DIN inventory with inhibitors

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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Nitrification rate (inhibitor method)
Approach: DIN accumulation in incubations with chemical inhibitors of nitrification
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours to days
Units: nmol N L-1 d-1; mol N cell-1 d-1
Community captured: bulk
Co-measurements: cell numbers

Method Overview

Nitrification rates are estimated by adding specific inhibitors of ammonia monooxygenase (AMO), such as allylthiourea (ATU), octyne, or N-serve (nitrapyrin), to dark seawater incubations. The inhibitors block the first step of nitrification (NH₄⁺ → NO₂^-), causing ammonium to accumulate in the inhibited bottles while control bottles show continued nitrification. The rate of NH₄⁺ accumulation relative to controls is used as a proxy for nitrification rate. Nitrite oxidation rates can similarly be estimated by tracking NO₂^- accumulation when nitrite oxidation is selectively inhibited.

Scale of measurement

Point sample; incubations are performed in the dark (to eliminate phytoplankton uptake) over hours to days. High spatial resolution sampling is possible when multiple discrete samples are incubated.

Data generated

Nitrification rates in nmol N L^-1 d^-1; when cell counts of nitrifying bacteria are available, cell-specific nitrification rates (mol N cell^-1 d^-1) can be derived.

Units & currency

Units are nmol N L^-1 d^-1 or mol N cell^-1 d^-1. The currency is nitrogen.

Sample size

Typical samples are ~1 L in volume.

Limitations

The key assumption is that the inhibitors are universally effective against all nitrifying organisms in the community. In practice, inhibitor effectiveness varies among nitrifier lineages (archaea vs. bacteria) and at different substrate concentrations. Long dark incubations required to accumulate measurable DIN changes can alter community composition and activity. Non-specific inhibition (e.g., ATU also inhibits methane monooxygenase) and incomplete inhibition introduce systematic errors.

Example Applications & Protocols

Classic examples

• Ward (2011) Measurement and distribution of nitrification rates in the oceans
• Bianchi et al. (1997) Nitrification rates, ammonium and nitrate distribution in upper layers of the water column and in sediments of the Indian sector of the Southern Ocean

Common calculations/conversions

• Nitrification rate = (Δ[NH₄⁺]_inhibited − Δ[NH₄⁺]_control) / incubation time.