidadian Forest Service assisting in field inventory data collection. Victoria-Mayaro Forest Reserve 2017. Photo: Bryce Currey

Nitrogen fixation has been shown to fuel tropical forest carbon storage in younger forests, a process that is increasingly crucial to carbon sequestrations as humanity plunges deeper toward worst-case scenario climate projections in the 21st century. Simultaneously, however, these same specialized trees have shown strong competitive effects, making it unclear which mechanism drives long term patterns in biomass accumulation.

Many tropical forests have excess N, but factors such as rising atmospheric CO2 or selective cutting practices might induce additional N demand. Without sufficient supply, these forests' ability to sequester crucial atmospheric CO2 may be limited.

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.

Measured rates of N fixation

We document sustained net biomass accumulation and high rates of N fixation in these forests, regardless of the timing of selective timber harvests, including within 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. This suggests that N-fixers could contribute to the long-term C sink in these forests via fertilizing non-fixers.

Field-technician extraordinaire, Andres, measuring N-fixation rates in the field using a custom designed manifold

Authors: Jack Brookshire, Nina Wurzburger, Bryce Currey, Duncan Menge, Mike

Oatham, Carlton Roberts


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