Soil aggregates, as noted in the section on soil structure in Chapter 1, play a central role in protecting pools of carbon and nitrogen, and are derived from a variety of sources. Amechanism particularly prevalent in many tropical soils is the physical aggregation process, which occurs abiotically as a physicochemical process (Oades and Waters, 1991). In both temperate and tropical soils, there are several biological processes
that result in the formation of "biological macroaggregates" (Fig. 3.12) (Six et al., 2002b). These include the following three processes: (1) Fresh plant- and root-derived residues form the nucleation sites for the growth of fungi and bacteria. Macroaggregate formation is initiated by fungal hyphae enmeshing fine particles into macroaggregates. Exudates from both bacteria and fungi, produced as a consequence of decomposition of fresh residues, form binding agents that further stabilize macroaggre-gates (tl>A). (2) Biological macroaggregates also form around growing roots in soils, with roots and their exudates enmeshing soil particles, thereby stimulating microbial activity (t0 B to ti)B). (3) A third principal mechanism of biological macroaggregate formation in soils in all climates is via the action of soil fauna, particularly earthworms, termites, and ants. For example, earthworms often produce casts that are rich in organic matter (tl,C) and are not stable when freshly formed and wet. During gut passage, the soil and organic materials are kneaded thoroughly and copious amounts of watery mucus are added as well. This molding process breaks bonds between soil particles, but can lead to casts that are quite stable upon drying. It is also worth noting that soil mesofauna, for example collembola and mites, are important in the SOM formation process through their production of copious amounts of fecal pellets. Effects of meso- and macrofauna on soil structure are discussed further in Chapter 4 on soil fauna.
The subsequent fate of macroaggregates follows a fascinating process over time, as noted by Six et al. (2002b) summarizing from several literature sources. At first (time t1), the young, freshly formed unstable macroaggregates (UA) are only stable when treated in very gentle fashion (that is, when the aggregates are taken from the field, brought to field capacity, subsequently immersed in water, and retained when gently sieved). The formation of water-stable aggregates (WSA) that can resist slaking (air drying and quick submersion in water before sieving) occurs by three processes (t1 to t2):
1. Under moist conditions ageing may increase stability by binding through microbial activity. Microbial activity is stimulated inside the biological macroaggregates, including worm casts because of their high organic matter content. In this process, substantial amounts of polysaccharides and other organics are deposited, serving to further stabilize the macroaggregates.
2. Dry-wet cycles can result in closer arrangements of primary particles, leading to stronger bonding and increased aggregate stability.
3. Biological and physicochemical macroaggregates, in the presence of active root growth, can become more stabilized by penetration of the aggregates by roots. This includes the roles of root exudates as
Active root growth (b)
TEMPERATE and TROPICAL Fungal and bacterial activity
"Biological" aggregate formation
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