Alternative Som Models

With a few exceptions, the pools used in the SOM dynamics models described above are functional and kinetically defined. While the microbial biomass and the litter inputs are relatively easy to measure, it is difficult to define a "slow" pool of organic matter in chemical or physical terms. While the multicompartmental models generally work well at predicting measured changes in total SOM, it is difficult to initialize models using independent measures and therefore they rely on site-specific calibrations based on total organic matter levels. Recent attempts have been made to link conceptual pools of SOM with measurable fractions, leading Elliott et al. (1996) to use the phrase "modeling the measurable or measuring the modelable." Such attempts include developing chemical or physical fraction-ation procedures to match measurable fractions to model pools (e.g., Sohi et al., 2001) or to construct new models that coincide with measurable fractions. Many of the model structures being proposed attempt to account for the various protection mechanisms that stabilize incoming organic matter. One such example is illustrated in Fig. 16.7. However, the development of models based on physical fractions is still in its infancy and has yet to overcome some of its challenges. Smith et al. (2002) have suggested that a measurable fraction and model pool are equivalent only when the measured fraction is unique and noncomposite. To date, few models that have been developed and tested would meet these criteria.

It is recognized that SOM is a mixture of a very large number of organic compounds with widely differing chemical and physical properties. The models described above partition SOM into only a few discrete pools. Each pool is treated as having constant characteristics, thus the changing dynamics of the whole organic matter follows from the changing distribution among the various pools. This leads to straightforward models, but their theoretical underpinning can be questioned. An alternative approach is to describe SOM as a continuum from fresh plant residues to refractory humic substances. Bosatta and Agren proposed that the heterogeneity of SOM could be described using a quality variable (q), which determines the decomposition rate (Agren and Bosatta, 1998; Bosatta and Agren, 1985, 1995). In their model, q for each cohort of new organic matter varies over time according to a continuous-quality equation. The equation describes changes in the molecular accessibility of a C atom to decomposers as the microbial population assimilates organic compounds and utilizes them to produce compounds of different quality. While the QSOIL model is conceptually satisfying, its complex mathematics has limited its broad application. Yang and Janssen (2000) propose using a single-pool, first-order model that allows the rate constant (k) to change. This approach is attractive because it is derived only from measured quantities of remaining organic material and there is no need for other variables that cannot be measured. The key feature of the model is that it takes into account the decrease

FIGURE 16.7 A soil organic matter dynamics model based on measurable fractions that account for various protection mechanisms (with permission from Six et al., 2002).

over time of the average and actual mineralization rates, which the authors refer to as "aging."

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Worm Farming

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