Dissolved organic matter
Organic matter in the soil organic pools described above is considered to be vertically immobile. The soil water in the profile normally contains an amount of dissolved organic matter originating from litter, faeces, humus or microbes. Consequently this organic matter can be passively transported vertically by water flows. The dissolved organic matter is generated from the immobile pools and can also be fixed again as humus. These processes are depth dependent, normally resulting in a release of matter to the dissolved organic pools close to the soil surface and a fixation of dissolved organics to the immobile pools at lower depths. It is possible to include such pools for dissolved organic matter for both carbon and nitrogen in the simulation, which then allows for vertical transport of organic matter by advection (see switch Dissolved Organics).
The initial carbon content in the dissolved organics pools is calculated from the initial concentration of dissolved carbon, given as a parameter iDOC, divided by the soil moisture content in the layer and the layer thickness. To calculate the initial dissolved nitrogen content, the dissolved carbon content is divided by the initial CN ratio of the humus pool, given as a parameter ih,CN.
The flux from the immobile pools to the dissolved organics pool, CDO, is determined by a rate parameter, dDOL1:
where f(T) and f(θ) are the common response functions for temperature and soil moisture. The same equation is used analogously for the litter 2, faeces and microbes pools as well as for all correspondent nitrogen pools.
Since dissolved organic material can be both released and fixed to the humus pool, the flux between these two pools is calculated slightly differently:
where dDOH is the rate parameter for formation of dissolved organic carbon, dDOD is the rate parameter for the fixation of dissolved organic carbon, f(T) and f(θ) are the common response functions for temperature and soil moisture, θ(z) is the soil moisture content and ∆z is the depth of the soil horizon. The same equation can be used analogously for the correspondent nitrogen flux.
The organic solutes are transported vertically by advection flows:
where qw is the vertical water flow. The equation is used analogously for nitrogen flows. If drainage and/or deep percolation is considered there will be associated flows of dissolved organic matter out of the soil profile.