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## Nitrification and Denitrification Losses of Nitrogen

The article discusses the processes of nitrification and denitrification, which can lead to nitrogen losses from soil systems.

Key points:

**Nitrification**
- Nitrification is the process of converting less mobile ammonium (NH4+) into more mobile nitrate (NO3-).
- Conditions favoring nitrification include high NH4+ concentrations, well-aerated soils, neutral pH, and warm temperatures.
- The potential nitrification rate in CLM is calculated as a function of NH4+ concentration, temperature, moisture, and pH.

**Denitrification**
- Denitrification is the process of converting nitrate (NO3-) into gaseous nitrogen (N2), with some production of NOx and N2O.
- Denitrification occurs under anaerobic conditions, when soil microbes use nitrate as an electron acceptor for respiration.
- The potential denitrification rate in CLM is co-limited by nitrate concentration and carbon consumption rates, and occurs only in the anoxic fraction of soils.
- The anoxic fraction is calculated based on soil properties like pore size, oxygen consumption, and water/air-filled pore space.

**N2O Emissions**
- The models also predict N2O emissions, with a constant fraction (6 x 10^-4) of the nitrification flux assumed to be N2O.
- The fraction of denitrification going to N2O (PN2:N2O) is variable, calculated based on the ratio of CO2 production to nitrate concentration and the water-filled pore space.