Consequences of warming and altered snowmelt timing on soil CO2, CH4, and N2O fluxes in the

Sierra Nevada rain-snow transition zone

Joseph C. Blankinship, Emma P. McCorkle, Matthew W. Meadows, Ryan G. Lucas, and Stephen C. Hart

AGU 2012 Fall MeetingB21I. When Winter Changes: Hydrological, Ecological, and Biogeochemical Responses I

Kearney Foundation of Soil Science 2006-

2011 Mission

Western North America is facing major climatic and hydrological changes

Stewart et al. 2005, Journal of Climate

more rain

Bales et al. 2006, Water Resources Research

II. Snowmelt Timing

I. Precipitation Type

Due to 3ºC warmingno changeWhat do these changes mean for soil microbial activities and

greenhouse gas (GHG) emissions during the warm season?

Connecting precipitation type & snowmelt timing to soil GHG fluxes

Warming in Seasonally Snow-Covered Ecosystems

More Rain Earlier Snowmelt

Timing of Soil Water Availability

Timing/Extent of Resources

Longer “Microbial Growing Season”?

↑ Microbial Stress?

↑ Soil GHG Emission? ↓ Soil GHG Emission?

FALL, WINTER, SPRING

SUMMER

Realistic simulation is difficult

Most studies confounded by summer precip and altered water input

Soil and plant heterogeneity

Research Questions

1. Will winter and spring warming exacerbate summer microbial stress in the rain-snow transition zone, thus reducing GHG emissions?

2. If water input stays the same, does snowmelt timing impact soil GHG fluxes for days, weeks, or months?

Warm & Rainy

Cold & Snowy

Image from The Mighty Sierra: Portrait of a Mountain World (1972) by Paul Webster and Francis Farquhar

Sierra Nevada climatic gradient

“Rain-snow transition zone”

Water Year 2011: 2nd wettest on record(~170% normal runoff; ~3.5 m max. snow depth)

Water Year 2012: 4th driest on record(~50% normal runoff; ~1.5 m max. snow depth)

Grant Grove, CA; Western Regional Climate Center

Little precip after snowmelt

Ideal, extreme seasonal and interannual variation in precipitation

Low elevation site (1,816 m ASL)MAT = 8.2 °C; MAP = ~1000 mm; 40-65% rain

To simulate warming, we moved forest soils across the rain-snow transition zone, twice

Downward Transfer

High elevation site (2,365 m ASL)MAT = 6.8 °C; MAP = ~1000 mm; 75-90% snow

* No difference in mean soil temperature during summer

Soil GHG fluxes:

Carbon Dioxide (CO2)

Methane (CH4)

Nitrous Oxide (N2O)

• Measured weekly or biweekly after snowmelt, and then monthly

• Not measured when snow depth > 5 cm

Winter warming accelerated summer GHG fluxes

CO2 emission CH4 uptake N2O emission

“Early Snowmelt”

n = 12

“Late Snowmelt”

3.1 m

3.1 m

n = 12

Snowmelt was manipulated by 2 to 3 weeks

Earlier snowmelt reduced CO2 emission and increased CH4 uptake

CO2

CH4

Wet Year Dry Year

Complete Snowmelt

Research Questions

1. Will winter and spring warming exacerbate summer microbial stress in the rain-snow transition zone, thus reducing GHG emissions? No , warming increased net GHG emission and seemed to relieve stress

2. If water input stays the same, does snowmelt timing impact soil GHG fluxes for days, weeks, or months?

CH4: WeeksCO2 (and N2O): Months

The warming effect…

Warming in Seasonally Snow-Covered Ecosystems

More Rain (Less Extreme Winter)

More Resources for Microbial Growth?

↑ Soil GHG Emission

…is likely constrained by earlier snowmelt.

Warming in Seasonally Snow-Covered Ecosystems

Earlier Snowmelt

↓ Soil Resources During Summer

↑ Microbial Stress?

↓ Soil GHG Emission

Questions?

EXTRA SLIDES FOLLOW:

* indicates significant elevation difference (P < 0.05 in one-way ANOVA); # indicates significant effect of snowmelt timing

Soil Temperature

Soil water content outside of experimental cores

n.s.

n.s.

CO2 fluxes

0

20

40

60

80

100

120

140So

il Ca

rbon

Dio

xide

Em

issio

n (m

g CO

2-C

m-2

h-1) Low Elevation

High Elevation

**

*

*

*

*