Soil organic C and total N contents of grazed virgin grasslands have been used as the comparison standard to assess the change in soil organic matter (OM) generated by cultivation in the Northern Great Plains. The assumption has been that grassland soil properties were not altered by livestock grazing and therefore reflect the native grassland condition at the time man began cultivating for crop production. In this study, soil properties of grazed and nongrazed (relict) virgin grasslands are compared to assess the effect of grazing. Four sites each of moderately coarse-, medium-, and fine-textured soils under grazed and under relict management were sampled at 0- to 0.076-, 0.076- to 0.152-, 0.152- to 0.305-, and 0.305- to 0.457-m depths. Analyses were made to compare organic C, total N, total P, organic P, and inorganic P contents, and bulk density between the management systems. Nutrient contents differed between the two management systems.
The largest content was not exclusively associated with either system; neither was the difference the same among textural groups or sampling depth. When averaged over all soil textures and depths, organic C and total P contents showed opposite trends from total N. Organic C and total P contents to 0.457 m were larger in relict grasslands by about 1.27 and 0.029 kg m−2, respectively, while the total N content was larger in grazed grasslands by about 0.163 kg m−2. Since the fencing of grasslands for livestock control about 75-yr ago, differences between the two systems have developed at an average annual rate of about 165 kg C, 20 kg N, and 4 kg P ha−1. Bulk densities were highest in grazed grassland in the uppermost 0.076 m. Based upon the organic C and total N contents in relict grasslands, reported losses of OM resulting from cultivation have been either over- or under-estimated, depending on whether organic C or total N content in grazed grasslands has been used as the comparison standard.