Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/10/2004
Publication Date: 3/1/2005
Citation: Campbell, C.A., Janzen, H.H., Paustian, K., Gregorich, E.G., Sherrod, L.A., Liang, B.C., Zentner, R.P. 2005. Carbon storage in soils of the north american great plains: effect of cropping frequency. Agronomy Journal 97:349-363. Mar/April 2005. Interpretive Summary: Changes in soil organic C depend on the degree to which the soil has been degraded; the greater the previous degradation, the greater the likelihood that a change in management will reverse the process. Most of the experiments reviewed were initiated on land in which the soil had already been degraded to some extent by frequent use of summer fallow. In most soils, soil organic carbon increased with cropping frequency; generally, this relationship was not linear. Soil organic C gains in no-till systems were greater than in tilled systems. In the semiarid Canadian prairie, soil organic C gains under no-till were about 250 kg/ha/yr greater than for tilled systems; in sub-humid environments, the advantage was about 50 kg/ha/yr for fallow-crop and fallow-crop-crop rotations but 250 kg/ha/yr for continuously cropped rotations. These gains require several years to accumulate (10-15). In the semiarid soils in Colorado, USA, a 12-yr no-tillage study on land that had been cropped to fallow-crop under conventional tillage for the previous 75 yr showed a positive effect of cropping frequency on soil organic C. Highest C gains were obtained at the low potential ET sites and lowest gains in the toeslope positions even though they had highest residue C inputs. The latter suggests that in more humid environments, the negative impact of mineralization on soil organic C gains may override the positive impact due to increased crop production and C inputs.
Technical Abstract: 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.