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Soil Surface
   GHG Fluxes

Leaf and Soil Surface Carbon Dioxide Exchange
Tim Arkebauer, Brigid Amos, Hui Shen and Dave Scoby

Fig. 1. Carbon dioxide flux components at the plant and soil level.

Project Goals

  • To quantify the gas exchange response of single leaves to relevant controlling factors throughout the growing season.
  • To quantify the soil surface CO2 flux throughout the year and relate the surface fluxes to relevant controlling factors.

Project Description

Single leaf gas exchange properties (e.g., CO2 assimilation rate, stomatal conductance, internal CO2 concentration) are being measured at regular intervals throughout the growing season using portable gas exchange systems. Our sampling scheme and data analyses emphasize the quantification of responses of net CO2 assimilation rate and stomatal conductance to relevant biophysical controlling factors (e.g., light intensity, leaf temperature, vapor pressure deficit, ambient CO2 concentration).

Soil surface CO2 fluxes are being measured with a portable gas exchange system connected to a stainless steel sampling chamber. Permanent collars are placed at selected locations within each IMZ in each study site. Surface fluxes are determined for each study site at approximately one week intervals.

Fig. 2 Brigid Amos measuring the flux of CO2 at the soil surface using a Li-Cor LI-6200
Portable Gas Exchange System and a prototype stainless steel chamber.

Fig. 3 Brent Holmquist measuring a light response curve for a maize leaf using a
Li-Cor LI-6400 Portable Photosynthesis System.


Results have shown higher single leaf rates of photosynthesis and higher stomatal conductances in the irrigated sites compared to the dryland site. In maize, photosynthetic rates decrease late in the season as leaf nitrogen concentrations decline from 3 4.5% to 1.5 2.5% (Fig. 4). Also, light response curves indicate more light saturation for leaves from the dryland site than for leaves from the irrigated sites. Maize leaf dark respiration rates have been similar at all sites with rates close to 0.1 mg CO2 m-2 s-1 at 25oC and Q10s near 2.0.

Fig. 4 Seasonal variability of light response curves in maize taken on the uppermost fully expanded leaf.

Results from 2001 showed much larger soil surface CO2 fluxes at the irrigated sites than at the dryland site. Typical midseason values were 0.4 0.5 mg CO2 m-2 s-1 for the irrigated sites and only 0.2 0.3 mg CO2 m-2 s-1 at the dryland site. As we have observed previously, the two primary factors influencing these fluxes are soil temperature (Fig. 5) and soil moisture content. Whilst overall spatial variability is high, there is a strong dependence of surface flux on proximity to the crop row at all three sites (Fig. 6). We have continued our empirical modeling of the soil surface CO2 fluxes and presently consider the influence of soil temperature (at 0.1 m), soil moisture content and leaf area index on modeled fluxes (e.g. Fig. 7).

Fig. 5 Soil surface CO2 flux in maize as a function of 0.1 m soil temperature.

Fig. 6 Soil surface CO2 flux in maize at within row and between row positions
in 2001 for the irrigated and dryland sites.

Fig. 7 Modeled daily soil surface CO2 flux in maize. The empirical model used
is based on soil temperature, soil moisture content, and leaf area index.
All sites in 2001 were planted to maize.

We have begun scaling our single leaf gas exchange data to the canopy level using a simple radiative model that considers direct beam and diffuse radiation penetration into the canopy. Single leaf net CO2 assimilation responses to incident light (light response curves) form the basis of the scaling procedure. We are also using our measured LAI and leaf angle distributions. The resulting canopy net CO2 exchange is then combined with the soil surface CO2 flux to estimate the above-canopy net CO2 exchange (NEE). A sample of the canopy results is shown in Figure 8 along with the concurrent eddy covariance NEE measurement. The agreement is quite good and thus, increases our confidence in the measured parameters (i.e., single leaf gas exchange, LAI, soil surface CO2 flux) that comprise the core of the scaled-up estimates.

Fig. 8 Comparison of daytime net ecosystem exchange measured by eddy
covariance and estimated by scaling single leaf measurements
in maize on a sunny day in July (DOY 190).

Tim Arkebauer is a professor in the Department of Agronomy and Horticulture. In addition to leaf and soil gas exchange and carbon sequestration he has interests in plant growth and development, plant water relations and evapotranspiration.

Dave Scoby is a research technologist in the Department of Agronomy and Horticulture with over twenty years experience in field-oriented physiological research.

Hui Shen is a graduate student in the Department of Agronomy and Horticulture and is pursuing a doctoral degree.

Brigid Amos

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