Abstract
Abstract. New Zealand's largest industrial sector is pastoral agriculture, giving rise to a large fraction of the country's emissions of methane (CH4) and nitrous oxide (N2O). We designed a system to continuously measure CH4 and N2O fluxes at the field scale on two adjacent pastures that differed with respect to management. At the core of this system was a closed-cell Fourier transform infrared (FTIR) spectrometer, which measured the mole fractions of CH4, N2O and carbon dioxide (CO2) at two heights at each site. In parallel, CO2 fluxes were measured using eddy-covariance instrumentation. We applied two different micrometeorological ratio methods to infer the CH4 and N2O fluxes from their respective mole fractions and the CO2 fluxes. The first is a variant of the flux-gradient method, where it is assumed that the turbulent diffusivities of CH4 and N2O equal that of CO2. This method was reliable when the CO2 mole-fraction difference between heights was at least 4 times greater than the FTIR's resolution of differences. For the second method, the temporal increases of mole fractions in the stable nocturnal boundary layer, which are correlated for concurrently emitted gases, are used to infer the unknown fluxes of CH4 and N2O from the known flux of CO2. This method was sensitive to “contamination” from trace gas sources other than the pasture of interest and therefore required careful filtering. With both methods combined, estimates of mean daily CH4 and N2O fluxes were obtained for 56 % of days at one site and 73 % at the other. Both methods indicated both sites as net sources of CH4 and N2O. Mean emission rates for 1 year at the unfertilised, winter-grazed site were 8.9 (±0.79) nmol CH4 m−2 s−1 and 0.38 (±0.018) nmol N2O m−2 s−1. During the same year, mean emission rates at the irrigated, fertilised and rotationally grazed site were 8.9 (±0.79) nmol CH4 m−2 s−1 and 0.58 (±0.020) nmol N2O m−2 s−1. At this site, the N2O emissions amounted to 1.21 (±0.15) % of the nitrogen inputs from animal excreta and fertiliser application.
Highlights
The accurate assessment of greenhouse gas (GHG) fluxes between the biosphere and the atmosphere is crucial to understanding the driving mechanisms of global climate change
For the gas-gradient ratio (GGR) method, we have demonstrated that the main one is to ensure that the mole-fraction gradients of the reference gas are a few times larger than instrument resolution, in order to contain random scatter within tractable limits
Continuous year-round measurements of the fluxes of CH4 and N2O at two neighbouring, contrasting pasture sites were obtained with the combination of GGR and nocturnal storage-ratio (NSR) methods, both using CO2 as the reference gas
Summary
The accurate assessment of greenhouse gas (GHG) fluxes between the biosphere and the atmosphere is crucial to understanding the driving mechanisms of global climate change. The accounting of full GHG budgets is especially important for agroecosystems as they are the largest global source of N2O and CH4 emissions (Montzka et al, 2011). Leahy et al (2004) showed that N2O and CH4 emissions on managed grasslands have the potential to fully counteract the CO2 sink strength in these ecosystems. This has been confirmed by a European wide synthesis study done at 10 different grasslands sites over 2 years (Soussana et al, 2007). Among soils from different land-use types, those from grasslands have been shown to have the highest rates of N2O emissions, due to high microbial activity stimulated by high soil C (carbon) and N (nitrogen) content (Schaufler et al, 2010)
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