Table 1.

Simulation models including nitrification rate and their treatment of controlling factors.

	DayCent (Parton et al., 2001; Del Grosso et al., 2009, 2016; Abdalla et al., 2010)	DNDC (DeNitrification DeComposition) (Li et al., 1992, 2012; Giltrap et al., 2010; Gilhespy et al., 2014)	Ecosys (Grant, 1994, 2001, 2014; Grant and Pattey, 2003; Grant et al., 2006)	MicroTrait–N (Bouskill et al., 2012)
PARAMETER
Nitrification rate	Nitrification rate is a function of ${\text{NH}}_4^{+}$, water content, temperature, pH, and texture	rate is a first order function of ${\text{NH}}_4^{+}$ concentration, nitrifier biomass, a temperature reduction factor, and a moisture reduction factor	substrate (NH3) oxidation under non-limiting O2 is calculated from active biomass and from NH3 and CO2 concentrations (same for ${\text{NO}}_2^{-}$)	Briggs Haldane kinetics for ammonia and oxygen for AO and for nitrite and oxygen for NO
Soil ammonia/ammonium	Model derived soil ammonium × maximum fraction nitrified	${\text{NH}}_4^{+}$ concentration used in Michaelis-Menten kinetics	Solution ${\text{NH}}_4^{+}$/NH3 drives rates	Dynamic solution NH3 driven by pH and consumption
Mineralization	Net mineralization fraction (.20)	Submodel of decomposition	Submodel of decomposition	Inputs but not linked
Nitrite	Not modeled	Not modeled	Modeled explicitedly	Product of AO
Oxygen in soil	Limited at high WFPS, soil physical properties control gas diffusivity and 02 demand	DOC Anaerobic balloon concept	Consumption by microbial groups, O2 uptake in competition with heterotrophs, roots; then diffusion to nitrifier	O2 use by nitrification reactions
Temperature	Ts estimated based on heat flux and soil heat capacity, used as T factor	Ts estimated based on heat flux and heat flow used as a T factor compared to optimum	Uses modeled Ts applied through Arrhenius function	Different temperature optima across guilds Optimum set to 25°C
Water	Optimum WFPS about 55% if low scales down nitrification from moisture stress, high scaled down by DOC	Soil moisture content converted to WFPS, Moisture reduction factor, optimum at 90% WFPS	Water film thickness from modeled water potential	Assumed in water films
Nitrifier abundance	Not modeled	Nitrifier biomass, Nitrifier-bacterial growth and death rate are functions of DOC and a T factor.	(Active) Nitrifier biomass growth by double Monod functions of CO2s and NH3s AO and NO separately	Growth and death of biomass through C and N equations
Nitrifier denitrification (N gas from nitrification)	Fraction of N nitrified	Function of water-filled pore space and quantity of N nitrified	Process included when O2 limits rate of NH3 oxidation	Decomposition of hydroxylamine or detoxification of ${\text{NO}}_2^{-}$ due to uncoupling

AO, ammonia oxidation; NO, nitrite oxidation; T, temperature; s, soil; WFPS, water filled pore space; DOC, dissolved oxygen concentration.