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Molybdenum and Phosphorus Interact to Constrain Asymbiotic Nitrogen Fixation in Tropical Forests

Author(s): Wurzburger, Nina; Bellenger, Jean Philippe; Kraepiel, Anne M. L.; Hedin, Lars O.

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Abstract: Biological di-nitrogen fixation (N2) is the dominant natural source of new nitrogen to land ecosystems. Phosphorus (P) is thought to limit N2 fixation in many tropical soils, yet both molybdenum (Mo) and P are crucial for the nitrogenase reaction (which catalyzes N2 conversion to ammonia) and cell growth. We have limited understanding of how and when fixation is constrained by these nutrients in nature. Here we show in tropical forests of lowland Panama that the limiting element on asymbiotic N2 fixation shifts along a broad landscape gradient in soil P, where Mo limits fixation in P-rich soils while Mo and P co-limit in P-poor soils. In no circumstance did P alone limit fixation. We provide and experimentally test a mechanism that explains how Mo and P can interact to constrain asymbiotic N2 fixation. Fixation is uniformly favored in surface organic soil horizons - a niche characterized by exceedingly low levels of available Mo relative to P. We show that soil organic matter acts to reduce molybdate over phosphate bioavailability, which, in turn, promotes Mo limitation in sites where P is sufficient. Our findings show that asymbiotic N2 fixation is constrained by the relative availability and dynamics of Mo and P in soils. This conceptual framework can explain shifts in limitation status across broad landscape gradients in soil fertility and implies that fixation depends on Mo and P in ways that are more complex than previously thought.
Publication Date: 21-Mar-2012
Electronic Publication Date: 21-Mar-2012
Citation: Wurzburger, Nina, Bellenger, Jean Philippe, Kraepiel, Anne M. L., Hedin, Lars O. (2012). Molybdenum and Phosphorus Interact to Constrain Asymbiotic Nitrogen Fixation in Tropical Forests. PLoS ONE, 7 (3), e33710 - e33710. doi:10.1371/journal.pone.0033710
DOI: doi:10.1371/journal.pone.0033710
EISSN: 1932-6203
Pages: e33710 - e33710
Type of Material: Journal Article
Journal/Proceeding Title: PLoS ONE
Version: Final published version. This is an open access article.

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