soil co 2 efflux from a subalpine catchment

Soil CO2 Efflux from a Subalpine CatchmentDiego A. Riveros-Iregui1, Brian L. McGlynn1, Vincent J. Pacific1, Howard E. Epstein2, Daniel L. Welsch, Kelsey Jencso1

1.diego.riverosiregui@montana.edu; Watershed Hydrology Lab, Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 597172.Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904. 3. Department of Geography, Frostburg State University, Frostburg, MD 21532

• Soil CO2 efflux is the single largest respiratory flux from forested

ecosystems. Understanding its spatial and temporal dynamics will

provide insight into C exchange at the ecosystem level.

• Evaluating and predicting soil respiration response to changing

hydrologic and biophysical conditions is currently constrained by

understanding of first-order controls and the methods used to

measure, model, and interpret soil respiration.

• We measured soil CO2 concentration, soil CO2 flux, soil moisture,

soil temperature, groundwater dynamics, and precipitation at 20-

minute intervals throughout two growing seasons at 4 sites and at

sub-weekly intervals at 62 sites covering the range of topographic

position, slope, aspect, and land cover.

• Our goal is to quantify watershed-scale heterogeneity in soil CO2

concentrations and surface efflux and gain understanding of the

relative contribution of biophysical controls on soil respiration.

Acknowledgments: This work is supported by the NSF Integrated Carbon Cycle Research Program, a Student Research Grant from the Montana Water Center and a MSU College of Graduate Studies Fellowship. We appreciate extensive logistic collaboration from the U.S. Forest Service.

Figure 2: Two growing season averages of soil respiration and soil water content at 34 of 62 total sites. These comparisons highlight the relative controls of water content on controlling soil respiration across the watershed. For example, note the high variability of water content in riparian areas, whereas high uplands remain dry during most of growing season. Note how increasing upslope contributing area (i.e., moving in the downhill direction) controls total soil respiration in high, dry uplands. Error bars are one S.D. Refer to colors in Figure 1 for location.

Figure 1. (A) Current instrumentation at the TCEF includes five real-time monitoring stations for soil moisture and temperature, >60 groundwater wells and piezometers, two eddy covariance towers, and >120 gas wells. We seek to understand the effect of watershed-scale heterogeneity on CO2 distribution and flux across

environmental gradients.

• Long-term monitoring of soil respiration, soil moisture, and soil temperature at 62 sites across the watershed.

• Continuous measurements of soil respiration in riparian and upland settings.

• Land cover analysis from Airborne Laser Swath Mapping (ALSM) sub-meter topographic and vegetation data.

• These methods are targeted to investigate the effect of heterogeneity of biophysical controls on soil CO2 efflux at the

watershed scale.

INTRODUCTION

METHODS

SITE

1. Understanding linkages between the biophysical controls

of soil respiration and hydrology from the point, to the plot,

to the watershed scale is critical to understanding

dynamics of net ecosystem CO2 exchange.

2. High elevation mountain landscapes play an important role

in the North American carbon cycle, yet the number of

studies of coupled C and water cycles in the Northern

Rocky Mountains is limited.

3. The relative sensitivity of soil respiration to biophysical

variables is poorly understood. However, catchment level

studies allow comparison of magnitude and variability of

biophysical controls across environmental gradients.

SIGNIFICANCE

Sites

T1

W4

T1

W3

T1

W2

T1

W1

T1

E1

T1

E2

T1

E3

T1

E4

T2

W4

T2

W3

T2

W2

T2

W1

T2

E1

T2

E2

T2

E3

T2

E4

SW

1S

W2

SW

3S

W4

SW

5S

W6

NW

Co

n6

NW

Co

n5

NW

Co

n4

NW

Co

n3

NW

Co

n2

NW

Co

n1

NW

Di6

NW

Di5

NW

Di3

NW

Di2

NW

Di1C

O2

Eff

lux

(g m

-2h

r-1)

0.0

0.5

1.0

1.5

2.0

Wa

ter

Co

nte

nt

(m3 m

-3)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Transect 1 Transect 2

Increase in upslope contributing area, (downhill)

A Stringer Creek(550 ha)

2090 m

2425 mElevation

LEGEND

Soil moisture & temperature

Biometry plots

Gas wells and respiration plots

Eddy covariance systems

Flume

SOIL RESPIRATION AND SOIL MOISTURE

MODELING SOIL CO2

Tenderfoot CreekExperimental Forest(TCEF)

SEASONALITY IN THE SOIL [CO2]- SOIL TEMP RELATIONSHIP

,,COCO 22

SHA TkPARkz

nDzt

Figure 3. (A) Relationship of soil [CO2] and soil temperature at 20-min

intervals at a riparian site with pronounced summer dry-down. (B) Highlighted evolution of hysteresis throughout the summer. Soil water content values are given in parentheses for each highlighted day. (From Riveros-Iregui et al. in review)

SOIL MOISTUREtop_ind

Value

High : 21.715910

Low : 1.928115

Figure 4: Triangle Topographic Index for Stringer Creek Watershed (Seibert and McGlynn, WRR 2007) based on a 3-m DEM. This is an estimate of soil wetness potential across the catchment.

tan

a ln.. IT

angle slope local :

area contr. upslope :a

SPATIAL AND TEMPORAL HETEROGENEITY

Contours are every 10 m

Stringer Creek Watershed(550 ha)