Читать книгу Wetland Carbon and Environmental Management - Группа авторов - Страница 60

Phenolic compounds can accumulate and inhibit decomposition under conditions that limit the activity of phenol oxidase, the enzyme that degrades phenolics. Because phenol oxidase requires O₂ to function, its activity generally is low in fully anaerobic soils, increases in surface soils, and is greatest in aerobic surface litter (Wright & Reddy, 2001). Although other extracellular enzymes involved in carbon mineralization exhibit low activities at lower O₂ concentrations (Freeman, Ostle et al., 2004; McLatchey & Reddy, 1998), this is probably not a direct effect of O₂ since hydrolytic enzymes do not require O₂ to function. Instead, low O₂ concentrations result in low phenol oxidase activity, allowing phenolic compounds to accumulate and inhibit hydrolytic enzymes (Fig. 3.3) (Fenner & Freeman, 2011; Freeman, Ostle et al., 2001). So, the O₂‐related inhibition of phenol oxidase activity does not just affect the decomposition of lignin and other phenolic compounds, it inhibits the breakdown of multiple classes of organic carbon and acts as an “enzymic latch” that preserves large quantities of carbon in organic wetland soils (Freeman, Ostle et al., 2001). The activity of phenol oxidase is also inhibited by moisture limitation, which may help limit carbon mineralization during droughts when soil O₂ concentrations increase (H. Wang et al., 2015).

The enzymic latch mechanism may be most important in wetlands with lignin‐poor vegetation (e.g., those dominated by Sphagnum mosses) and/or those with low soil iron contents (Y. Wang et al., 2017). Although phenol oxidase activity increases in some wetlands with water table drawdown (that is, increased O₂ penetration into the soil), this is not a universal response. Instead of being restricted by low soil O₂, the activities of phenol oxidase and hydrolytic enzymes can be enhanced in the presence of Fe²⁺ (Van Bodegom et al., 2005; Hall & Silver, 2013; Liu et al., 2014) and, therefore, may decline following a sustained water table drawdown (Y. Wang et al., 2017). This “iron gate” mechanism differs from the enzymic latch and suggests that increasing soil oxidation in mineral soil wetlands may help protect against the decomposition of lignin (Y. Wang et al., 2017).

Figure 3.3 Effects of O₂ availability on enzyme activity and organic matter decomposition. A cascade starts when increased O₂ supply stimulates microbial aerobic respiration (A), triggering increased phenol oxidase synthesis (B) and a decline in inhibitory phenolics (C). Lower inhibition causes higher hydrolase activity (D) and organic matter decomposition (E) releasing CO₂ and nutrients (F), which can feed back on microbial activity (G) through pH effects related to CO₂ production and nutrition effects due to release of nitrogen, phosphorus, and other nutrients.

Source: Modified from Fenner & Freeman (2011).