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Symposium Papers

Carbon Storage in Soils of the North American Great Plains

Effect of Cropping Frequency

^a Agric. and Agri-Food Canada, Cent. Exp. Farm, Ottawa, ON K1A 0C6, Canada
^b Agric. and Agri-Food Canada Res. Cent., Lethbridge, AB T1J 4B1, Canada
^c Dep. of Soil and Crop Sci., Colorado State Univ., Fort Collins, CO, 80523
^d Great Plains Syst. Res. Unit, USDA-ARS, P.O. Box E, Fort Collins, CO, 80522
^e Pollution Data Res., Environ. Canada, 851 St. Joseph Blvd., 9th Floor, Hull, QC K1A 0H3, Canada
^f Agric. and Agri-Food Canada Res. Centre, Swift Current, SK S9H 3X2, Canada

^* Corresponding author (campbellca{at}agr.gc.ca)

Received for publication December 15, 2003. Summer fallow (fallow) is still widely used on the North American Great Plains to replenish soil moisture between crops. Our objective was to examine how fallowing affects soil organic carbon (SOC) in various agronomic and climate settings by reviewing long-term studies in the midwestern USA (five sites) and the Canadian prairies (17 sites). In most soils, SOC increased with cropping frequency though not usually in a linear fashion. In the Canadian studies, SOC response to tillage and cropping frequency varied with climate—in semiarid conditions, SOC gains under no-till were about 250 kg ha^–1 yr^–1 greater than for tilled systems regardless of cropping frequency; in subhumid environments, the advantage was about 50 kg ha^–1 yr^–1 for rotations with fallow but 250 kg ha^–1 yr^–1 with continuous cropping. Specific crops also influenced SOC: Replacing wheat (Triticum aestivum L.) with lentil (Lens culinaris Medikus) had little effect; replacing wheat with lower-yielding flax (Linum usitatismum L.) reduced SOC gains; and replacing wheat with erosion-preventing fall rye (Secale cereale L.) increased SOC gains. In unfertilized systems, cropping frequency did not affect SOC gains, but in fertilized systems, SOC gains often increased with cropping frequency. In a Colorado study (three sites each with three slope positions), SOC gains increased with cropping frequency, but the response tended to be highest at the lowest potential evaporation site (where residue C inputs were greatest) and least in the toeslope positions (despite their high residue C inputs). The Century and the Campbell et al. SOC models satisfactorily simulated the relative responses of SOC although they underestimated gains by about one-third.

Abbreviations: Cont W, continuous wheat • F-W, fallow–spring wheat (rotation) • PET, potential evapotranspiration • SOC, soil organic carbon • W-Lent, spring wheat–lentil

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