The Story of Organic Matter Decay in Soils
To most people, humus is a garlic-y chickpea spread for sandwiches or pita bread. However, to soil scientists, its something entirely different. Soil humus is a mixture of dark, colloidal organic compounds relatively resistant to decomposition. These compounds result from the decay of organic litter and accumulate in the O and A horizons of soils. Soil humus helps glue mineral particals into aggregates, giving structure to the soil and affecting soil stability.
There are three main classifications of humus: fulvic acid, humic acid and humin. Humin is insoluble but fulvic and humic acids are soluble in dillute NaOH solution. Humic acids precipitate in acidic solution, but fulvic acids remain soluble. Humic molecules are incredibly varied in composition, but generally are characterized by:
1) many active chemical functional groups exposed to the surrounding solution for reaction with other substances in the solution.
2) a very large cross-linked and "folded" molecule with molecular weights in the hundreds of thousands of grams per molecule" (Miller and Donahue, 1990, p. 185).
Humus has a large surface area per unit of mass and is highly charged (similar to clay), and individual humus molecules are dynamic and constantly changing form (but may remain humus for several thousand years). Humus includes sugar amines, nucleic acids, phospholipids, vitamins, sulfolipis, polysaccharides and many other unclassified compounds (Miller and Donahue, 1990). Figure 1 shows a hypothetical structure of a humic acid with many of the characteristic functional groups. Fulvic acid and humin have similar structures. The COOH and phenolic OH groups are weakly acidic, which give humus its pH buffering ability, pH dependent charge and cation chelating ability. In addition, the toxicity of the phenolic subgroups which make up humus contributes to its resistance to microbial decomposition. (Singer and Munns, 1996) The branched structure of humic molecules may also make them more resistant (Stott and Martin, 1990).
Humus is formed during the decomposition of organic "litter" (including pine needles, leaves, and animal droppings) in soils. This decay is mediated by microbes and the enzymes they excrete (which break certain, specific bonds in organic matter). The main reactions of this decomposition are:
aerobic conditions: carbohydrate + O2 --> CO2 + H2O + energy
anaerobic conditions: carbohydrate --> CO2 + acid or alcohol + energy (Singer and Munns, 1996).
These reactions are much complicated by the complex structure of the litter, and the products of decomposition include various nutrients, organic acids and amines (depending on the conditions and starting materials) in addition to the "resistant residues" or humus (Miller and Donahue, 1990). Between 60 and 80 percent of the carbon from most plant residues is evolved as CO2 within a year of deposition; 5 to 15 percent is incorporated into the...