Amanda Brennan1, Elise Pendall1, Dave Risk2, Yolima Carrillo3

1 University of Wyoming, Department of Botany and Program in Ecology, Laramie, WY 82071, contact: abrenna3@uwyo.edu 2 Dept. of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada; 3 University of Sydney, Australia

AcknowledgementsWe thank Dan LeCain and Jack Morgan for technical support. This project was supported by USDA-CSREES Soil Processes Program , DOE and NSF, with travel support from UW Program in Ecology and Department of Botany.

B51B-0547

Continuous soil respiration at the Prairie Heating and CO2 Enrichment site using forced diffusion chambers

Introduction

Variation in soil respiration is regulated by plant and microbial

responses to environmental changes, such as soil moisture. Our first

objective was to determine the reliability of forced diffusion chambers

housing non-dispersive infrared CO2 sensors for continuous fluxes in

semi-arid grasslands (Risk et al. 2011). We also compared the

response of soil respiration to environmental drivers under different

climatic treatments. We hypothesized that elevated CO2 would

stimulate both total soil and microbial respiration, due to increased

belowground allocation, whereas warming would reduce soil and

microbial respiration as a result of soil drying. We further hypothesized

that total soil respiration would be more sensitive to soil moisture,

whereas microbial respiration in the plant-free plots would be more

sensitive to changes in soil temperature.

Study site• Prairie Heating and CO2 Enrichment (PHACE) is located in northern mixed-grass prairie near Cheyenne, WY, USA (http://www.ars.usda.gov/npa/rrru/phace) • Vegetation dominated by cool season (C3) grasses with an important warm season (C4) species• Mean annual precipitation is 370 mm; mean temperature is 7°C• PHACE rings are 3-m diameter, elevated [CO2] is raised to 600 ppm by direct injection in daytime during the growth season, and temperature is warmed to +1.5/+3°C day/night year-round with ceramic heaters. [CO2] treatment started in 2006, warming in 2007.

Observations

Conclusions and Future Work•Prairie soil respiration was not consistently affected by climate change treatments, but was largely influenced by environmental drivers.•Treatment effects were stronger on microbial respiration with less effects of environmental drivers.•The drift in CO2 concentration over time needs to be addressed before flux-driver relationships can be better understood.

Infra-red Heaters

Free Air CO2 Enrichment Ring

PHACE Treatments

ct = ambient [CO2] and temperature

cT = ambient [CO2] and elevated temperature

Ct = elevated [CO2] and ambient temperature.

Soil Efflux and Environmental Controls

CO2 ConcentrationsContinuous Timeseries- Forced Diffusion•Soil Emissions pass into a dynamic chamber along a slightly restrictive membrane with known diffusivity; background air is monitored in a chamber sealed off from the soil•Vaisala GMP-222 IRGAs inside the chambers continuously measure [CO2]

Plant-free plots had glyphosate added in May 2008 and a root barrier to 25 cm is maintained by manual removal.

Daily Averages for Background Air [CO2]

[CO

2]

pp

m

400

500

600

700

800

900 CtcTct

Daily averages for plant-free chamber [CO2]

[CO

2]

pp

m

400

500

600

700

800

900

Daily averages for mixed grass prairie chamber [CO2]

Day of Year

0 100 200 300

[CO

2]

pp

m

400

500

600

700

800

900

ReferencesRisk D, Nickerson N, Creelman C, McArthur G, Owens J (2011). Forced Diffusion soil flux: A new technique for continuous monitoring of soil gas efflux. Agricultural and Forest Meteorology 151: 12, 1622-1631.

Mixed grass prairie soil respiration

Soil

Re

sp

iratio

n (

mo

l m-2

s-1)

2

4

6

8

10

12

14

Plant-free soil respiration

Soil

Re

sp

iratio

n (

mo

l m-2

s-1)

2

4

6

8

10

12

14

CtcTct

Soil Temperature at 3 cm depth

Tem

pera

ture

o C

-10

0

10

20

30

Volumetric Water Content at 0-10 cm

Day of Year

0 100 200 300

VW

C (

%)

4

6

8

10

12

14

16

18

20

22

Risk et al. 2011

Mixed-grass chamber

Plant-free chamber

Air Chamber