Soil Organic C Changes in Response to 10-Yr of Increasing Cropping System Intensity in Montana.

Soil organic C (SOC) in agricultural soils can benefit from increasing cropping intensity in semiarid climates. The objective of this study was to determine the impact of annual cropping practices on SOC levels in Montana semiarid climate.  Field studies were conducted at five farm-managed sites in northern Montana and on the property of the MSU-Post Farm near Bozeman between 2002 and 2012. Estimates of SOC mass for different profile depths were calculated for the farm-managed sites (i.e. 0-20 cm and 0-50 cm) and MSU-Post Farm (0-30 cm). Estimates of SOC mass were contrasted with baseline numbers in 2002 (or 2004) where comparison could be reliably made. This study found that terrestrial C sequestration was possible in Montana’s dryland cropland soils by replacing fallow-wheat with more intensive cropping practices such as annual, pulse-wheat rotations.  The impact of a reduction in tillage on C sequestration was somewhat unclear but it appeared to be considerably less important than cropping intensity.Estimated SOC sequestration response averaged 0.32 MT C ha^-1 yr^-1, or 1.17 MT CO₂ equivalent ha^-1 yr^-1 over five sites where responses to increasing cropping intensity were observed.   As anticipated there was considerable variance (0.18 to 0.60 MT C ha^-1 yr^-1) in soil C sequestration rates among the responsive sites.  The largest C sequestration was response to cropping intensity was observed at a field site which was characterized by high soil clay content (60%).   Current, Montana ag-statistics indicate that approximately 1.29 million hectares of farm land (3.19 million acres) are in fallow on an annual basis.  Approximately,   72% of the fallow-wheat acreage (or 925,000 hectares) is found in Montana’s Golden Triangle plus three other neighboring counties.  If we apply our mean soil C sequestration rate to this acreage, then the potential of terrestrial C sequestration in Montana following conversion of fallow-wheat to annual cropping equates to approximately 1.0 million MT of CO₂per year.