**3. How does vermicomposting work?**

The vermicomposting process includes two different phases regarding earthworm activity: (i) an active phase during which earthworms process the organic substrate, thereby modifying its physical state and microbial composition (Lores et al., 2006), and (ii) a maturation phase marked by the displacement of the earthworms towards fresher layers of undigested substrate, during which the microorganisms take over the decomposition of the earthworm-processed substrate (Aira et al., 2007; Gómez-Brandón et al., 2011b). The length of the maturation phase is not fixed, and depends on the efficiency with which the active phase of the process takes place, which in turn is determined by the species and density of earthworms (Domínguez et al., 2010), and the rate at which the residue is applied (Aira & Domínguez, 2008).

More specifically, the impact of earthworms on the decomposition of organic waste during the vermicomposting process is initially due to *gut associated processes* (GAPs) (Figure 3), i.e., via the effects of ingestion, digestion and assimilation of the organic matter and microorganisms in the gut, and then casting (Gómez-Brandón et al., 2011a). Specific microbial groups respond differently to the gut environment (Schönholzer et al., 1999) and selective effects on the presence and abundance of microorganisms during the passage of organic material through the gut of these earthworm species have been observed. For instance, some bacteria are activated during passage through the gut, whereas others remain

Fig. 3. Earthworms affect the decomposition of organic matter during vermicomposting through ingestion, digestion and assimilation in the gut and then casting (*gut associated processes*); and *cast associated processes,* which are more closely related with ageing processes.

The vermicomposting process includes two different phases regarding earthworm activity: (i) an active phase during which earthworms process the organic substrate, thereby modifying its physical state and microbial composition (Lores et al., 2006), and (ii) a maturation phase marked by the displacement of the earthworms towards fresher layers of undigested substrate, during which the microorganisms take over the decomposition of the earthworm-processed substrate (Aira et al., 2007; Gómez-Brandón et al., 2011b). The length of the maturation phase is not fixed, and depends on the efficiency with which the active phase of the process takes place, which in turn is determined by the species and density of earthworms (Domínguez et al., 2010), and the rate at which the residue is applied (Aira &

More specifically, the impact of earthworms on the decomposition of organic waste during the vermicomposting process is initially due to *gut associated processes* (GAPs) (Figure 3), i.e., via the effects of ingestion, digestion and assimilation of the organic matter and microorganisms in the gut, and then casting (Gómez-Brandón et al., 2011a). Specific microbial groups respond differently to the gut environment (Schönholzer et al., 1999) and selective effects on the presence and abundance of microorganisms during the passage of organic material through the gut of these earthworm species have been observed. For instance, some bacteria are activated during passage through the gut, whereas others remain

> *GAPs*: Gut associated processes *CAPs*: Cast associated processes

Fig. 3. Earthworms affect the decomposition of organic matter during vermicomposting through ingestion, digestion and assimilation in the gut and then casting (*gut associated processes*); and *cast associated processes,* which are more closely related with ageing processes.

Casts

CAPs

**3. How does vermicomposting work?** 

Domínguez, 2008).

Organic matter

GAPs

unaffected and others are digested in the intestinal tract and thus decrease in number (Drake & Horn, 2007; Monroy et al., 2009). Such selective effects on microbial communities as a result of gut transit may alter the decomposition pathways during vermicomposting, probably by modifying the composition of the microbial communities involved in decomposition, as microbes from the gut are then released in faecal material where they continue to decompose egested organic matter. Indeed, as mentioned above, earthworm casts contain different microbial populations to those in the parent material (Domínguez et al., 2010), and in turn it is expected that the inoculum of those communities in fresh organic matter promotes modifications similar to those found when earthworms are present, altering microbial community levels of activity and modifying the functional diversity of microbial populations in vermicomposting systems (Aira & Domínguez, 2011).

Upon completion of GAPs, the resultant earthworm casts undergo *cast associated processes* (CAPs; Figure 3), which are more closely related to ageing processes, the presence of unworked material and to physical modification of the egested material (weeks to months). During these processes the effects of earthworms are mainly indirect and derived from the GAPs (Aira et al., 2007). In addition, during this aging, vermicompost is expected to reach an optimum in terms of its biological properties, thereby promoting plant growth and suppressing plant diseases (Domínguez et al., 2010). However, little is yet known about when this "optimum" is achieved, how we can determine it in each case and if this "optimum" has some kind of expiration date.
