Global Change and the Southern Plains Mixed Prairie

It=s no secret to those involved in agriculture that the weather has a tremendous effect on our activities. Scientists who study past climates have found that large climate changes have occurred many times in the history of the earth. The advance and retreat of glaciers provide ample evidence of changes in the relatively near past. Recently, however, concerns have arisen that human activities may be having an appreciable effect on many global processes including climate. Since the last ice age there have been periods when the climate was cooler and wetter and periods when it was warmer and drier. Many climatologists believe that we have already begun to impact some important aspects of climate such as the temperature ranges near the earth=s surface, length of growing seasons, amounts and patterns of precipitation, and perhaps the frequency and severity of storms.

Greenhouse Gases

Although the atmosphere may seem to be too large for us to have a significant impact on, concentrations of some important atmospheric gases such as carbon dioxide, methane and nitrous oxide have increased significantly during the last 200 years (Table 1 below), especially since the rapid increases of population and fossil energy use which accompanied the industrial revolution. It has been estimated that these atmospheric gasses have increased at rates ranging from .25 to 0.6 percent annually. Carbon dioxide concentrations have increased by about one fourth during this time period. Even at current rates of emission, the concentration will double over the next century.

Since plants depend on carbon from atmospheric CO₂ during photosynthesis, it would seem that increasing the CO₂ levels (currently 350 parts per million) would be beneficial. For example, increased CO₂ has been shown to decrease the amount of soil water that many plants use. However, CO₂ and the other gases which are increasing may have other effects which not be entirely beneficial. All of the >greenhouse gasses= mentioned above absorb infrared radiation (heat) given off by the earth=s surface which would otherwise escape into space. The resultant increased heating of the atmosphere could cause changes in the earth=s weather patterns.

Implications of Climate Change

The implications of these climate changes has heightened the interest of policy makers and researchers around the world. An International Panel on Climate Change (IPCC) are working world-wide to predict future climate conditions. One vital piece of this puzzle is more information on the global carbon cycle. Carbon is constantly being moved through organisms, minerals, soils, oceans, and the atmosphere. The first step in understanding the process is to quantify all of the carbon >sources= and >sinks.=

Carbon Sinks

Current best estimates, using all the known sources and sinks for carbon, indicate that CO₂ should be increasing even faster than 1.8 ppmv annually. This indicates that there may be important sinks for carbon which are unknown or have not been adequately quantified. Increases in atmospheric carbon comes primarily from combustion of fossil fuels, farmland tillage, forest and grassland burning, volcanic activity, and the respiration of plants and animals. Only about half of the carbon emitted is accounted for by changes in the atmospheric carbon and by growth in carbon sinks such as oceans and plants which absorb CO₂ and convert it into the carbon in foliage and roots. The remaining >missing= carbon (about 3 gigatons/year) is currently being actively pursued. Earlier investigations of the global CO₂ cycle indicates that the missing sink may be the rangelands in the western part of North America.

Carbon Flux on Rangelands

We hypothesize that healthy rangelands may be responsible for at least a part of this activity. Good condition rangelands in many areas have been shown to maintain relatively high levels of soil organic matter and it has been shown that farmed soils converted back to grasslands can result in rebuilding of soil organic matter levels. Major rangeland ecosystems such as the prairies and grasslands of the Great Plains, the deserts of the southwest, or the shrub-steppe of the Great Basin might be key carbon sinks.

Woodward is one of 11 ARS research centers across the U.S. currently making direct measurements of the carbon flux through the atmosphere. The SPER is representative of the southern mixed-grass prairie which covers about 70 million acres. Figure 1 shows carbon accumulation in three typical sites in this area in 1995.

The first site is a brush controlled pasture in good condition, the second is a sagebrush dominated pasture, and the third is a wheat field. These studies show marked contrasts in the amount and timing of CO₂ flux that occurred during the growing season. Both the sagebrush dominated and sagebrush controlled rangeland accumulated about the same amount of carbon, but the pattern of accumulation varies depending upon the mix of brush, cool-season and warm-season plants. For example the sagebrush dominated site began accumulating carbon earlier in the year due to the presence of the sagebrush and cool-season grasses such as Texas bluegrass and the annual bromes (Figure 2). The pasture which had no sagebrush had fewer cool-season plants and did not became a net carbon fixer until late in April. As one would expect from the large amount of production on a wheat field, it fixed carbon at a very high rate until the grain ripened at the end of May.

These studies are ongoing and have shown similar patterns in other years, but the quantity of carbon fixed can differ depending on growing conditions. In the near-normal precipitation year of 1995, the two pastures fixed about 350g of CO₂ per meter square during the growing season. They fixed about 800g in 1997 which had above normal precipitation. These observations will be used to determine carbon flux and >sequestration= in rangelands. If the yearly net flux is positive, it indicates that more carbon is being fixed by the mixed-prairie than is being lost. If so, then rangelands may account for a part of the global carbon sinks which are currently missing.