Ames Iowa NACP Research

Objective: Develop infrastructure for long-term monitoring of spatial and temporal variation of CO₂ / H₂O vapor exchange and C storage within corn/soybean systems through implementation of a combination field-scale/boundary layer measurement site.

Site description

A long-term measurement site will be located and maintained in central Iowa, near the center of the Upper Midwest Corn Belt. Over 90% of the land in this area is devoted to privately owned production corn/soybean fields. The farming systems and associated tillage and nutrient management practices for corn/soybean production are typical of those throughout the Upper Midwest Corn Belt. The area is intensively cultivated in corn and soybeans. The topography is characterized by flat to gently rolling terrain. Portions of the landscape are poorly drained due to low relief and relatively young geologic development of the soil. There are closed depressions within the landscape that have accumulated soil from eroded upslope positions. Soils within depression areas (potholes) are characterized by poorly drained clay material while the upslope soils are better drained. The proposed site is located in Story County in the Walnut Creek Watershed, approximately 10 km south of Ames, Iowa. The watershed is transected by Walnut Creek and its accompanying gentle to moderate sloping landscapes of the uplands. Walnut Creek drains into the Des Moines River and is part of the Mississippi River watershed. The site is divided into two fields to allow for measurements over both crops within the same growing season. Clarion-Nicollet-Webster soils dominate the site and are representative of the fine-textured soils with moderate to high organic matter content typical of the region. A digital elevation model (DEM) and basic soil characterizations (e.g. texture, pH, C and nutrient content, microbial biomass, thermal and hydraulic properties) will be completed at a scale and level of detail commensurate with the flux measurements.

Tower Installation "

Three towers will be installed in two adjacent fields. Two 5 m-tall towers will be in the fields with sufficient fetch to be representative of the corn and soybean surface exchange processes. A 20 m tower will be located on the border of the fields to monitor exchange processes at a height where the influence of individual corn and soybean surfaces on the vertical profiles or fluxes becomes horizontally blended. The 20 m tower will be a permanent installation set in a concrete base with appropriate guy wires tied to concrete anchors. The 5 m towers will be semi-permanent and appropriately anchored to the soil. These 5 m towers will have the capability to be easily removed and redeployed twice each year to accommodate spring and fall management practices. In addition, the mobility of the semi-permanent towers will allow for annual redeployment within the corn and soybean fields at positions to account for different soil types and/or management treatments. Cellular telephone telemetry will enable real-time data access and daily instrument quality assurance checks.

Instrumentation

All three towers will be instrumented with an eddy covariance system (EC) to measure turbulent fluxes of sensible heat (H), latent heat (LE) and CO₂. The EC instrumentation consists of a three dimensional sonic anemometer (CSAT3, Campbell Scientific Inc., Logan, UT) and a fast response H₂O vapor and CO₂ density open path infrared gas analyzer (IRGA) (LI7500, LI-COR Inc., Lincoln, NE). The EC instruments will be mounted on the 5 m towers at 4 m above the ground level (AGL) and at the top of the 20 m tower. Ancillary instrumentation on the 5 m towers will include net radiation (R_n) (Q^*7.1 Radiation Energy Balance Systems, REBS, Seattle, WA), soil heat flux plates (G) (HFT-1.1, REBS, Seattle, WA), type T soil thermocouples, a high precision infrared radiometric temperature sensor (IRT, 15º field of view) (IRTS-P, Apogee Instruments, Inc., Logan, UT) and an air temperature (T_a) / relative humidity (RH) sensor (Vaisala HMP-35, Campbell Scientific Inc., Logan, UT). Two soil heat flux plates will be buried at 0.06 m below the soil in both soybean and corn fields, one within the plant row the second in the inter-row space. Soil temperature will be measured with type T thermocouples above each soil heat flux plate. Ancillary instrumentation on the 20 m tower will include two (north and south view) IRT sensors mounted near the top of the tower and 5 pairs of T_a/RH sensors and cup anemometers (R.M. Young, Traverse City, MI) mounted in a logarithmic configuration beginning at 2 m AGL. A USDA-NRCS Soil Climate Analysis Network (SCAN) site is located within 2 km of the experimental area, which provides some redundancy for the meteorological measurements. Below canopy profile measurements of H₂O/CO₂ vapor density in corn or soybeans will be made at 3-6 heights (crop dependent) with a closed path infrared gas analyzer (LI6262, LI-COR Inc., Lincoln, NE). The sampling frequency will be 10 Hz with CO₂ and H₂O vapor density values output as 30-min averages. These measurements allow evaluation of CO₂ and H₂O gas density dynamics in canopy air space of as a function of time and turbulent energy exchange above the canopy. The data from these sites will be coupled with data from intensive sampling of crop growth and development to relate net ecosystem exchange (NEE) of CO₂ with the crop biomass.

Measurements of net ecosystem exchange

Net ecosystem exchange of CO₂ will be measured with a combination of techniques including eddy covariance, soil chambers, and diffusion-gradient measurements. Simultaneous independent measurement of the total (eddy covariance) and soil (chamber and diffusion-gradient) CO₂ flux allows for the determination (by difference) of the plant contribution to the total CO₂ flux. Annual estimates of NEE from these techniques will be compared with direct measurement of soil and plant C pools on an annual time step.

a) Eddy covariance of total surface CO₂ flux. Eddy covariance measurements represent the vertical flux of a transported scalar (CO₂) at a fixed point above a surface and represent the net exchange of the scalar between the surface and the atmospheric boundary layer above. This is accomplished by correlating the fluctuations of the scalar of interest with fluctuations of vertical wind speed. The 5 m towers will represent NEE for corn and soybeans while the 20 m tower will represent a more regional estimate of NEE comprised of a mixed footprint of corn and soybeans. These continuous, long-term measurements will provide unique and valuable data set that will aid in improving understanding of the complex processes for CO₂/H₂O exchange at the field and boundary layer scale for the Upper Midwest.

b) Chamber measurements of soil CO₂ flux. Soil chamber measurements provide a measure of CO₂ flux from microbial and plant root respiration, which is a component of the total CO₂ flux measured by the eddy covariance systems. Soil CO₂ flux will be measured using automated chambers similar to the design of Parkin and Kaspar (2003). The chambers are 60 cm x 60 cm x 15 cm stainless steel open ended boxes pressed into the soil approximately 5 cm. The top of each steel box is fitted with a wooden framework that supports a sliding cover. The covers are supported by casters riding on steel tracks attached to the sides of the chambers. Linear actuators driven by gear motors attached to the frames serve to open and close the covers at hourly intervals. Soil respiration is measured every hour by sliding the cover over the chamber top to allow CO₂ to accumulate in the chamber headspace. Carbon dioxide is measured during a 6-min period by pumping the chamber gas through an infrared gas analyzer (LI-820, LI-COR Inc., Lincoln, NE). After 6 min the chambers are opened. This cycle is repeated for a total of five measurements per hr. Respiration rates are calculated from the CO₂ flux data using the algorithm of Hutchinson and Mosier (1981). Each chamber will be instrumented with thermocouples to measure air temperature, surface soil temperature, and soil temperature at 5 cm. Frequency domain probes will be used to measure soil water content (0-6 cm), and a small fan is placed in each chamber to recirculate air during the respiration measurements. Three systems will be deployed over a field to measure CO₂ fluxes.

c) Diffusion-gradient measurement of soil CO₂ flux. An ARS headquarters-funded postdoctoral Research Associate will deploy and evaluate an innovative approach to the diffusion-gradient technique of trace gas emission measurement. Profiles of soil air CO₂ concentrations will be sampled simultaneously in the corn and soybean fields by placing 15-m loops of 15.8 mm-diameter silicone tubing at the soil surface and at 5 and 10 cm-depths. Small, solid-state CO₂ sensors (GMT222, Vaisala Inc., Woburn, MA) will be installed within each of the loops with a small pump to circulate the air and measure CO₂ concentration at 30-min intervals. This approach couples the silicone tubing method for sampling soil air (Jacinthe and Dick, 1996) with evolving sensor technology for measurement of CO₂. Distinct advantages of this approach are that CO₂ concentrations in the soil atmosphere are measured from a large area without withdrawing air samples to an analyzer, which disturbs the pressure fields in the soil. Soil water content (CS615, Campbell Scientific, Inc., Logan, UT), temperature (type T thermocouples), and atmospheric pressure fluctuations (model 264 differential pressure transducer, Setra Systems Inc., Boxborough, MA) will also be measured in the 0-10 cm soil layer. Carbon dioxide fluxes at the soil surface will be calculated from the measured CO₂ concentration gradients and diffusivity values measured independently using the in-situ method of MacIntyre and Philip (1964). Soil gas diffusivity will be measured with 6 chambers located adjacent to the profile measurements in each field at intervals dictated by changes in soil water content and surface conditions. Carbon dioxide fluxes will be calculated using Fick’s Law with the measured CO₂ gradients and the mean soil gas diffusivities. Fluxes measured with the diffusion-gradient technique will be compared with chamber and eddy covariance measurements of CO₂ fluxes to assess the relative accuracy and feasibility of this new approach.

Integration of spatial and temporal scales "

Trace gas flux measurements represent points within a field and, as part of the scaling process from a single soil map unit into a field scale and ultimately to a regional scale, understanding of the spatial and temporal variability is required. To address this aspect we propose to evaluate a system that will allow the determination of the spatial variation of CO₂ fluxes from soil across a range of landscape positions. Two CO₂ sampling chambers would be mounted onto a track system that would be automatically controlled to sample the CO₂ evolution from soil at a given spatial interval. Samples could be collected every 2-5 meters along the track at a time interval of 15-30min for each sample. This unit would be deployed to traverse along a track of 30-50 m within a field to collect a series of spatial data throughout a day and over several days. The units would be mounted on a track system that would allow for sampling to be completed regardless of the soil water conditions at the surface. The unit would be constructed to provide an airtight seal when the chamber is lowered onto the soil surface. Ancillary data to be collected during these observations would be air surface temperature along with surface soil water content, precipitation, wind speed, and wind direction. The data for each transect would be evaluated for spatial variability using spatial statistics and then evaluate the stability of these relationships over time intervals. These analyses would provide an understanding of whether the patterns of CO₂ flux are random or systematic across a landscape and will aid in scaling approaches to extend point measurements to larger scales.