Boundary-layer isotope dilution/mass balance methods for measurement of nocturnal methane emissions from grazing sheep

Following advances with methods for ¹³C/¹²C isotopic analysis of methane in small (≤4L) air samples, new isotope dilution techniques are proposed for measurement of methane emissions at the paddock scale from grazing ruminant animals. These techniques combine measurement of the isotopic δ¹³CH₄ composition of air samples with a non-intrusive mass balance method applied in the nocturnal boundary layer. Flux estimates from trials using the isotope dilution techniques are compared with estimates based on scaling up individual animal emission measurements using a rumen gas tracer technique. The methane flux assessed by the latter technique ranged from 35 to 70mg (CH₄) m^-2d^-1 with a stocking density between 10 and 20 sheep ha^-1. The isotope dilution based nocturnal boundary-layer estimates generally agreed to better than a factor of 2 and usually to within 20% of the average of individual animal emission rate per unit area of paddock. Both static and advecting mass balance methods are developed. In the advecting case, the upwind/downwind contrast in δ¹³C was typically 0.2-0.5‰. Care was necessary with air sampling to avoid error in this small contrast contributing to error in the flux. Agreement between concentration- and isotope-based nocturnal boundary layer methods and the sheep breath measurements indicated that sample representativeness was generally good. Factors which affect the accuracy of the method are examined and include variability in nocturnal mixing height, the assumed δ¹³CH₄ composition of the source sheep breath and diurnal patterns in sheep emission. This paper establishes new techniques useful in the paddock to landscape scale although widespread application awaits further development of technology for rapid and repeatable field analysis of δ¹³CH₄ in small samples.