The Nitrogen Balance. Manure Crop Residue. Fertiliser. Denitrification. Soil. Mineral Nitrogen (NH 4 , NO 3 , Urea). Organic Matter. Leaching. The Nitrogen Balance. The APSIM Soil N Module
(NH4, NO3, Urea)
The Nitrogen Balance
The soil organic matter pools in APSIM Soil N Module and the transformations occurring in each layer.
The transformations are: mineralization, immobilization, nitrification and denitrification. [Leaching is dealt with in the water balance module]
Mineralisation and Immobilisation
Mineralization (of N) is the release of mineral N from the breakdown of organic matter.
Immobilization is the tying up of mineral N by soil microbes as they endeavour to use substrates as an energy source .
An inadequate supply of mineral N to meet any immobilization demand results in a slowing of decomposition.
APSIM Soil N assumes both nitrate- and ammonium-N are available for immobilization, though ammonium-N is used preferentially.
Soil water factor for decomposition.
Soil temperature factor for decomposition
Decomposition rate reaches unconstrained limit at 32oC
The APSIM SurfaceOMModule
All above ground residues are defined in terms of:
• C:N ratio
• C:P ratio
• Cover it provides for the soil surface
Residues for each crop type are accounted separately.
Main Parameters for Modelling C and N
•Organic Carbon (%) (measured)
•Total Nitrogen (%) (measured)
•Finert – the inert C fraction (hard to measure – so estimate)
•Fbiom – the biomass fraction (hard to measure – so estimate)
•Starting mineral N pools (NO3, NH4) (measured)