Decomposition of organic matter

The decomposition of organic matter is variously referred to as oxidation, metabolism, degradation and mineralisation. Organic matter is first oxidised by molecular oxygen, and the products (or metabolites) of the reaction are carbon dioxide and recycled nutrients (i.e. the reaction is the reverse of photosynthesis) [1,2].

For example, the aerobic respiration of Redfield organic matter yields 106 moles of carbon dioxide (CO₂), 16 moles of ammonia (NH₃) and one mole of orthophosphate (PO₄) (reaction 1). If sufficient dissolved oxygen is present, bacteria oxidise the NH₄ produced in reaction 1 to nitrite (NO₂) (reaction 1a) and then to nitrate (NO₃) (reaction 1b) in a process called nitrification.

(1) Oxygen reduction

106(CH₂O)16(NH₃)(H₃PO₄) + 106O₂ = 106 CO₂ + 16 NH₃ + PO₄

(1a) 16 NH₄⁺ + 24 O₂ = 16 NO₂^– + 16 H₂O + 32 H⁺

(1b) 16 NO₂^– + 8 O₂ = 16 NO₃

106(CH₂O)16(NH₃)(H₃PO₄) + 138 O₂ ——- 106 CO₂ + 16 HNO₃ + H₃PO₄ + 122 H₂O

When dissolved oxygen is depleted, oxidation proceeds in a series of reactions which represent successively lower pE levels (or redox states; Figure 1). Reactions 2 – 4 occur under sub-oxic conditions.

(2) Manganese reduction

106(CH₂O)16(NH₃)(H₃PO₄) + 236 MnO₂ + 472 H⁺ ——- 106 CO₂ + 236 Mn²⁺ + 8 N₂ + H₃PO₄ + 366 H₂O

(3a) Nitrate reduction by denitrification

106(CH₂O)₁₆(NH₃)(H₃PO₄) + 94.4 HNO₃ ——-106 CO₂ + 55.5 N₂ + H₃PO₄ + 177 H₂O

(3b) Dissimilatory nitrate reduction to ammonia

106(CH₂O)₁₆(NH₃)(H₃PO₄) + 84.8 HNO₃ ——-106 CO₂ + 42.4 N₂ + 16 NH₃ + H₃PO₄ + 148.4 H₂O

Iron and sulfate are the last oxidants in the series. Sulfate reduction (5) only occurs under anoxic conditions.

(4) Iron oxyhydroxide reduction

106(CH₂O)16(NH₃)(H₃PO₄) + 212 Fe₂O₃ (or 424 FeOOH) + 848 H⁺ ——- 106 CO₂ + 16NH₃ + H₃PO₄ + 742 H₂O + 424 Fe²⁺

(5) Sulfate reduction

106(CH₂O)16(NH₃)(H₃PO₄) + 53SO₄^2- ——- 106 CO₂ + 16 NH₃ + H₃PO₄ + 106 H₂O + 53 S^2-

Figure 1. Schematic representation of chemical processes with depth in sediment and changes in Eh range (Modified after Millero, 1996) [3]

1. Froelich, P. N., Klinkhammer, G. P., Bender, M. L., Luedtke, N., Heath, G. R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B., and Maynard, V. (1979). Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic; suboxic diagenesis. Geochimica et Cosmochimica Acta Pergamon, Oxford. 43, 1075-1090.
2. Drever, J.I. 1982. The Geochemistry of Natural Waters, Prentice-Hall, Inc., Englewoods Cliffs, N.J., pp. 388.
3. Millero, F.J. 1996. Chemical Oceanography. CRC Press LLC. pp. 469.