Decomposition

Decomposition is a general term used to describe the interrelated processes by which organic matter is broken down to CO₂ and humus with a simultaneous release of nutrients. These processes are a critical link responsible for recycling of nutrients in the intrasystem nutrient cycling. The decomposition process often begins with soil macrofauna physically breaking down the particle-size of organic matter and predigesting organic materials. Action by microorganisms results in the progressive breakdown of organic matter through the litter ---> partially decomposed organic matter ---> humus continuum releasing CO₂ and nutrients along the way.

The rate of decomposition is dependent on litter quality and environmental conditions. Litter quality factors important to decomposition and mineralization include: nutrient content (e.g., C/N ratio), composition of organic matter, especially lignin concentrations (lignin/nitrogen ratio), and concentrations of polyphenols (including tannins). Litter with higher concentrations of nutrients and lower concentrations of lignin and polyphenols will decompose more rapidly and net mineralization begins earlier. Availability of nutrients from other soil pools also enhances decomposition rates if nutrient concentrations are low in litter.

Soil temperature and moisture content are very important factors affecting decomposition rates. At favorable moisture conditions, increasing temperature results in an exponential increase in decomposition rates (Q₁₀ of approx 2). At a constant temperature, soil moisture content shows a parabolic affect on decomposition rates with a maximum rate at intermediate levels of moisture. High moisture content limits soil gas exchange leading to low oxygen concentrations and potentially anaerobic conditions. At low moisture content, the lack of water limits microbial metabolism; however, many microorganisms can remain active to much lower soil water potentials (at -3.0 MPa) than plants (at -1.5 MPa). In addition to temperature and moisture conditions, soil acidity extremes (pH <4 or>9) may severely reduce decomposition rates.

The mean residence time of forest floor organic matter and its nutrients for contrasting forest types is shown in this Table. Mean residence times show a trend of increasing with increases in latitude and of conifer litter having greater mean residence times compared with deciduous litter within the same climatic region. Warmer temperatures are responsible for greater decomposition rates at low latitudes while litter quality of deciduous forests is generally higher than coniferous forests leading to greater decomposition rates for deciduous forest litter. A comparison of mean residence times for major nutrients shows the generalized trend: N > P > Mg = Ca > K.