Framework for Understanding

OM in the soil consists of non-living organic matter (NLOM; Zepp and Sonntag, 1995) and a living component composed of plant roots and soil microorganisms and animals. The NLOM is from either native plant material or remnants of microorganisms and soil animals, or NLOM that has been partially processed by soil organisms. OM coming from herbivores or their predators can for this discussion be regarded as plant material, since that is its source.

©CAB International 2000. Climate Change and Global Crop Productivity (eds K.R. Reddy and H.F. Hodges)

These different kinds of OM appear in a variety of structures and consist of a variety of chemical compounds (sugars, celluloses, lignin, etc.) in different proportions.

SOM dynamics can be discussed within the general framework shown in Fig. 17.1, which is divided into NLOM and decomposer biomass. The two, of course, are not physically separated as in the figure and therefore can be difficult to measure separately. However, most estimates of decomposer biomass suggest that it makes up around 4—5% of the total SOM (Wardle, 1992).

The soil is supplied with new NLOM primarily from plant litter input, a source that will change both quantitatively and qualitatively with climatic change. In the soil, the litter is mixed with old NLOM and provides the food for the primary decomposers (mainly fungi and bacteria; Berg et al, 1998). The rate at which the primary decomposers consume NLOM depends both on its availability (chemical structure and association with the soil matrix) and on the physical (temperature, water) and chemical climate in the soil. Respiration associated with growth and sustenance of the decomposers leads to losses of C from the soil in the form of CO2. When decomposers die, their biomass is included in NLOM, thus becoming a food source for other decomposers. An important aspect of this cycle between NLOM and decomposers is that the chemical composition of NLOM assimilated by the decomposers is different from that returned via the dead decomposers.

OM that has not been part of the decomposer biomass proper but that has been modified by decomposer activity should also be included in this cycle. The decomposer community consists of a complete food web: primary decomposers, fungivores and bacterivores and predators on them. Conceptually, this does not alter Fig. 17.1, but considerations of the soil community structure may be important for rates of transformation (e.g. Zheng et al., 1997), although the food web aspect can usually be ignored (Andren et al, 1999).

Fig. 17.1. A general framework for describing soil organic matter dynamics. (See text for further explanation.)

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