Long-term soil nitrogen and vegetation change on sandhill rangeland

The effect of livestock grazing on organic C and N in rangeland soils is not well defined. In this study on sandy rangeland in western Oklahoma, we sampled 8 pastures moderately grazed by cattle and 8 adjacent exclosures ungrazed by livestock for 50 years. The sagebrush was largely controlled by herbicide in the study areas. The C and N concentrations in the surface 5 cm of soil, total herbage production, and total N uptake by vegetation were similar (P > 0.05) in grazed and nongrazed areas. Carbon and N concentrations in soils sampled to a constant mass to a depth of 5 cm or less were not (P > 0.05) different from concentrations determined on soil sampled to a constant depth of 5 cm. When calculated on a content basis, grazing increased (P < 0.001) the bulk density (1.35 g cm(-3)) compared to nongrazed pastures (1.19 g cm(-3)) and had a significant (P < 0.01) effect on C and N in the surface 5 cm of soil. Litter and total N in litter were greater (P < 0.01) on nongrazed areas. Little bluestem (Schizachyrium scoparium (Michx.) Nash) and sand bluestem (Andropogon hallii Hack.) produced more herbage and had greater frequency on nongrazed areas, whereas blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Griffiths], sand dropseed [Sporobolus cryptandrus (Torr.)Gray], and western ragweed (Ambrosia psilostachya DC.) increased in frequency on grazed areas. Thus, 50 years of moderate grazing by cattle had no measurable effect on C and N concentrations in the surface 5 cm of the sandy soil or on total N uptake by plants as compared with nongrazed areas; however, significant differences occurred in species composition which may alter mechanisms of C and N balance.