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Greenhouse gas exchange above potato and pea fields in the lower Fraser Valley in British Columbia, Canada Quan, Ningyu

Abstract

The three main biogenic greenhouse gases (GHGs), nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂), are associated with agricultural production and strongly affected by environmental factors, which makes mitigation of GHG emissions a more challenging task in intensive agricultural system in the context of global climate change. Quantification of GHG emissions is of great interest in agroecosystems to better understand flux exchange between agroecosystems and the atmosphere and to provide knowledge for climate change related policy making. However, in Canada, these studies are largely limited to Ontario and the Canadian Prairie provinces, and GHG emissions in the main cropping systems in British Columbia have scarcely been studied. Therefore, this study is aimed at making continuous measurements of N₂O, CO₂ and CH₄ from potato and pea crops in the lower Fraser Valley using the eddy covariance (EC) technique. Furthermore, to cope with the issue of slight spatial inhomogeneity at this study site, flux footprint analysis was used, coupled with EC and closed-chamber measurements. I found that flux footprint correction had the largest effect on N₂O flux compared with CO₂ flux because of more pronounced difference of N₂O flux between two different land surfaces (crop and edge areas). After flux footprint correction, the potato and pea crops were both weak CO₂ sinks with annual net ecosystem exchange values being -57 and -97 g C m⁻² yr⁻¹, respectively. After taking carbon (C) export via crop harvest into account, the potato crop shifted from being a C sink to a C source of 375 g C m⁻² yr⁻¹, while the pea crop became near neutral sequestering only 19 g C m⁻² yr⁻¹. Annual GHG budget was quantified by converting N₂O and CH₄ to CO₂ equivalents using global warming potential on a 100-year timescale, which is 298 for N₂O and 34 for CH₄. The annual GHG budgets were 481 and 200 g CO₂e m⁻² yr⁻¹ for the potato and pea crops respectively. For both potato and pea crops, N₂O contributed the largest proportion to annual total CO₂e and outweighed the CO₂ uptake from the atmosphere, making both crop fields net sources of GHGs.

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