No abstract

Carbon dioxide and nitrous oxide are two important greenhouse gases (GHG) released from cropping systems. Their emissions can vary substantially with climate, soil, and crop management. While different methods are available to account for GHG emissions in life cycle assessments (LCA) of crop production, there are no standard procedures. In this study, the objectives were: (i) to compare several methods of estimating CO 2 and N 2 O emissions for a LCA of cropping systems and (ii) to estimate the relative contribution of soil GHG emissions to the overall global warming potential (GWP) using results from a field experiment located in Manitoba, Canada. The methods were: (A) measurements; (B) Tier I and (C) Tier II IPCC (Intergovernmental panel on Climate Change) methodology, (D) a simple carbon model combined with Intergovernmental Panel for Climate Change (IPCC) Tier II methodology for soil N 2 O emissions, and (E) the DNDC (DeNitrification DeComposition) agroecosystem model. The estimated GWPs (-7.2 to 17 Mg CO 2 eq ha-1 y-1 ;-80 to 600 kg CO 2 eq GJ-1 y-1) were similar to previous results in North America and no statistical difference was found between GWP based on methods D and E and GWP based on observations. The five methods gave estimates of soil CO 2 emissions that were not statistically different from each other, whereas for N 2 O emissions only DNDC estimates were similar to observations. Across crop types, all methods gave comparable CO 2 and N 2 O emission estimates for perennial and legume crops, but only DNDC gave similar results with respect to observations for both annual and cereal crops. Whilst the results should be confirmed for other locations, the agroecosystem model and method D can be used, at certainly one selected site, in place of observations for estimating GHGs in agricultural LCA.

Ecosystem-scale methane (CH₄) flux (F_CH4) over a subarctic fen at Churchill, Manitoba, Canada was measured to understand the magnitude of emissions during spring and fall shoulder seasons, and the growing season in relation to physical and biological conditions. F_CH4 was measured using eddy covariance with a closed-path analyser in four years (2008–2011). Cumulative measured annual F_CH4 (shoulder plus growing seasons) ranged from 3.0 to 9.6 g CH₄ m⁻² yr⁻¹ among the four study years, with a mean of 6.5 to 7.1 g CH₄ m⁻² yr⁻¹ depending upon gap-filling method. Soil temperatures to depths of 50 cm and air temperature were highly correlated with F_CH4, with near-surface soil temperature at 5 cm most correlated across spring, fall, and the shoulder and growing seasons. The response of F_CH4 to soil temperature at the 5 cm depth and air temperature was more than double in spring to that of fall. Emission episodes were generally not observed during spring thaw. Growing season emissions also depended upon soil and air temperatures but the water table also exerted influence, with F_CH4 highest when water was 2–13 cm below and lowest when it was at or above the mean peat surface

Net ecosystem exchange of carbon was measured using eddy covariance for four growing seasons at a subarctic hummocky fen in northern Manitoba, Canada. Over a 115 day measurement period each year, cumulative net ecosystem exchange of carbon ranged from a gain of 49 g C m −2 to a loss of 16 g C m −2 with a mean loss of 6 g C m −2 from the fen, with an uncertainty of about ±34 g C m −2. Ecosystem respiration decreased with higher water tables (r 2 = 0.3), especially in one summer when flooding occurred to 0.12 m above the peat surface. Additional methane emissions previously documented for the site of 4-5.7 g C m −2 year −1 added to the carbon loss. Carbon loss was measured from this same fen in the 1990s and it is likely that the carbon gain (peat accumulation) during past centuries has not continued in recent decades. Scaling to annual greenhouse gas emissions as a 100 year global warming potential showed that this fen is currently a source of 192-490 g CO 2 -equivalents m −2 year −1 based on both carbon dioxide and methane flux measurements, indicating that peat is decomposing. . La respiration de l'écosystème diminuait lorsque les nappes phréatiques étaient plus hautes (r 2 = 0.3), particulièrement un été durant lequel le niveau d'inondation a atteint 0,12 m au-dessus de la surface de la tourbe. Des émissions supplémentaires de méthane précédemment documentées pour le site de 4 à 5,7 g C m −2 a −1 ont contribué davantage à la perte de carbone. On avait mesuré la perte de carbone à ce même fen dans les années 1990 et il est probable que le gain de carbone (l'accumulation de tourbe) pendant des siècles passés n'a pas continué dans les décennies récentes. Une mise à l'échelle aux émissions de gaz à effet de serre annuelles comme un potentiel de réchauffement climatique sur 100 ans a montré que ce fen est actuellement une source de 192 à 490 g équivalents de CO 2 m −2 a −1 d'après les mesures tant du flux de dioxyde de carbone que de méthane, indiquant que la tourbe se décompose. [Traduit par la Rédaction]

Ecosystem-scale methane (CH₄) flux (F_CH4) over a subarctic fen at Churchill, Manitoba, Canada was measured to understand the magnitude of emissions during spring and fall shoulder seasons, and the growing season in relation to physical and biological conditions. F_CH4 was measured using eddy covariance with a closed-path analyzer in four years (2008–2011). Cumulative measured annual F_CH4 (shoulder plus growing seasons) ranged from 3.0 to 9.6 g CH₄ m⁻² yr⁻¹ among the four study years, with a mean of 6.5 to 7.1 g CH₄ m⁻² yr⁻¹ depending upon gap-filling method. Soil temperatures to depths of 50 cm and air temperature were highly correlated with F_CH4, with near surface soil temperature at 5 cm most correlated across spring, fall, and the whole season. The response of F_CH4 to soil temperature at the 5 cm depth and air temperature was more than double in spring to that of fall. Emission episodes were generally not observed during spring thaw. Growing season emissions also depended upon soil and air temperatures but water table also exerted influence with F_CH4 highest when water was 2–13 cm below and least when it was at or above the mean peat surface