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

B W Sullivan; W K Smith; A R Townsend; M K Nasto; S C Reed; Robin L Chazdon; C C Cleveland

doi:10.1073/pnas.1320646111

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

Journal article

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

B W Sullivan, W K Smith, A R Townsend, M K Nasto, S C Reed, Robin L Chazdon and C C Cleveland

Proceedings of the National Academy of Sciences of the United States of America, Vol.111(22), pp.8101-8106

2014

DOI: https://doi.org/10.1073/pnas.1320646111

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Abstract

adaptive cluster sampling

free-living nitrogen fixation

nitrogen deposition

symbiotic nitrogen fixation

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.

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Title: Spatially robust estimates of biological nitrogen (N) fixation imply substantial human alteration of the tropical N cycle
Authors: B W Sullivan (Author) - University of Montana, United States
W K Smith (Author) - University of Montana, United States
A R Townsend (Author) - University of Colorado, United States
M K Nasto (Author) - University of Montana, United States
S C Reed (Author) - Southwest Biological Science Centre, United States
Robin L Chazdon (Author) - University of Connecticut, United States
C C Cleveland (Author) - University of Montana, United States
Publication details: Proceedings of the National Academy of Sciences of the United States of America, Vol.111(22), pp.8101-8106
Publisher: National Academy of Sciences
Date published: 2014
DOI: 10.1073/pnas.1320646111
ISSN: 0027-8424
Organisation Unit: Tropical Forests and People Research Centre; University of the Sunshine Coast, Queensland; Forest Research Institute
Language: English
Record Identifier: 99451090302621
Output Type: Journal article

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