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Processes Affecting Carbon Fluxes of Grassland Ecosystems Under Elevated CO₂

Clenton Ownnsby
Agronomy Department
Kansas State University
Manhattan, KS
USA      
http://spuds.agron.ksu.edu/co2.htm

This project seeks to extend our knowledge of ecosystem-level carbon and water vapor fluxes. A major goal of the proposed study is to determine the extent to which C₄-dominated grasslands will sequester carbon and act as a buffer against future increases in atmospheric CO₂ through a better understanding of the carbon and nitrogen interactions with soil microbes. Our approach centers on water use efficiency changes associated with elevated CO₂. We continue to monitor above- and belowground biomass production and monitor net carbon exchange and evapotranspiration at the chamber level with natural precipitation in the original nine plots and by manipulating water availability in four experimental chambers. Grazing impact cannot be assessed in the current study, but, because of the importance of herbivory in natural systems, we will determine the effect of elevated CO₂ on regrowth biomass accumulation, water relations, and physiology following top removal.

The experimental site for the tallgrass prairie research is located in pristine tallgrass prairie north of and adjacent to the Kansas State University campus. Vegetation on the site was a mixture of C₃ and C₄ species and was dominated by big bluestem (Andropogon gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash]. Subdominants included Kentucky bluegrass (Poa pratensis L.), sideoats grama [Bouteloua curtipendula (Michx.) Torr.], and tall dropseed [Sporobolus asper var. asper (Michx.) Kunth]. Members of the sedge family made up 5-10% of the composition. Principal forbs included ironweed [Vernonia baldwinii subsp. interior (Small) Faust], western ragweed (Ambrosia psilostachya DC.), Louisiana sagewort (Artemesia ludoviciana Nutt.), and manyflower scurfpea [Psoralea tenuiflora var. floribunda (Nutt.) Rydb.]. Average peak biomass occurs in early August at 425 g m-2 of which 35 g m_-2 is from forbs. Soils in the area are transitional from Ustolls to Udolls (Tully series: fine, mixed, mesic, montmorillonitic, Pachic Argiustolls). Slope on the area is 5%. Fire has been infrequent, occurring two to three times in 10 years. Past history has included primarily winter grazing by cow-calf pairs. The 30-year average annual precipitation is 84 cm, with 52 cm occurring during the growing season.

We continue fumigation with double ambient CO₂ on the same plots that have received that treatment for a 5-year period and compare those plots to ones with ambient CO₂, with and without a chamber. Use of these plots will be of particular importance in assessing the long-term effects that elevated CO₂ has on ecosystem function and carbon storage in a grassland ecosystem. We have four additional chambers which have been used to characterize canopy-level evapotranspiration and net carbon exchange associated with CO₂ enrichment and chamber effect at the plant canopy level. We will manipulate water availability in those chambers to better characterize biomass production and physiologic processes. Data collected in previous studies as well as data collected in the ongoing study will be used in model development and paramaterization to evaluate potential landscape and mesoscale impacts of elevated CO₂ on carbon source/sink relationships and ET.

The fumigation chambers are a scaled-up version of a design described by Heagle et al. (1979) as modified by the USDA-ARS Air Quality Field Laboratory at Raliegh, NC. The chambers are 4.5 m in diameter by 3.25 m in height with a cone-top baffle that restricted the top opening to 1.5 m. The baffle adds 0.75 m to the height of the open-top chamber for a total height of 4 m. The cone-top baffle increases stability of the CO₂ concentration and reduces CO₂ required to maintain the desired concentration. Chamber air is changed approximately two times per minute using a 91.4-cm dia, 6-bladed fan powered by a 1.5 hp, 3-phase 220 v electric motor. Air delivery inside the chamber is via a parabolic plenum consisting of a 75-cm dia polyethylene tube with 3.2-cm dia holes on 18-cm centers on the inner half the tube. Chamber air flow rate is sensed using a differential pressure transducer and the Bernoulli equation (Ham et al. 1993).

The structural framework is covered by 6 mil, UV-resistant polyethylene film. The cone-top baffle placed atop each chamber reduces the opening to 1.5 m, thereby restricting the precipitation that enters the chamber. A gutter system collects the precipitation into a reservoir located on the ground where it is pumped to a sprinkler located in the center of the chamber which is adjusted to cover the diameter of the chamber. Aluminum edging is placed around the upslope bottom edge of the chamber to prevent run-off from entering the chamber. On the four chambers with water availability manipulation, a barrier will be placed to a depth of 1.5 m to eliminate water exchange from surrounding soil.

Control of CO₂ concentration in open-top chambers is by computer-controlled mass flow controllers. During each scan of the 15 plots CO₂ concentration is determined and for CO₂-enriched plots the CO₂ concentration for the last ambient CO₂ plot is doubled and compared with the current value for an enriched chamber. If they are not equal, a new setpoint is calculated and sent to the mass flow controller. Data are taken hourly for temperature, PAR, dewpoint temperature, and CO₂ concentration.