Spatially robust estimates of biological nitrogen (N) fixation imply substantial human alteration of the tropical N cycle

Proceedings of the National Academy of Sciences of the United States of America
By: , and 

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Abstract

Biological nitrogen fixation (BNF) is the largest natural source of exogenous nitrogen (N) to unmanaged ecosystems and also the primary baseline against which anthropogenic changes to the N cycle are measured. Rates of BNF in tropical rainforest are thought to be among the highest on Earth, but they are notoriously difficult to quantify and are based on little empirical data. We adapted a sampling strategy from community ecology to generate spatial estimates of symbiotic and free-living BNF in secondary and primary forest sites that span a typical range of tropical forest legume abundance. Although total BNF was higher in secondary than primary forest, overall rates were roughly five times lower than previous estimates for the tropical forest biome. We found strong correlations between symbiotic BNF and legume abundance, but we also show that spatially free-living BNF often exceeds symbiotic inputs. Our results suggest that BNF in tropical forest has been overestimated, and our data are consistent with a recent top-down estimate of global BNF that implied but did not measure low tropical BNF rates. Finally, comparing tropical BNF within the historical area of tropical rainforest with current anthropogenic N inputs indicates that humans have already at least doubled reactive N inputs to the tropical forest biome, a far greater change than previously thought. Because N inputs are increasing faster in the tropics than anywhere on Earth, both the proportion and the effects of human N enrichment are likely to grow in the future.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Spatially robust estimates of biological nitrogen (N) fixation imply substantial human alteration of the tropical N cycle
Series title Proceedings of the National Academy of Sciences of the United States of America
DOI 10.1073/pnas.1320646111
Volume 111
Issue 22
Year Published 2014
Language English
Publisher National Academy of Sciences
Contributing office(s) Southwest Biological Science Center
Description 6 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Proceedings of the National Academy of Sciences of the United States of America
First page 8101
Last page 8106