Soil systems recalcitrance

Estimation of the relative effectiveness of nutrient sources can be a useful way in which to estimate their efficacy as fertilisers (Barrow 1985). The relative effectiveness of alternative nutrient sources is usually calculated by comparing the yield plateau of the response curve of the fertiliser in question to a soluble source of the same nutrients (Barrow 1985). For minerals used as nutrient inputs in organic farming systems their relative effectiveness is almost always <1 due to low solubility in soil. Organic matter inputs can also be evaluated in terms of their relative effectiveness based on their recalcitrance, but of equal importance is the extent to which they are physically protected from degradation in soil aggregates (Strong et al. 1999), which would be different in different soil types. 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

The aim of many of the described anaerobic bioreactors was only to test the potential use of this technology to remediate soil, water or air streams contaminated with organochlorines. In this regard, most of them were successful, even without reaching 100% removal. However, partial reductive dechlorination in other systems led to low removal rates (<30%) and to the formation of other, equally or more, toxic compounds like VC, biphenyl or monochlorobenzene, making the efficiency of these processes questionable, especially for extremely recalcitrant compounds like HCB, Polychlorinated Biphenyls or CF. Finally, it is important to note that all these reactors were designed and operated on lab-scale, where processes can be more easily... 

It has recently been found that obligately anaerobic microbial consortia can mineralize many recalcitrant chemicals (toluene, chloroform, benzene, chlorophenols, etc.) that had been considered essentially nonbiodegradable in the absence of oxygen (19, 20, 21). Extensive work at the University of Idaho with obligately anaerobic microbial consortia indicates that these systems are capable of complete biodegradation of nitroaromatic pollutants. Work with 2-.s c-butyl-4,6-dinitrophenol (dinoseb) has shown complete fermentation of this nitroaromatic pollutant in soils by anaerobic consortia without buildup of aromatic biotransformation products (16, 17). Similar results were observed with a variety of nitro-toluenes and munitions residues, including TNT and trimethylenenitramine (RDX) under appropriately controlled conditions (12, 13). In the work with TNT and RDX, hydroxyaro-... 

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