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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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## Soil

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**The Nitrogen Balance**Manure Crop Residue Fertiliser Denitrification Soil Mineral Nitrogen (NH4, NO3, Urea) Organic Matter Leaching**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)

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