ARS | Publication request: SOIL AGGREGATION AND GLOMALIN UNDER PASTURES IN THE SOUTHERN PIEDMONT USA

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 16, 1999
Publication Date: N/A

Interpretive Summary: Soil aggregation is important in maintaining soil surface integrity and allowing water to infiltrate, rather than runoff and cause erosion. Soil productivity declines with erosion. Soils of the southeastern USA are particularly susceptible to erosion because of the frequency of high-intensity thunderstorms that can wash away exposed soil by overland flow of water on gently to steeply sloping land. Covering soil with permanent vegetation such as grass has been shown to reduce soil erosion. Despite potential benefits of grass establishment on soil organic matter and aggregation, cattle grazing off pastures raises concerns of increased erosion because of the potential for reducing vegetative cover by soil trampling. The long-term effects of grazing versus haying on soil aggregation have not been well documented. We compared water-stable aggregation and glomalin under grazed pastures to hayland, conservation-tillage cropland, and forestland. Glomalin is a specific compound produced by soil fungi that acts as a glue to bind soil particles together into aggregates. Our data indicate that grazing of pastures in the Southern Piedmont USA has equally effective impacts on soil aggregation and glomalin compared with other land conservation strategies, such as no-tillage cropping and forestry.

Technical Abstract: We determined dry- and water-stable aggregate distribution, stability of macroaggregates and mean-weight diameter, and total and immuno-reactive glomalin under pasture management systems in the Southern Piedmont USA. Water-stable macroaggregation, mean-weight diameter, and their stabilities were (1) similar between 24-year-old conservation-tillage cropping and 20-year-old tall fescue-common bermudagrass pasture, (2) similar between 15- to 19-year-old grazed and hayed hybrid bermudagrass, (3) 7 to 14% greater in 30-year-old than in 10-year-old grazed tall fescue and hybrid bermudagrass pastures, (4) similar among long-term grazing, haying, and forest and (5) 5 to 11% lower under cattle grazing than under monthly hayed or unharvested management during the first 4 years following conversion of cultivated cropland to pastureland. Total glomalin of the 1.0-4.75-mm aggregate class was highly related to whole-soil organic C content, but neither of these properties was well related with macroaggregation, mean-weight diameter, and their stabilities. We conclude, overall, that grazing of pastures in the Southern Piedmont USA has no detrimental effects on soil aggregate distribution and stability and is comparable with other land conservation strategies.