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
Model contents