The APSIM Nitrogen Balance

The Nitrogen Balance
Denitrification Crop Residue
Mineral Nitrogen
(NH4, NO3, Urea)
Organic Matter
The Nitrogen Balance
The APSIM Soil N Module
The module is essentially a model of soil carbon dynamics, which permits the
simulation of nitrogen because of the close association of C & N.
The model considers the dynamics in terms of four main soil organic matter
pools (as well as surface residues)
•Inert – Organic Carbon not decomposing
•FOM - fresh organic matter (crop residues, dead roots)
•BIOM – the more labile, soil microbial biomass
•HUM – the bulk of soil organic matter
… in a layered soil profile. The soil layer structure for Soil N is the same as for
Soil Water
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]
The Nitrogen Balance
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
The Nitrogen Balance
Soil water factor for decomposition.
The Nitrogen Balance
Soil temperature factor for decomposition
Decomposition rate reaches unconstrained limit at 32oC
The Nitrogen Balance
The APSIM SurfaceOM Module
All above ground residues are defined in terms of:
• Mass
• C:N ratio
• C:P ratio
• Cover it provides for the soil surface
Residues for each crop type are accounted separately.
The Nitrogen Balance
The Nitrogen Balance
Residue Decomposition
The decomposition of surface residue is calculated as:
Decomposition Rate = Potential Decomposition Rate
x Moisture Factor
x Temperature Factor
x C:N Factor
x Contact Factor
The Soil N module tests whether any immobilization
demand can be met. If availability of mineral N is limiting, a
new soil-limited decomposition rate is calculated.
The Nitrogen Balance
Residue cover
APSIM SurfaceOM keeps track of
specific area (ie area of cover per unit mass)
Increasing amounts of residue have diminishing effects on
the proportion of the surface that is covered because
additional residue may overlay other residue rather than
cover bare soil.
Hence C = 1 – exp (-S)
where C is effective residue cover
and S is total surface area of residues per unit area
The Nitrogen Balance
Main Parameters for Modelling C and N
•Organic Carbon (%) (measured)
•Total Nitrogen (%) (measured)
•pH (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)

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