Soil methanotrophs - a novel methane mitigation technology?
Authors: Adrian Walcroft, Sally Price, Kevin Tate. Rob Sherlock, David Whitehead
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Executive summary
Methane (CH4) is a potent greenhouse gas (GHG) produced globally by both biotic and abiotic processes. There is still some uncertainty in the global CH4 budget, highlighted recently by the discovery of a possible new biotic source in growing plants (Keppler et al., 2006).
The most important known sources of CH4 are natural wetlands (21%); fossil fuel related to natural gas, coal mines, and the coal industry (16%); enteric fermentation (16%); rice paddies (11%); biomass burning (7%); landfills (7%); and animal waste (5%). The major sinks for CH4 are biological oxidation at or near the sites of production (~700 Tg y-1), and photochemical oxidation in the atmosphere (~450 Tg y-1). However, oxidation of atmospheric CH4 by aerobic soils also provides a significant additional sink (20–60 Tg y-1). The net flux between soils and the atmosphere results from the balance between two microbiological processes: methanogenesis (production of CH4) and methanotrophy (consumption of CH4). Because the soil sink for atmospheric CH4 is microbially mediated it is sensitive to environmental factors (e.g., moisture, temperature), and disturbance by management (Reeburg et al., 1993).
Methane oxidation by forest soils, particularly oxidation kinetics, location in soil, and inhibition, has been studied extensively because these soils represent a major part of the soil sink in the global CH4 budget. The highest CH4 oxidation activity in forest soils is usually measured in subsurface soil layers (Adamsen and King 1993; Koschorreck and Conrad, 1993; Roslev et al., 1997; Whalen et al., 1992). This localisation of methanotrophs in deeper soil layers has been attributed to inhibition of methanotrophs by ammonium or terpenes present in the organic surface layer of forest soils (Amaral and Knowles, 1997; 1998).
Recently, Saggar et al. (2008) reviewed soil CH4 oxidation rates measured in New Zealand pastures under cattle- and sheep-grazed pasture, pine and beech forests, regenerating shrubland, and cropland. These datasets suggested all soils oxidise CH4 but soil oxidation rates vary markedly with land use. Methane oxidation rate was highest for a New Zealand beech forest soil (10.5 kg CH4 ha-1 yr-1). The pine soils had intermediate oxidation rates (4.2–6.4 kg CH4 ha-1 yr-1), while the oxidation rates for all pasture and cropped soils were mostly <1 kg CH4 ha-1 yr-1 (Table 1).
Ungrazed and unimproved pasture soils had similar CH4 oxidation rates to those in improved and intensively managed dairy- and sheep-grazed pasture soils, suggesting increased intensification of agriculture from sheep to dairying has little impact on soil CH4 sink capacity. On the other hand, the capacity of some soils to oxidise CH4 may have declined from the effects of high N inputs from fertiliser, dung, and urine. This could imply that, since 1990, the CH4 sink strength of New Zealand soils has declined, effectively increasing net CH4 emissions. Deforestation in the North Island to make more land available for dairying is another reason to believe CH4 oxidation in soil may be decreasing, because this land-use change is causing large increases in N inputs to soil, and these are known to affect soil CH4 oxidation.
Table 1. Soil methane oxidation estimates for different land cover and land use (from Saggar et al. 2008)
| Land Use | Annual consumption (kg CH4 ha-1 yr-1) |
Reference |
|---|---|---|
| Native Beech | 10.5 | Price et al. (2004a,b) |
| Pine | 4.2-6.4 | Tate et al. (2006) |
| Shrub land | 2.3 | Tate et al., (2007) |
| Dairy pasture | 0.5-0.6 | Saggar et al. (unpublished) |
| Sheep pasture | 0.6-1.0 | Saggar et al.(2007) |
| Ungrazed pasture | 0.85 | Saggar et al. (2007) |
| Cropping | 1.5 | Van der Werden (1999) |
However, there is considerable uncertainty about the effects of N on CH4 emissions, with recent in vitro research (Kruger and Frenzel 2003) suggesting a positive effect of additional N on methanotrophic bacteria for potential CH4 oxidation rates in soil and root samples. These findings contrast with those of earlier work (Steudler et al. 1989; Mosier et al. 1991) showing negative effects of ammonium-N on CH4 oxidation. We have found little evidence of an N effect on CH4 oxidation in New Zealand soils (Tate et al., 2007), indicating that soil aeration status and the nature of the methanotroph population in soils together explain most of the changes observed in CH4 oxidation with land-use change in New Zealand.
Based on the limited data available, Saggar et al. (2008) estimated a possible increase in national soil CH4 oxidation due to afforestation since 1990 of 0.8–7 Gg CH4 that, if proven, could offset 1–8% of increased agricultural emissions during this period. Although small, this offset would nonetheless be useful for national reporting purposes. The wide range of the potential offset is mainly because data are limited.
This report provides new data on soil CH4 oxidation in reverting shrubland. We also describe new research, using our recently acquired knowledge of the mechanisms regulating soil CH4 oxidation (Tate et al 2007; Singh et al. 2007), to develop a practical technology for capturing emissions from housed animals, effluent ponds, and landfills. This latter research has been made possible through collaboration with the Macaulay Land Use Research Institute (Aberdeen, UK) and the University of Victoria, B.C., Canada.
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