Soil greenhouse gas fluxes in the High Sierra: Lessons from a hydrological gradient in a subalpine meadow in Yosemite National Park

Background/Question/Methods

Seasonally snow-covered ecosystems in the western US are facing major hydrological change as a consequence of climatic warming, but little is known about the relationships between soil moisture and greenhouse gas fluxes in remote, high-elevation ecosystems in the Sierra Nevada Mountains of California.  Biogeochemical processes in Sierra Nevada ecosystems may be particularly sensitive to water stress because the summer growing season is also the dry season.  During the summer of 2010, we studied a hydrological gradient in a subalpine meadow (el. 2860 m) in Yosemite National Park in order to better understand: (1) the contribution of high-elevation meadows to greenhouse gas emissions; and (2) the environmental controls on emissions.  The gradient spanned 50 meters, from the wet center of the meadow to the dry edge.  Fluxes of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) were measured using a static chamber technique at 10-meter intervals along the gradient, both during (i.e., July) and at the end of the growing season (i.e., September).

Results/Conclusions

Surprisingly, this relatively wet meadow was not a source of CH₄ or N₂O on either sampling date.  Soil CH₄ consumption was strongly related to soil moisture on both sampling dates (r²_adj = 0.41-0.56), with higher consumption on the drier side of the meadow indicative of diffusional limitation.  The wet side of the meadow exhibited net CH₄ uptake despite a wetter-than-average preceding winter and spring.  Net N₂O consumption was commonly measured in wet soils in July (r²_adj = 0.11) and on the colder sampling date in September.  CO₂ emission was weakly related to soil moisture in July, but was positively related to soil moisture in September (r²_adj = 0.44).  Soil CO₂ emission was best explained (positively) by plant species richness (r²_adj = 0.25).  This subalpine meadow with small-statured plants is likely a source of CO₂ and a sink of CH₄ and potentially N₂O during the summer.  All three gas fluxes, especially CO₂ and CH₄, were controlled by soil moisture.  If spatial variation is used as a surrogate for time, then our results suggest that warmer and drier summers will cause less CO₂ emission, more CH₄ uptake, and more N₂O emission in high-elevation Sierra Nevada ecosystems.