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Crested wheatgrass (Agropyron cristatum [L.] Gaertn.) and Russian wildrye (Elymus junceus Fisch.) are commonly used for reseeding in the more xeric Mixed Prairie of the Canadian prairies because they are perceived to be more productive than native species. However, they have been implicated in soil deterioration. The objectives of our study were to compare the aboveground net primary production and soil organic carbon (C) among monoculture communities of selected native grass species, crested wheatgrass, and Russian wildrye and to compare the native grass monocultures with their mixtures. In 1995, a 5-year study was initiated on Dark Brown Chernozemic (Typic Haploboroll) soil near Lethbridge, Alberta. Ten treatments consisting of monocultures of introduced and selected native species and mixtures of native species were established in a randomized complete block design with 4 replications. Aboveground net primary production and soil organic C were measured. Monocultures of 2 native species, green needlegrass (Stipa viridula Trin.) and blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Steud.), were more productive than crested wheatgrass or Russian wildrye under both normal moisture and drought conditions. Monocultures of these native species also tended to be more productive than their mixtures. The western wheatgrass (A. smithii Rydb.) monoculture and the western wheatgrass–blue grama mixture experienced the greatest yield reduction as a result of drought. Treatment effects on soil organic C were not detected (P . 0.05) 5 years after seeding. Soils of the June grass (Koeleria macrantha [Ledeb.] J.A. Schultes f.) community had less (P , 0.05) macro-organic C than most other treatments. Resumen

The regional scale response of soil carbon mass to long-term grazing exclusion was investigated in the Canadian Great Plains. Vegetation, litter, macro-organic matter and soil were sampled in paired grazed and ungrazed treatments from 9 independent locations along an environmental gradient in southern Alberta. Vegetation and litter carbon mass were greater on ungrazed treatments, but no consistent grazing effect was observed for macro-organic matter (roots, subsurface litter) or soil (fine particles < 2mm) carbon mass per equivalent soil mass. Soil carbon in mixed grass prairie was positively correlated with clay content, but no grazing effect could be detected when this subset (n = 7) was analyzed by ANCOVA. Comparison of multiple sites with a consistent sampling and reporting method revealed no general trend in the response of soil carbon to grazing. Current range management practices to maintain range types in good to poor condition appear to be consistent with maintaining the soil organic mat...

Differences in 13C inputs between two adjacent Canadian mixed grass prairie communities were used to estimate turnover of two soil organic matter fractions with increasing depth below the surface. Partial cultivation and abandonment of a C4-dominated grassland ca. 1930, and natural recovery to a native C3-dominated grassland changed the 13C/12C ratio (reported in units of δ13C‰) of plant residue inputs to soil organic matter. After 70 years, macro-organic matter carbon (MOMC) differed up to 5.6‰, and fine soil organic carbon (FSOC) by 2.9‰ near the surface; differences declined to a depth of 1 m. Mean residence time estimates differed among fractions from a short 35 years for MOMC 0–5 cm, to a longer 413 years for FSOC 5–10 cm. The background carbon pool on this site was substantial, and the minimum change in soil carbon of 10.29 Mg C ha−1 over 70 years represented less than 7% of the carbon pool to a 1-m depth. Considerable vertical and horizontal variation in soil organic carbon mass and 13C content were observed. Variation due to small sample size was a more substantial limitation than cumulative analytical variation for calculating turnover estimates >500 years or at depths >20 cm. Corrections for bias were also necessary since imperfect physical separation of soil organic matter fractions, or CO32− contamination, were issues.