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, 368 (1621), 20130119

Biological Nitrogen Fixation: Rates, Patterns and Ecological Controls in Terrestrial Ecosystems

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Biological Nitrogen Fixation: Rates, Patterns and Ecological Controls in Terrestrial Ecosystems

Peter M Vitousek et al. Philos Trans R Soc Lond B Biol Sci.

Abstract

New techniques have identified a wide range of organisms with the capacity to carry out biological nitrogen fixation (BNF)-greatly expanding our appreciation of the diversity and ubiquity of N fixers-but our understanding of the rates and controls of BNF at ecosystem and global scales has not advanced at the same pace. Nevertheless, determining rates and controls of BNF is crucial to placing anthropogenic changes to the N cycle in context, and to understanding, predicting and managing many aspects of global environmental change. Here, we estimate terrestrial BNF for a pre-industrial world by combining information on N fluxes with (15)N relative abundance data for terrestrial ecosystems. Our estimate is that pre-industrial N fixation was 58 (range of 40-100) Tg N fixed yr(-1); adding conservative assumptions for geological N reduces our best estimate to 44 Tg N yr(-1). This approach yields substantially lower estimates than most recent calculations; it suggests that the magnitude of human alternation of the N cycle is substantially larger than has been assumed.

Keywords: biogeochemistry; biological nitrogen fixation; nitrogen cycle.

Figures

Figure 1.
Figure 1.
Box diagram for the calculation of terrestrial biological nitrogen fixation (BNF) by difference; values in parentheses are annual fluxes in Tg N. Hydrologic losses to the ocean (HLO) and losses along the flowpath between soils and oceans (HLF), which together constitute total hydrologic losses (HL), are obtained as described in the text. We calculate the fraction of total losses that occur by gaseous pathways (fG) as: (δ15NTBδ15NI + ɛH)/(ɛHɛG), where δ15NTB and δ15NI represent the natural abundance of 15N in the terrestrial biosphere and in N inputs to the terrestrial biosphere, and ɛH and ɛG are the isotope effects on terrestrial N from fractionations associated with hydrologic losses and gaseous losses (GL), respectively. Inputs via lightning (LNF) and atmospheric transport of reactive N from oceans to land (Nrl) are obtained as described in the text. BNF can then be calculated as: HL/(1 – fG) – LNF Nrl.

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