Managing Rangelands to Sequester Carbon

Rangelands make up the most extensive terrestrial ecosystem on the planet. Over eight hundred million acres of rangeland are located in 19 western states, more than twice the area of cultivated cropland in the U.S. Unlike annual crops, the vegetation of most rangelands is predominantly perennial and often consists of a mixture of cool-season and warm-season species which lengthens the amount of time during the year that plants are actively growing and taking up atmospheric CO2. Perennial grasses, a major component of most rangeland plant communities, enhance C storage into the soil because grasses have a much higher portion of total plant biomass in below ground tissues than do trees, most shrubs, or annual crops. The magnitude of land area involved, plus attributes of rangeland vegetation that promote high levels of C storage, make rangelands a potentially important sink for atmospheric C.

Until recently, little consideration has been given to the possibility that rangelands can be managed to enhance C storage. Potential improvements in management to promote higher vegetation productivity and higher levels of soil C include fertilization, seeding improved plant varieties, and manipulating the timing, duration and intensity of grazing.

Potential
Rangelands are located predominately in arid or semi-arid regions, where both the production of vegetation and its response to fertilizer are limited by water. In these ecosystems, intensive management practices such as fertilization, irrigation, or seeding improved plant varieties usually are economically impractical. Improvements in management to promote higher vegetation productivity and increased soil C will therefore generally be restricted to modifying stocking rates and the timing of livestock grazing. For rangelands in good condition with no serious ecological or management problems, we can assume that there is little potential for further C storage because the C content of these soils is relatively stable and already at or near maximum expected concentrations. In these healthy rangelands, the goal is to preserve existing C stocks by maintaining or establishing optimal grazing strategies to avert C losses. Most rangelands, however, cannot be classified as in good condition. Because of excessive grazing and poor management in the past, approximately two-thirds of western rangelands are classified as in fair to poor condition. These degraded rangelands have lost much of their plant diversity, productivity, and native soil C, and would benefit from improved management. However, the rate at which C can be added to the soil by improved grazing management will be relatively low because of the low natural productivity of arid and semi-arid ecosystems. Improvements in grazing management usually will induce gradual rather than rapid improvements in plant species composition and production, so the impact on soil C may not be measurable for years.

Strategies
Research to develop grazing management strategies that optimize C storage in rangelands is in its infancy, and the current literature suggests no clear general relationship between grazing and C sequestration. Some studies have reported no effect of grazing on soil C, while others have reported increases or decreases in soil C as a result of grazing. Several generalizations concerning the impact of grazing on ecosystem C can be made, however:

(1) Grazing management that leads to a shift in plant species composition causes changes in soil C stocks.

This is because plant roots are the major source of C to the soil, and plant species differ in the distribution, mass and turnover of their root systems. Change in plant community composition as the result of grazing management is a major reason for the lack of a clear relationship between grazing and soil C sequestration.

Heavy grazing without adequate rest periods to allow regrowth weakens plants and decreases biomass production, which decreases inputs of C into the soil from the roots. With overgrazing, especially in conjunction with drought, plants begin to die and the removal of plant cover exposes the soil to loss of organic matter C by wind and water erosion.

Studies on a mixed-grass prairie near Cheyenne, Wyoming showed that C stocks in the top 12 inches of the soil were lower in 40-year-old livestock enclosures compared to adjacent pastures that had been lightly grazed. Excluding grazing by livestock tied up a large amount of C in excessive above ground plant litter, and caused an increase in annual forbs and grasses which lack dense fibrous rooting systems conducive to soil organic matter formation and accumulation.

Studies on a mixed-grass prairie near Cheyenne, Wyoming showed that grazing at light-to-moderate stocking rates resulted in a stable plant community dominated by desirable forage grasses and maximum plant biodiversity. Light-to-moderate grazing also stimulated early season photosynthesis and earlier spring green-up, and enhanced C and nutrient cycling among plants, animals and the soil, all of which contribute to building C stocks in the soil.

Most rangeland ecosystems evolved under grazing by large herbivores, and for these ecosystems, grazing appears to be a necessary component to overall health of the ecosystem. Developing livestock grazing strategies that optimize the stability and diversity of the plant community will also optimize soil C sequestration. The challenges in developing a best management grazing strategy for a given rangeland ecosystem are (1) determining the optimal length and timing of rest "heavy" for a given ecosystem, and (3) fine-tuning the stocking rate and the timing and duration of grazing to take into account annual fluctuations in precipitation and temperature.

We can expect the rate of C sequestration to be low as the result of improving grazing management on arid and semi-arid rangelands. However, because of the vast land areas occupied by rangelands, very small changes in the amounts of C lost or gained in rangeland soils become extremely important.

Jean Reeder, Soil Scientist,
USDA-ARS,
Fort Collins, CO