ammoniumnitrogen

Ammonium nitrogen (commonly reported as NH4+-N or as part of "ammoniacal nitrogen") is the portion of nitrogen present as ammonium (NH4+) and, in equilibrium with it, un-ionized ammonia (NH3). The relative amounts of NH4+ and NH3 depend on pH and temperature: higher pH and temperature shift the balance toward NH3. Concentrations are frequently expressed in water quality and soil testing as milligrams per liter (mg/L) or milligrams per kilogram (mg/kg) of nitrogen.

In aquatic systems and wastewater, ammonium nitrogen is an important indicator of organic matter decomposition, sewage contamination, and fertilizer runoff. Un-ionized ammonia (NH3) is toxic to fish and invertebrates at relatively low concentrations, so environmental guidelines often set limits for total ammoniacal nitrogen with adjustments for pH and temperature. In treated wastewater, control methods include biological nitrification, air stripping, and chemical treatment.

In soils and agriculture, ammonium is a plant-available nitrogen form and a product of microbial ammonification of organic nitrogen. Ammonium is held on cation exchange sites and is less prone to leaching than nitrate, but it can be converted to nitrate through nitrification or lost as gaseous ammonia under high pH. Analytical determination commonly uses colorimetric assays, ion-selective electrodes, or automated instrumentation; samples for ammonium analysis are often preserved by cooling or acidification to limit transformation.

Ammonium nitrogen plays a central role in the nitrogen cycle, influencing nutrient availability, water quality, and ecosystem health.