CSIRO PUBLISHING
Functional Plant Biology, 2006, 33, 613–615
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Rapid communication: A comment on scaling methane emissions from vegetation and grazing ruminants in New Zealand Francis M. Kelliher A,E , Harry Clark B , Zheng Li C , Paul C. D. NewtonB , Anthony J. ParsonsB and Gerald Rys D A Landcare
Research, PO Box 69, Lincoln, New Zealand. Research, Private Bag 3127, Hamilton, New Zealand. C AgResearch, Private Bag 11008, Palmerston North, New Zealand. D Ministry of Agriculture and Forestry, PO Box 2526, Wellington, New Zealand. E Corresponding author. Email:
[email protected] B Landcare
Abstract. Keppler et al. (2006, Nature 439, 187–191) showed that plants produce methane (CH4 ) in aerobic environments, leading Lowe (2006, Nature 439, 148–149) to postulate that in countries such as New Zealand, where grazed pastures have replaced forests, the forests could have produced as much CH4 as the ruminants currently grazing these areas. Estimating CH4 emissions from up to 85 million ruminants in New Zealand is challenging and, for completeness, the capacity of forest and pastoral soils to oxidise CH4 should be included. On average, the CH4 emission rate of grazing ruminants is estimated to be 9.6 ± 2.6 g m−2 year−1 (± standard deviation), six times the corresponding estimate for an indigenous forest canopy (1.6 ± 1.1 g m−2 year−1 ). The forest’s soil is estimated to oxidise 0.9 ± 0.2 g m−2 year−1 more CH4 than representative soils beneath grazed pasture. Taking into account plant and animal sources and the soil’s oxidative capacity, the net CH4 emission rates of forest and grazed ecosystems are 0.6 ± 1.1 and 9.8 ± 2.6 g m−2 year−1 , respectively.
Introduction Keppler et al. (2006) report that methane (CH4 ) is emitted by plants in aerobic environments. In the absence of an understanding of the mechanism involved, it is a moot point how this notable discovery should be scaled. Kirschbaum et al. (2006) discussed scaling CH4 emissions from vegetation, and obtained much lower values than those calculated by Keppler et al. (2006). Here we consider another extrapolation inspired by Keppler et al. (2006). This is the comment by Lowe (2006) suggesting that, in countries where CH4 from ruminants forms a large part of the national emissions, such as Ireland and New Zealand, ‘it is possible that the forests that once occupied pasture may have produced as much methane as ruminants and grasses on the same land’. We test Lowe’s hypothesis by compiling and analysing vegetation, ruminant and soils data from New Zealand and estimating net CH4 emission rates from typical indigenous forest and grazed pasture ecosystems. We do not consider the atmosphere’s oxidising power beyond an assumption of its invariance (Manning et al. 2005). Methods Briefly we begin by obtaining representative data for leaf mass per unit ground area that are required to scale the postulated leaf CH4 emissions data to the ecosystem for indigenous forest and grazed grassland. We © CSIRO 2006
then analyse measurements of soil CH4 oxidation for these two land uses and estimate the required scaling factors. Finally, from New Zealand’s ruminant CH4 emissions inventory (Ministry for the Environment 2005), we obtain a national, annual average rate and estimate the associated uncertainty. For vegetation, our method to scale methane emissions to an ecosystem, representative of a land use, was on the basis of leaf mass per unit ground area. The method is consistent with the leafmass based approach described by Kirschbaum et al. (2006). For an indigenous forest, we used data from a 100-year-old mountain beech [Nothofagus solandri var. cliffortioides (Hookf.) Poole] site (Hollinger 1989). The forest’s wood mass was not measured and including an emissions estimate for the wood fraction, following Parsons et al. (2006), suggested only a small increase over that from the leaf mass alone. Hollinger’s Craigieburn site was located 100 km north-west of Christchurch, New Zealand (43.1 S, 171.7 E) and later chosen for soil CH4 oxidation measurements (Price et al. 2004). For grazed pasture, we used an average from data compiled by the Ministry of Agriculture and Forestry (www.maf.govt.nz/statistics/primaryindustries/land-useand-farm-counts/index.htm; verified 17 May 2006). The soil CH4 oxidation rate is denoted negative to indicate net uptake from the atmosphere. To determine the rate for indigenous forest, a land use currently covering 65 000 km2 of New Zealand’s total area of 269 000 km2 , we used soil chamber measurements reported by Price et al. (2004). For these data, the standard deviation was ± 20% of the average. Pastoral agriculture is a land use currently covering 117 000 km2 (D Lillis, personal communication). To determine the rate for soil beneath grazed pasture, we further analysed chamber measurements in freely and poorly drained soils for 100 d after cattle urine application (Li and Kelliher 2005). For the freely drained soil,
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Table 1. Annual net CH4 emissions from indigenous forest and grazed pasture ecosystems in New Zealand (± standard deviation) including soil oxidation denoted negative to indicate net uptake from the atmosphere For the vegetation, we assumed the growing season was 300 d long and the hourly sunshine rate averaged 6 h d−1 . The vegetation leaf mass method calculations were consistent with those described by Kirschbaum et al. (2006). Other calculations are described in the text
Vegetation
Leaf mass (g m−2 )
Vegetation CH4 emissions (g m−2 year−1 )
Soil CH4 oxidation (g m−2 year−1 )
Ruminant CH4 emissions (g m−2 year−1 )
Net ecosystem CH4 emissions (g m−2 year−1 )
1219 250
1.6 ± 1.1 0.3 ± 0.2
−1.05 ± 0.21 −0.14 ± 0.02
0.0 9.6 ± 2.6
0.55 ± 1.12 9.76 ± 2.61
Indigenous forest Grazed grassland
in control and urine application plots, CH4 oxidation rates averaged −0.18 ± 0.02 (± standard deviation) and −0.11 ± 0.02 g m−2 year−1 , respectively. For the poorly drained soil, the corresponding rates were −0.06 ± 0.01 and −0.02 ± 0.01 g m−2 year−1 . From these data, the standard deviation was considered ±16% of the average. To scale these data to a grazed ecosystem, we estimated that urine covered 25% of the pastoral agriculture area following Haynes and Williams (1993). To be representative, using a geographic information system, we combined national spatial layers of land use and soil drainage class to estimate areas of pastoral agriculture soils yielding 75% as freely drained, 16% imperfectly drained and 9% poorly drained based. For ruminants, annual CH4 emissions were obtained from New Zealand’s official inventory including dairy cattle, beef cattle, sheep, deer and goats (Ministry for the Environment 2005). The inventory is compiled by an Intergovernmental Panel on Climate Change (IPCC)-compliant ‘Tier 2’ method described by Clark et al. (2003). To obtain an average rate, national CH4 emissions reported for the year 2003 (3-year running average) was divided by 117 000 km2 , the associated land area. The associated uncertainty is expressed by a standard deviation estimated to be ±27% of the average based on Clark et al. (2003).
Results and discussion Our purpose is to test Lowe’s hypothesis that indigenous forest may produce as much CH4 as pasture grazed by ruminants, predicated on a new, significant source of CH4 emission from plants in aerobic environments. In the absence of data, we do not know whether non-leaf plant organs emit CH4 , so vegetation emission rates must be speculative. Our leaf-mass estimates suggest indigenous forest emits around five times more CH4 per unit area than the pasture that replaced it (Table 1). However, the soil’s capacity to oxidise CH4 should also be considered when comparing forest and grazed ecosystems. Once the soil’s oxidative capacities are taken into account (Table 1) not only does the forest ecosystem’s net emissions rate fall to approximately one-third of the leaf-mass rate (1.6 to 0.55 g m−2 year−1 ), it becomes only around three times larger than the pasture ecosystem (0.55 v. 0.16 g m−2 year−1 ) without grazing ruminants. Estimating CH4 emissions from ruminants, especially free-grazing ruminants, is a challenging task. Direct measurements can be made on small groups of animals for short periods (Johnson et al. 1994; Lassey et al. 1997; Boadi et al. 2002) but given the variability in farm and animal conditions and the paucity of data at national and annual
scales, there is considerable uncertainty around estimates including large-scale integrations (Clark et al. 2003). However, for freely grazing ruminants, scaled-up animal CH4 emission measurements agree reasonably well with flock, herd and regional emissions from micrometeorological measurements averaged over approximately month-long campaigns (Judd et al. 1999; Wratt et al. 2001; Harvey et al. 2002; Laubach and Kelliher 2004). The discovery that plants also emit CH4 is undoubtedly exciting, but our calculations suggest that CH4 emissions produced by ruminant animals grazing temperate grassland are close to 16 times greater than the net CH4 emissions of an indigenous forest ecosystem (0.6 ± 1.1 g m−2 year−1 compared with 9.8 ± 2.6 g m−2 year−1 ), exceeding the uncertainty inherent in all national, annual emission estimates. These data therefore do not support Lowe’s (2006) extrapolation from Keppler et al. (2006) that New Zealand’s indigenous forest produced as much CH4 as that of ruminants currently grazing the land. Acknowledgments We gratefully acknowledge funding by the Foundation for Research, Science and Technology and Ministry of Agriculture and Forestry, valuable discussions with Tim Clough, Johannes Laubach and Sally Price and spatial data analysis by Peter Newsome. Two anonymous reviewers constructively criticised our original submission. References Boadi DA, Wittenberg KM, Kennedy AD (2002) Validation of the sulphur hexafluoride (SF6) tracer gas technique for measurement of methane and carbon dioxide production by cattle. Canadian Journal of Animal Science 82, 125–131. Clark H, Brookes I, Walcroft A (2003) Enteric methane emissions from New Zealand ruminants 1990–2001 calculated using an IPCC Tier 2 approach. Report to the Ministry of Agriculture and Forestry, New Zealand. Harvey MJ, Brailsford GW, Bromley AM, Lassey KR, Mei Z, Kristamen IS, Reisinger AR, Walker CF, Kelliher FM (2002) Boundary-layer isotope dilution / mass balance methods for measurement of nocturnal methane emissions from grazing sheep. Atmospheric Environment 36, 4663–4678. doi: 10.1016/S13522310(02)00410-7
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Haynes RJ, Williams PH (1993) Nutrient cycling and soil fertility in the grazed pasture ecosystem. Advances in Agronomy 49, 119–199. Hollinger DY (1989) Canopy organization and foliage photosynthetic capacity in a broad-leaved evergreen montane forest. Functional Ecology 3, 53–62. Johnson K, Huyler M, Westberg H, Lamb H, Zimmerman P (1994) Measurement of methane emissions from ruminant livestock using an SF6 tracer technique. Environmental Science & Technology 28, 359–362. doi: 10.1021/es00051a025 Judd MJ, Kelliher FM, Ulyatt MJ, Lassey KR, Tate KR, Shelton D, Harvey MJ, Walker CF (1999) Net methane emissions from grazing sheep. Global Change Biology 5, 647–657. doi: 10.1046/j.13652486.1999.00264.x Keppler F, Hamilton JTG, Braß M, R¨ockman T (2006) Methane emissions from terrestrial plants under aerobic conditions. Nature 439, 187–191. doi: 10.1038/nature04420 Kirschbaum MUF, Bruhn D, Etheridge DM, Evans JR, Farquhar GD, Gifford RM, Paul KI, Winters AJ (2006) A comment on the quantitative significance of aerobic methane release by plants. Functional Plant Biology 33, 521–530. Lassey KR, Ulyatt MJ, Martin RJ, Walker CF, Shelton ID (1997) Methane emissions measured directly from grazing livestock in New Zealand. Atmospheric Environment 31, 2905–2914. doi: 10.1016/S1352-2310(97)00123-4 Laubach J, Kelliher FM (2004) Measuring methane emission rates of a dairy cow herd by two micrometeorological techniques. Agricultural and Forest Meteorology 125, 279–303. doi: 10.1016/j.agrformet.2004.04.003
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Manuscript received 13 April 2006, accepted 16 May 2006
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