Figure 3.

Schematic of process models used for simulation of N₂O emission with different degrees of complexity: (a) simplified, (b) conceptual, (c) complex. Black arrows and components are accounted for in the models, grey arrows and components are optional, red arrows indicate exchange of components between anaerobic (denitrification) and aerobic (nitrification) micro-sites in the soil. Simplified process models use potential denitrification rates which are decreased by reduction factors related to soil environmental conditions for calculation of N₂O emission. In addition, conceptual models also include N₂O emission from nitrification mostly by use of fixed fractions. However, both simplified and conceptual models follow the theory that N₂O production in the soil equals N₂O flux at the soil–atmosphere interface. Complex process models calculate N turnover via nitrification and denitrification considering the dynamics of microbes. Nitrification and denitrification N turnover is weighted by calculation of anaerobic-aerobic volume fractions as function of soil oxygen concentrations. For this complex process models take into account diffusion processes which also determine the N₂O flux at the soil–atmosphere interface, thus in contrast to simplified and conceptual models emission is not equal to production.