Consequences of Warming and Altered Snowmelt Timing On Soil N Cycling and Greenhouse Gas Fluxes in the Sierra Nevada Rain-Snow Transition Zone.

The legacy of winter warming on soil nitrogen cycling and summer soil greenhouse gas emissions remains unclear.  Effects of winter warming on summer hydrology and biogeochemistry may be most obvious in rain-snow transition zones (i.e., areas between more severe and less severe winters) and in a Mediterranean-type climate with almost no summer precipitation.  We therefore simulated climatic warming in the rain-snow transition zone of the Sierra Nevada Mountains in California by: (1) moving a mixed conifer forest soil down in elevation from predominantly snow to equal amounts of precipitation as rain and snow; and (2) manipulating the timing of spring snowmelt, both earlier using black sand and later using white Tyvek cloth.  Increasing fall, winter, and spring mean soil temperature by 1.4 °C by moving soil cores down in elevation, with little change in summer temperature or total precipitation amount, increased soil CO₂ emission by 32% during snow-free periods between August 2010 and June 2012.  Warming also increased CH₄ uptake by 48%, but had no effect on N₂O fluxes.  Our field manipulations of snowmelt timing significantly affected soil moisture and CO₂ emission during the growing season.   The 2-week (WY 2011) and 3-week (WY 2012) advancement of snowmelt at the high elevation site caused drier soils and 10-35% less CO₂ emission during the subsequent growing season.  Snowmelt timing had no effect on the other two greenhouse gases measured. Our results suggest that climatic warming in snow-dominated ecosystems will likely increase net greenhouse gas emission from the soil to the atmosphere in the short-term.  However, continued advancement of the snowmelt date, without a concomitant increase in precipitation, will likely constrain the extent of the temperature-induced increase in greenhouse gas fluxes.