Keith R. Lassey

METHANE EMISSIONS MEASURED DIRECTLY FROM GRAZING LIVESTOCK IN NEW ZEALAND

Abstract We report measurements of methane emissions from individual ruminant livestock-both sheep and dairy cows-grazing pasture typical of New Zealand lowlands in the temperate southwest Pacific. These are the first measurements reported from grazing sheep, and among the first from grazing cattle. The measurement technique, developed at Washington State University, enables emission rates to be determined from analyses of “breath” samples collected while grazing. More than 250 measurements of daily methane emission from 50 sheep (8 months old) were made, with flock-mean emission 18.9 ± 0.8 g hd −1 d−1. Although emissions were weakly correlated with feed intake, they represented a 4.6 ± 0.1 % average loss of gross dietary energy. The corresponding mean emission based on 40 measurements of daily emissions from 10 lactating dairy cows was 263 ± 10 g hd−1 d−1, approximately 6.2% of estimated gross energy intake. A notable feature was the large inter-sheep variability in daily methane emission (factor of 1.4 range) that could not be attributed to variable intake. This would appear to suggest an appreciable diversity of methanogenetic response to digestion, and may be significant in the search for strategies to control emissions of this greenhouse gas.

Concentration and 13C records of atmospheric methane in New Zealand and Antarctica : evidence for changes in methane sources

Measurements of 13C in atmospheric methane made at Baring Head, New Zealand (41°S), over the 4-year period, 1989–1993, display a persistent but highly variable seasonal cycle. Values for δ13C peak in summer at about −46.9‰ and drop to around −47.5‰ in the late winter. Methane concentration shows a similar cycle, with winter peaks and summer minima. Similar features are observed at the New Zealand Antarctic station, Scott Base, at 78°S. While the phase of the δ13C cycle is consistent with a kinetic isotope effect that preferentially leaves methane enriched in 13C in the atmosphere after oxidation by OH, the amplitude of the cycle is much larger than expected from published laboratory measurements of the effect. We interpret the origin of this cycle and its inter-annual variability to be due to episodic southward transport of isotopically heavy methane from large-scale tropical biomass burning, possibly in conjunction with changes in the rate of interhemispheric transport in the troposphere. The Baring Head 13C data show no significant secular trend from 1989 to mid-1991, followed by a rapid trend toward methane less enriched in 13C. This indicates a major shift in the balance of the sources of atmospheric methane and precludes an increased sink strength. The trend in 13C since mid-1991 coincided with significant changes to the methane growth rate observed at Baring Head and at Scott Base: an elevated growth rate of about 15 parts per billion by volume (ppbv) during 1991 gave way to less than 3 ppbv yr−1 thereafter. A 2-box model of atmospheric methane (one box per hemispheric reservoir) suggests that (1) the recent decline in 13C in methane observed at Baring Head and Scott Base cannot have a solely northern hemispheric origin and (2) the most plausible origin is a recent reduction in methane released by biomass burning in the southern hemisphere, combined with a lower release rate of fossil methane in the northern hemisphere.

The trend in atmospheric methane δ13C and implications for isotopic constraints on the global methane budget

A recent paper by Tans [1997] has drawn attention to the isotopic disequilibrium that inevitably prevails when atmospheric methane is not in steady state with its sources, noting in particular the very slow adjustment of the isotopic signature δ13C toward its steady state. Our aim in this paper is to clarify the nature of disequilibrium effects on δ13C(CH4) and to assess their likely magnitudes in the global atmosphere over recent decades. We use a simple model simulation incorporating a plausible scenario of the global methane source history over 1700–2010, which includes an unchanged source since 1990. The simulation of both mixing ratio and δ13C compare favorably with the secular features of a 10-year data set (1988–1998) from Baring Head, New Zealand, and of a 17-year data set (1978–1995) in air archived from Cape Grim, Australia. This corroborates a recent analysis of those data sets and their compatibility with stabilized sources. We show that the slow adjustment of δ13C toward steady state arises from the effect of isotope fractionation on the cancellation of contributing terms to δ13C. We explore the implications of disequilibrium for the usual practice of relating δ13C values in the atmosphere to those in the aggregate source through a shift induced by fractionation and quantify the flaws in this practice. Finally, we examine the sensitivity of the atmospheric secular response, in both mixing ratio and δ13C, to sustained changes in source and sink and show that δ13C is a potentially powerful diagnostic of such changes.

Livestock methane emission and its perspective in the global methane cycle

Over the past three centuries, the atmospheric methane burden has grown 2.5-fold, reaching levels unprecedented in at least 650 000 years. Agricultural expansion has played a large part in this anthropogenic signal, with enterically fermented methane emitted by farmed ruminant livestock accounting for about one quarter of all anthropogenic emissions. This paper summarises the range of measurements that give confidence in estimates of the emission per animal and per unit feed intake and in their extrapolation to national and global emission inventories, while noting also some of the inherent uncertainties. Global emissions are discussed in the context of the evolving global methane cycle.

Methane emission from dairy cows and wether sheep fed subtropical grass‐dominant pastures in midsummer in New Zealand

Abstract Methane emission was measured from 10 dairy cows and 12 wether sheep grazing kikuyu grass‐ (Pennisetum clandestinum) dominant pastures at Waimate North, Northland, in February 1997 and March 1999, and from 10 dairy cows grazing summer grass‐ (Digitaria sanguinalis) dominant pasture at Edgecumbe, Bay of Plenty, in March 2000. Methane emission was measured from each animal for 5 consecutive days in each measurement period using the sulphur hexafluoride (SF 6 ) tracer gas technique. Analysis of variance of the kikuyu grass chemical composition with sheep and cow data combined showed that the 1999 pastures were significantly higher in protein (P < 0.01), soluble sugars (P < 0.001), lipid (P < 0.01), and dry matter (DM) digestibility (P < 0.001) and lower in ash (P = 0.023), acid detergent fibre (ADF) (P < 0.001), and neutral detergent fibre (NDF) (P < 0.001) than the 1997 pastures, presumably as a consequence of urea topdressing in 1999. The chemical composition of the summer grass‐dominant pasture grazed by cows in 2000 was similar to the kikuyu grass, except that DM digestibility was higher. Daily methane emissions from kikuyu grass were 363 and 167 g/day for the cows and 15.6 and 4.4 g/day for the sheep in 1997 and 1999, respectively. These corresponded to methane yields (MYs, methane energy as a percentage of gross energy) of 7.1 and 3.8% for the cows and 6.3 and 1.9% for the sheep in 1997 and 1999, respectively. There was clearly a significant inhibition (P < 0.001) of methane production in 1999 in both species fed kikuyu grass. Methane emission was 422 g/day and MY 6.7% in the cows fed summer grass, values that were similar to the kikuyu‐fed cows in 1997. Methane emitted in g/kg digestible DM intake was 33.8 and 38.2 for the 1997 kikuyu grass‐fed cows and sheep and 33.3 for the summer grass‐fed cows. This suggests that methane emitted per unit of digested DM is higher for ruminants fed subtropical (C4) grasses than those fed temperate (C3) grasses and we believe it to be related to the higher cell wall content of C4 grasses.

Seasonal variation in methane emission from dairy cows and breeding ewes grazing ryegrass/white clover pasture in New Zealand

Abstract Daily methane emission from 12 Romney‐cross‐bred ewes and 10 lactating Friesian dairy cows, rotationally grazed on perennial ryegrass/white clover dominant pastures, was measured during four seasons of a year (September, November, March, and June/July). Methane emission was measured from each animal for 5 consecutive days in each measurement period using the sulphur hexafluoride tracer gas technique. The pastures varied significantly in chemical composition between seasons, generally decreasing in protein, soluble sugars, and digestibility and increasing in acid detergent fibre (ADF) and neutral detergent fibre (NDF) as the grasses flowered in November, with an increase in protein and a decrease in soluble sugars in March, a trend that continued through to June/July. Methane emission (g/day) from dairy cows was significantly different (P < 0.001) between seasons, being high at peak lactation in September (430.6) and declining with milk yield and feed intake in November (247.6) and March (181.5) and maintaining its level in June (137.4) when the cows were not lactating. Methane emission expressed per unit of feed intake was significantly higher in September and June than in November and March, suggesting some inhibition in the latter 2 months. There was less seasonal variation in methane emission (g/day) from the breeding ewes: March (27.0) and July (27.9) were significantly lower (P < 0.05) than in November (33.2), but neither differed from September (30.6). July was significantly lower (P < 0.001) than the other periods in methane emission per unit of feed intake, presumably because of its higher feed intake. It is suggested that for inventory purposes a methane emission factor of 26 g/kg digestible dry matter intake (DDMI) would be suitable for sheep and dairy cows grazing fresh temperate pasture.

On the performance of SF6 permeation tubes used in determining methane emission from grazing livestock

In a technique for measuring methane emitted by grazing ruminant livestock, a calibrated source of inert tracer sulfur hexafluoride (SF6) is inserted into the rumen of each participating animal prior to collection of “breath” samples for gas analysis. Each source comprises a “permeation tube” from which an SF6 charge slowly escapes through a permeable membrane, to be sampled with the breath. This paper reports analyses of the permeation characteristics of such tubes and provides evidence that the permeation rate slowly changes rather than stays constant as the technique supposes. This feature has been observed routinely over several generations of tube fills in our laboratory. Failure to take account of a changing permeation rate can lead to a systematic error in the inferred methane emission rate of up to about 15%. A quality control strategy is proposed that enables permeation rates to be extrapolated with confidence, based on the monitored performances of control tubes as proxies for inserted tubes.

Modeling the variation of δ13C in atmospheric methane: Phase ellipses and the kinetic isotope effect

We use the TM2 three-dimensional atmospheric tracer model with a methane source-sink budget based on existing literature to simulate small spatial and temporal variations in the 13C/12C ratio of atmospheric methane. The results show that δ13C varies markedly with wind direction everywhere outside the extratropical Southern Hemisphere (ETSH). Within the ETSH, both methane mixing ratio and δ13C have regular seasonal cycles with differing and latitude-dependent phases. Phase diagrams constructed from these seasonal cycles, showing changes in δ13C versus changes in mixing ratio, have elliptical shapes. The slope of the major axis of these ellipses is determined by the kinetic isotope effect (KIE) of the single atmospheric methane removal process used in the model. The ellipse eccentricity is determined by seasonal variation in the source δ13CH4, which is dominated by the biomass burning source because of its isotopic enrichment relative to other sources. Comparison of the model results, for a KIE based on CH4 + OH oxidation, with observations in the South Pacific region shows significant discrepancies in both the ellipse major axis slopes and eccentricities. We suggest that this is an indicator of an additional sink process that discriminates strongly against 13CH4. Such a sink could be active chlorine in the marine boundary layer.

Atmospheric methane and its carbon isotopes in the southern hemisphere: Their time series and an instructive model

We report data from the clean air monitoring station at Baring Head, New Zealand on concentrations of atmospheric methane and its 13C12C and 14C12C ratios. The record for methane concentration features a recent (post-July 1991) and persistent elevation above expected levels. The 2-year δ13C record shows a surprisingly large seasonal cycle (ca 0.3‰ peak to peak) about a mean value of −47.14 ± 0.03‰ (2 standard deviations) with no discernible trend. The 14C12C record is relatively featureless, corresponding to 119.7 ± 0.7 percent modern carbon (pMC) in January 1990 with a small upward trend of 1.3 ± 0.8 pMC/yr over the past three years. A simple model depicting the southern hemisphere atmosphere as a single methane reservoir with seasonally-modulated sources and sinks is used to examine methane dynamics in this hemisphere. According to this model, approximately half the methane in the southern hemisphere atmosphere is delivered from the northern hemisphere. The model can interpret the δ13C seasonal cycle only as a large injection of isotopically heavy methane during the austral spring, such as might result from biomass burning.

Methane emission from sheep grazing four pastures in late summer in New Zealand

Abstract Four groups of sheep were grazed on four late summer/autumn pastures: southern North Island summer moist hill country (Ballantrae); good quality perennial ryegrass/white clover dominant pasture in the Manawatu (Aorangi); severe late summer drought pasture in Hawkes Bay (Poukawa); and after drought conditions in Canterbury (Springston). Mature ewes were used at Springston, while young wethers were used at all the other sites. Methane emission from each animal was measured using the sulphur hexafluoride (SF6) tracer technique and feed intake was also determined. The pastures used were chosen for their expected chemical compositions at that time of the year. However, unseasonal rain fell just before the measurements were made at Ballantrae, Aorangi, and Springston. Those three pastures, although different in botanical composition, were similar in chemical composition and dry matter (DM) digestibility. The Poukawa pasture was dead and had low protein, soluble carbohydrate and DM digestibility, and high cell wall content. For the Ballantrae, Aorangi, Poukawa, and Springston pastures respectively, methane emissions were: 19.3, 21.9, 21.4, and 35.2 g/day; 13.8, 12.9, 17.8, and 21.1 g/kg DM intake; and the methane yields (methane energy as a percentage of gross energy intake) were 4.1, 3.9, 5.3, and 6.3%. The results support the view that young wether sheep have a lower methane yield than mature sheep and that methane yield is higher from pastures of poor feeding value.