Saturday, 15 July 2006

Glomalin and Water-Stable Aggregation Measurements in Northern Plains Rangeland Management Studies.

Kristine A. Nichols, USDA-ARS, Northern Great Plains Research Laboratory, P.O. Box 459, Mandan, ND 58554, John R. Hendrickson, USDA-ARS, Northern Great Plains Research Laboratory, 1701 10th Avenue SW, P.O. Box 459, Mandan, ND 58554, and James E. Millar, USDA-NRCS, 1386 Lincoln Ave SW, Huron, SD 57350.

In the Northern Plains region of the U.S., rangeland covers 43% of the landscape or 180 million acres of the total ca. 526 million acres of non-federal rangeland in the U.S. The soils in over 60% of this region are rated with either high wind erosion potential or fragile soils with tolerable soil-loss levels of less than 3 tons per acre annually. Management of livestock and plant species composition may impact soil quality. Glomalin, a glycoproteinaceous substance produced by arbuscular mycorrhizal fungi, is a soil quality indicator that is sensitive to management allowing changes to be measured on a short time scale and is a major component of soil organic carbon. Arbuscular mycorrhizal fungi receive carbon, in the form of simple sugars, directly from the plant. Therefore, the hyphal growth and glomalin production are influenced by the dynamics of plant growth. Two studies are highlighted here to compared glomalin and water-stable aggregation (WSA) in rangeland soils. The first study conducted near Platte, SD had three treatments: native grass with rotational grazing, tame grass with heavy grazing, and tame grass with rotational grazing. The second study (Rangeland Restoration Study) at the Northern Great Plains Research Laboratory (NGPRL) near Mandan, ND compared the ability of two treatments (chemical application and burning) to alter plant species composition and promote native plant species with a control on five different vegetative communities. Community type was determined by a visual estimate of dominate vegetation in 2002. Communities were dominated by 1) native grasses; 2) Kentucky bluegrass (Poa pratensis); 3) smooth bromegrass (Bromus inermus); 4) Kentucky bluegrass and smooth bromegrass; and 5) Kentucky bluegrass, smooth bromegrass and other introduced species. Burning treatments were conducted in all communities in late April, 2003 and repeated in the native communities in late April 2004. Chemical treatments were applied using 6 oz ai/acre of Plateua® (imazapic) when smooth brome was 10 to 16 cm tall in the spring of 2003 and 2004. Results from the study at Platte, SD showed glomalin concentrations and WSA were highest in the native grass pasture with rotational grazing treatment. The tame grass pasture with heavy grazing had the lowest WSA, while the tame grass pasture with rotational grazing had the lowest glomalin values. Preliminary data in the Rangeland Restoration study showed that the glomalin concentrations were higher in the chemical and burn treatments than the control and highest in the burn treatment with native plant species composition. For all three treatments – burn, chemical, and control, glomalin concentrations were highest when the plant species composition was dominated by natives. The results from these two studies indicate that grazing management strategies and plant species composition impact glomalin concentrations and WSA. These measurements will be expanded to other rangeland studies in the Northern Plains region including three long-term agroecosystem experiments at NGPRL related to different grazing management - two native vegetation moderately grazed pasture and heavily grazed pasture established in 1916, and one seeded crested wheatgrass [Agropyron desertorum (Fisch. ex. Link) Schult.] pasture established in 1932 and a study near Streeter, ND with three grazing management treatments – idle (no grazing), rotational, and season-long.

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