The Nitrogen Balance
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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

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The Nitrogen Balance


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 preferentially.

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 SurfaceOMModule

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

  • 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

      • RESIDUE_WT and

      • 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)