Carbon dioxide, methane, and nitrous oxide exchanges in an age-sequence of temperate pine forests
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We investigated soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) exchanges in an age-sequence (4, 17, 32, 67 years old) of eastern white pine (Pinus strobus L.) forests in southern Ontario, Canada, for the period of mid-April to mid-December in 2006 and 2007. For both CH₄ and N₂O, we observed uptake and emission ranging from -160 to 245 μg CH₄ m⁻² h⁻¹ and -52 to 21 μg N₂O m⁻² h⁻¹, respectively (negative values indicate uptake). Mean fluxes from mid-April to mid-December across the 4, 17, 32, 67 years old stands were similar for CO₂ fluxes (259, 246, 220, and 250 mg CO₂ m⁻² h⁻¹, respectively), without pattern for N₂O fluxes (-3.7, 1.5, -2.2, and -7.6 μg N₂O m⁻² h⁻¹, respectively), whereas the uptake rates of CH₄ increased with stand age (6.4, -7.9, -10.8, and -23.3 μg CH₄ m⁻² h⁻¹, respectively). For the same period, the combined contribution of CH₄ and N₂O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO₂ and aggregated soil exchanges of CH₄ and N₂O was on average 4%, <1%, <1%, and 2% for the 4, 17, 32, 67 years old stand, respectively. Soil CO₂ fluxes correlated positively with soil temperature but had no relationship with soil moisture. We found no control of soil temperature or soil moisture on CH₄ and N₂O fluxes, but CH₄ emission was observed following summer rainfall events. LFH layer removal reduced CO₂ emissions by 43%, increased CH₄ uptake during dry and warm soil conditions by more than twofold, but did not affect N₂O flux. We suggest that significant alternating sink and source potentials for both CH₄ and N₂O may occur in N- and soil water-limited forest ecosystems, which constitute a large portion of forest cover in temperate areas.
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