Phosphatase activity and nitrogen fixation reflect species differences, not nutrient trading or nutrient balance, across tropical rainforest trees.

Sarah A Batterman,Jefferson S Hall,Pete Sheldon,Lars O Hedin,J Kimiko Lahaela Walter,Benjamin L Turner,Michiel Van Breugel,Lingli Liu

doi:10.1111/ele.13129

Abstract

A fundamental biogeochemical paradox is that nitrogen-rich tropical forests contain abundant nitrogen-fixing trees, which support a globally significant tropical carbon sink. One explanation for this pattern holds that nitrogen-fixing trees can overcome phosphorus limitation in tropical forests by synthesizing phosphatase enzymes to acquire soil organic phosphorus, but empirical evidence remains scarce. We evaluated whether nitrogen fixation and phosphatase activity are linked across 97 trees from seven species, and tested two hypotheses for explaining investment in nutrient strategies: trading nitrogen-for-phosphorus or balancing nutrient demand. Both strategies varied across species but were not explained by nitrogen-for-phosphorus trading or nutrient balance. This indicates that (1) studies of these nutrient strategies require broad sampling within and across species, (2) factors other than nutrient trading must be invoked to resolve the paradox of tropical nitrogen fixation, and (3) nitrogen-fixing trees cannot provide a positive nitrogen-phosphorus-carbon feedback to alleviate nutrient limitation of the tropical carbon sink.

Highlights

The observation that nutrients may limit the ability of tropical forests to serve as a sustained sink for atmospheric carbon dioxide underscores the importance of resolving the mechanisms and plant strategies that govern how tropical trees acquire nutrients (Wurzburger & Hedin 2016)
We found some evidence that trees capable of nitrogen fixation differ in phosphatase activity from non-fixing trees (ANOVA with functional type as a fixed variable and species as a nested random variable for n = 6–10 trees for each of our seven species: F(1,5) = 7.6, P < 0.05)
We found no clear support for the nutrient trading hypothesis, including that we could not identify a positive feedback between fixation and phosphatase activity

Summary

Introduction

The observation that nutrients may limit the ability of tropical forests to serve as a sustained sink for atmospheric carbon dioxide underscores the importance of resolving the mechanisms and plant strategies that govern how tropical trees acquire nutrients (Wurzburger & Hedin 2016). The ability to fix nitrogen may allow for higher biomass growth rates and trigger phosphatase production. This distinction matters for resolving whether nitrogen fixers have a unique ability to trade fixed nitrogen for enhanced competitive success (relative to non-fixers) in low phosphorus and high nitrogen soils. Such a mechanism can be central for resolving the distribution of fixers across soil conditions in tropical forests (Houlton et al 2008)

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Results

Discussion

Conclusion