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Garden with Insight v1.0 Help: Soil patch next day functions: calculate mineralization of fresh organic N
Fresh organic N is found in flat residue and live microbial biomass, so mineralization of fresh organic N is the release of nitrogen into
inorganic forms during decomposition of flat residue and dead microbes to organic matter. Fresh organic N mineralization is the product
of the amount of fresh organic N in any soil layer and a variable called
the decay rate constant. The decay rate constant (which is not
really a constant) depends on soil temperature (warmer = more decay),
soil water (wetter = more decay), and the ratio of carbon to nitrogen in
the soil.
Microbes in the soil need carbon and nitrogen in specific ratios to grow, so they extract a specific ratio of
C:N from the organic matter. When the microbes die, they release C and N in the same ratio that they
took them up. If there is more nitrogen in the organic matter than what the microbes need, some nitrogen
will be left over in an inorganic state. Mineralization includes both the release of N after the death of the
microbes and the nitrogen the microbes don't need. The simulation uses a minimum ratio of 25:1 C:N for
fresh organic N mineralization. If the C:N ratio is less than 25:1, meaning there is excess nitrogen,
mineralization will proceed at its fastest pace. If the C:N ratio is greater than 25:1, the amount of
mineralization is reduced by a curve that reaches down to about 20% of maximum mineralization at a C:N
ratio of 100:1.
Nitrogen mineralized from fresh organic N is divided into two portions. Eighty percent is released by the
breakdown of the organic matter (the N the microbes don't need) and goes directly into the nitrate compartment. The other twenty percent represents nitrogen in dead
microbial biomass and goes into the active organic N compartment in
the organic matter. A corresponding addition to the organic matter in each layer is made for the dead
microbes.
This is a good place to point out that this simulation does not model immobilization, or the
trapping (making immobile) of nutrients in the bodies of microbes while they are alive, though it does
model immobilization by plants. Immobilization is indirectly simulated by making mineralization and
several other processes depend on some of the factors that affect microbial and bacterial life (temperature,
moisture, pH), but the actual amount of organic material and nutrients
held in living microbial bodies is not simulated. Probably the amount of this biomass is relatively small and so it can be ignored without too much loss
of accuracy, but there are other considerations. For example, immobilization of nutrients tends to create a
lag time after organic fertilizers are added (sometimes a few weeks) before they are available to plants,
and that lag is not simulated here.
EPIC N Mineralization
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