CH4 exchange

Woodland soils can act as effective sinks for both atmospheric CH4 and CH4 produced in deeper soil layers. CH4 is predominantly used by bacteria in the soil (methano-trophs), which use it as a source of carbon in a process called CH4 oxidation. The 'high capacity-low affinity' methanotrophs are adapted for growth at high CH4 concentrations (>1000 ppm in air), occurring, for example, in wetlands and in waterlogged soil layers. The 'low capacity-high affinity' methanotrophs are able to make use of the trace amounts of CH4 in the atmosphere (~1.8 ppm in air) (Bender and Conrad, 1992; see also Dunfield, Chapter 10, this volume).

The production and consumption of CH4 within the soil profile is controlled by several factors, of which pO2, CH4 partial pressure (pCH4) and temperature are the most important. The key as to whether a soil acts as a sink or source of CH4 is pO2. The availability of oxygen in (parts of) the soil profile regulates whether microbial decomposition of substrates follows the aerobic pathway where CH4 is consumed and the end product is CO2, or the anaerobic pathway where the end products are CH4 and CO2. Again, pO2 is regulated through soil diffusivity, which is a function of the water-filled pore space, rainfall and the depth of the groundwater table. In general, forest soils tend to be sinks for CH4 because trees keep the water table well below the surface and allow the methanotrophs to grow (e.g. Keller et al., 1983; Yavitt et al., 1990a,b; Adamsen and King, 1993; Castro et al., 1995). The CH4 uptake by well-aerated forested soils has been related to soil texture (Striegl, 1993), soil pH (Brumme and Borken, 1999), atmospheric nitrogen deposition (Butterbach-Bahl and Papen, 2002), soil acidification (Brumme and Borken, 1999) and forest management (Borken and Brumme, 1997; Prieme and Christensen, 1997). The balance shifts from a net CH4 sink (uptake exceeds production) to a net CH4 source (production by methanogens exceeds uptake by meth-anotrophs) in water-saturated forest soils, as may sometimes happen during winter in drained soils, and more frequently in gley soils, riparian forest soils or forested bogs and fens (e.g. Crill et al., 1988; Bartlett et al., 1990; Roulet et al., 1992).

Increased atmospheric nitrogen deposition increases [NH+] in the soil and usually decreases CH4 uptake by well-drained soils (Steudler et al., 1989; Gulledge and Schimel, 1998; Van den Pol-van Dasselaar et al., 1999; Le Mer and Roger, 2001). Three mechanisms have been postulated for the partial inhibition (slowdown) of CH4 uptake by well-drained soils in response to increased nitrogen input: (i) competitive inhibition of the CH4 monooxygenase by ammonia; (ii) inhibition of CH4 consumption by toxic intermediates and end products of methanotrophic ammonia oxidation such as hydroxylamine and nitrite; and (iii) osmotic stress due to high concentrations of nitrate and/or ammonium (Schnell and King, 1994; Bradford et al., 2001; Bodelier and Laanbroek, 2004; Re ay and Nedwell, 2004). Bradford et al. (2001) suggested that the nitrate effect on CH4 oxidation may be mediated through aluminium toxicity. However, Whalen and Reeburgh (2000) concluded that increased nitrogen deposition did not decrease CH4 oxidation in boreal forests soils. Similarly, Borken et al. (2002) studied the long-term effect of reduced atmospheric nitrogen inputs on a coniferous forest and concluded that CH4 oxidation did not increase. In contrast, positive effects of increased nitrogen availability on atmospheric CH4 uptake have even been reported for severely nitrogen-limited forests (Goldman et al., 1995; Borjesson and Nohrsted, 2000; Steinkamp et al., 2000). Bodelier and Laanbroek (2004) hypothesized that such an increase of the methanotrophic activity of a soil due to nitrogen additions may be directly linked to nitrogen limitation of the methanotrophic bacterial community or of the biosynthesis of enzymes involved in CH4 oxidation. An increase in the size and activity of the nitrifying population, co-oxidizing atmospheric CH4, has also been proposed as a possible mechanism. However, the potential rate of such co-oxidation of CH4 by soil nitrifiers has been found to be insignificant in forest soils in the UK (Reay et al., 2005).

In addition to the potential for chronic atmospheric nitrogen deposition to alter the size of this sink, other sources of nitrogen input to soils have been found to have a significant effect on the forest soil CH4 sink. For instance, Reay et al. (2005) reported vastly differing CH4 oxidation potentials in soils under different vegetation types, with soils under alder having almost no capacity for CH4 oxidation even under optimal conditions. This was apparently due to inhibition of CH4 oxidation by the elevated nitrogen concentrations in the soils that result from the nitrogen-fixing Frankia spp. in the alder root nodules. Similarly, the practice of nitrogen fertilization, as used in many commercial forests to increase productivity, has been shown to significantly reduce CH4 consumption rates (Castro et al., 1994; Chan et al., 2005).

In summary, the mechanisms ofincreased nitrogen availability through increased atmospheric nitrogen deposition on CH4 oxidation are still poorly understood. They may differ from site to site, tipping the balance from inhibition to no effect or even an increase in CH4 oxidation (see Fig. 17.1).

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