Soil aggregation and glomalin under pastures in the Southern Piedmont USA

Soil aggregation is important for maintaining soil surface integrity and allowing water to infiltrate, rather than runoff and cause erosion. The effect of grazing animals on soil aggregation compared with other conservation management strategies in the Southern Piedmont USA is not well known. We tested a hypothesis that grazing animals might negatively affect soil aggregation characteristics. Water-stable macroaggregates (>0.25 mm), mean-weight diameter, and their stabilities were (i) similar between conservation-tillage cropping and tall fescue (Festuca arundinacea Schreb.)-common bermudagrass (Cynodon dactylon L.) pasture; (ii) similar between 15- to 19-yr-old grazed and hayed hybrid bermudagrass; (iii) 7 to 14% greater in 30-yr-old than in 10-yr-old grazed tall fescue and hybrid bermudagrass pastures; (iv) similar among long-term grazingland, hayland, and forestland; and (v) 5 to 11% lower under cattle grazing than under monthly haying or unharvested management during the first 4 yr following conversion of cultivated cropland to pastureland. Water-stable aggregate distribution at a depth of 0 to 50 mm was 0.30 +/- 0.07 g g(-1) in the 1.0- to 4.75-mm class, 0.46 +/- 0.07 g g(-1) in the 0.25- to 1.0-mm class, 0.15 +/- 0.02 g g(-1) in the 0.05- to 0.25-mm class, and 0.07 +/- 0.01 g g(-1) in the <0.05-mm class, averaged across management systems and replications (n = 56). Total glomalin of the 1.0- to 4.75-mm dry-stable aggregate class was highly related to whole soil organic C content, but neither of these properties was particularly well related with water-stable macroaggregation, mean-weight diameter, or their stabilities. We conclude, overall, that grazing of pastures in the Southern Piedmont USA has little detrimental effect on soil aggregate distribution and stability and is comparable in soil conservation with other land conservation strategies.