Roger Gates: Restoring carbon for soil health
April 8, 2011
There has been little disagreement over the value of perennial vegetation for soil conservation. There has, however, been uncertainty about the most beneficial management of the vegetation. Research from the USDA-ARS Station near Watkinsville, GA, was recently reported that evaluated the contribution of grazing management to soil conservation of grassland. While experimental findings are not entirely applicable to native rangelands of the Northern Great Plains, this research suggests some principles which may apply.
Land that had been farmed for several decades using conventional tillage was planted to a perennial grass, Coastal bermudagrass. After establishment, bermudagrass was overseeded with Georgia 5 tall fescue. Three management alternatives were examined: grazing, haying, and non-use were compared during a twelve year experiment. Two levels of grazing were evaluated, either heavy (2.4 yearling Angus steers per acre; 1340 pounds per acre residual dry matter) or light (3.5 yearling Angus steers per acre; 2,680 pounds per acre residual dry matter) utilization. Season long stocking extended as long as 10 months in some years. Soil characteristics measured included bulk density, soil carbon and soil nitrogen. Soil parameters were examined at increasing depths and sampling was repeated periodically during the 12-year trial. Interval sampling allowed researchers to estimate the rate at which change occurred.
Ironically, less than 200 miles southwest of this research location is an erosional feature which documents the disturbance which can result from removal of perennial cover. Nineteenth century development in southwest Georgia precipitated rapid conversion of pine forest to farmland. Cultivation, associated primarily with cotton production, exposed the soil surface and led to massive overland flow of water during heavy rains. Soil erosion, that began with rills and incidental channels, soon led to gullies and increasingly concentrated erosive flow. Ultimately, ravines 150-feet deep were formed creating a landscape reminiscent of the Badlands. Contrasting colors of exposed soil layers have created spectacular scenery, leading the state of Georgia to identify the area as “Providence Canyon” and establish a state park. Creation of this geologic “wonder,” alternatively referred to as the “Little Grand Canyon,” in less than a century serves as a warning about pervasive risks of careless land management.
Research results indicated that increasing soil carbon was closely associated with decreasing bulk density. Soil carbon remained essentially constant for the haying treatment. For the non-use treatment, soil carbon increased slightly. Both heavy and light grazing utilization resulted in conspicuous increases in soil carbon. No difference in soil carbon change could be attributed to the degree of grazing use. Increases in soil carbon resulting from grazing were most rapid early in the study and changed more gradually after several years. The contribution of increasing levels of soil carbon (and associated organic matter) to soil productivity is well established, benefitting water infiltration (enhancing the moisture environment of growing plants) and enhancing availability of nutrients for plant growth.
It is important to recall that this research was initiated on depleted cropland. Perennial forage plants were established in an effort to improve soil conditions. In contrast to these depleted southern cropland soils, prairie soils of the Northern Great Plains generally have not been depleted and are not devoid of carbon. There has been great interest in the impact of grazing on soil carbon levels. This research suggests that, when soil carbon is low, establishment and grazing of perennial forage plants can increase soil carbon levels, perhaps quite dramatically for a few years. The rate of soil carbon increase clearly slowed over the length of the 12-year experiment. It is likely that the rate at which soil carbon increases will be moderated by initial levels and gradually slow as some threshold carbon level is approached.