Response of soil efflux to experimental warming and increased precipitation intensity depends upon latitudinal climate gradient in Pacific Northwest grasslands

Background/Question/Methods

Efflux of CO₂ from soils is expected to increase as global temperatures rise, releasing stored soil organic carbon (SOC) and potentially amplifying climate forcing.  However, the response of CO₂ efflux to experimental warming varies between regions and has been shown to decrease over time, which has been termed soil acclimation.  It is unclear to what degree the observed response is dependent on local site factors, as most experimental studies are conducted at a single site.  To deconvolve local versus regional climatic controls, we examined the response of CO₂ efflux to simulated climate change along a 520-km latitudinal climate gradient in three Pacific Northwest (PNW) grasslands.  We are also assessing if there is an attenuation of CO₂ efflux in warmed treatments and, if so, is this transient response to heating regionally consistent.  The three sites are located in southern Oregon (SOR), central Oregon (COR), and Central Washington (WA), respectively.  At each site we implemented a fully-factorial treatment combination of 2.5-3^oC warming and 20% added precipitation intensity.  Climate treatments have been continuous since Fall 2010 and CO₂ efflux has been measured monthly from January 2011.

Results/Conclusions

The response of CO₂ efflux varied seasonally among sites, consistent with soil temperature and moisture trends imposed by the regional climate gradient of increasing Mediterranean-like conditions from north to south.  Soil efflux was highest in the wettest/warmest conditions (Spring), lower in cool/wet conditions (Fall and Winter), and lowest when dry (Summer).  The climate gradient influenced both the timing of seasonal transitions and the response magnitude.  Efflux in SOR was stimulated by experimental warming in January only, in COR it was stimulated through April, and in WA through May.  Efflux in SOR was suppressed by warming in May and June, in June for COR, and in July for WA, indicating that warming accelerated moisture limitation.  Precipitation stimulated efflux in SOR during May and July, consistent with efflux in SOR being primarily moisture limited.  Treatment effects diminished in August through December, which may indicate soil acclimation.  The primary limitations on efflux varied seasonally in a predictable manner along the climate gradient and emphasize the importance of considering regional responses to climate change within an ecosystem type. Also, drier sites in a Mediterranean-like climate may show smaller annual responses to warming because of their greater moisture limitation during much of the growing season.