CHAPTER Interactions Between Soil Biota, Soil Organic Matter, and Soil Structure J Hassink, F J Matus, C Chenu, and J W Dalenberg INTRODUCTION Physical protection mechanisms are important determinants of the stability of organic matter in soils This is partly based on the observation that the turnover rates of easily decomposable compounds are much higher in liquid microbial cultures than in soils (Van Veen and Paul, 1981), and on electron microscopy studies that have demonstrated sites of accumulation of organic residues of clearly cellular origin (Foster, 1988) The phenomenon that drying of soil samples and disruption of soil aggregates can increase C and N mineralization (Richter et al., 1982; Gregorich et al., 1989) is indirect evidence of the existence of physical protection in soil It has been concluded that there is more physical protection in fine-textured soils than in coarse-textured soils, leading to higher carbon (C) contents and larger amounts of microbial biomass (Jenkinson, 1988; Amato and Ladd, 1992; Hassink, 1994) Several mechanisms have been suggested to explain the physical protection of organic matter in soil against decomposition Tisdall and Oades (1982) have mentioned the adsorption of organics on or coating by clay particles Another explanation is the entrapment of organics in small pores in or between aggregates, which renders them inaccessible to the decomposing microbial community (Elliott and Coleman, 1988) Soil structure may also control decomposition processes by its effect on the grazing intensity of the soil fauna on microbes The soil fauna may stimulate microbial growth rates through grazing (Clarholm, 1985; Coleman et al., 1978), and predation of microbes by protozoa and nematodes has been suggested as an important mechanism of nutrient turnover in soil (Coleman et al., 1978; Elliott et al., 1980) A large proportion of bacteria may occupy © 1997 by CRC Press LLC pores