Revisiting nitrification in the Eastern Tropical South Pacific: A focus on controls

Xuefeng Peng, Clara A. Fuchsman, Amal Jayakumar, Mark J. Warner, Allan H. Devol, Bettie Ward

Research output: Contribution to journalArticle

40 Scopus citations

Abstract

Nitrification, the oxidation of ammonium (NH4+) to nitrite (NO2-) and to nitrate (NO3-), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with 15N-labeled substrates to measure rates of NH4+ and NO2- oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions of NH4+ and NO2- oxidation rates were primarily controlled by NH4+ and NO2- availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer, NH4+ oxidation was negligible or below detection, but high rates of NO2- oxidation were observed. NH4+ oxidation displayed extremely high affinity for both NH4+ and oxygen. The positive linear correlations between NH4+ oxidation rates and in situ NH4+ concentrations and ammonia monooxygenase subunit A (amoA) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situ NH4+ concentrations or supply by adjusting their population size, which determines the NH4+ oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N2O concentration, along with independently reported simultaneous direct N2O production rate measurements, suggests that AOA were predominantly responsible for NH4+ oxidation, which was a major source of N2O production at oxygen concentrations > 5 μM.

Original languageEnglish (US)
Pages (from-to)1667-1684
Number of pages18
JournalJournal of Geophysical Research: Oceans
Volume121
Issue number3
DOIs
StatePublished - Mar 1 2016

All Science Journal Classification (ASJC) codes

  • Forestry
  • Aquatic Science
  • Soil Science
  • Water Science and Technology
  • Earth-Surface Processes
  • Geochemistry and Petrology
  • Geophysics
  • Oceanography
  • Palaeontology
  • Ecology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Atmospheric Science

Keywords

  • ammonia oxidation
  • eastern tropical South Pacific
  • nitrification
  • nitrite oxidation
  • oxycline
  • oxygen minimum zone

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