No-till and Continuous Cropping System Studies in Semiarid Montana to Measure Soil C Sequestration.

Carbon sequestration capabilities of cropland soils in Montana have received little attention due to the lack of measured sequestration rates. Our objective is to measure the effects of no-till and increased cropping intensity on soil organic carbon (SOC) levels in dryland cropping via long-term (10+ yr) experiments. In fall 2002, farmer-managed field-scale trials (8 - 12 ha) were begun at six sites in north central Montana and a plot-scale study (0.016 ha) was begun near Bozeman. The field-scale trials compare tilled wheat-fallow, no-till wheat-fallow, tilled wheat-legume, and no-till wheat-legume systems. Change in SOC among systems in the field-scale trials was not detected after 4 yr. Previous studies have shown that significant changes in SOC are often undetected until after 6 yr following change in soil management.  The replicated plot trial compares tilled fallow-wheat, no-till fallow-wheat, no-till continuous wheat, no till legume-wheat, and unmanaged perennial alfalfa-grass (i.e. CRP) systems. Soil organic C levels (0-30 cm) after 4 yr averaged 35.3 and 36.3 MT/ha for fallow-wheat and annual cropping systems, respectively. Thus, soil organic C (0-30 cm) was sequestered at a rate of approximately 0.25 MT/ha/yr in the annually cropped no-till systems compared to fallow-wheat systems (P=0.02). The perennial alfalfa-grass system sequestered SOC at a rate of 0.67 MT/ha/yr compared to fallow-wheat systems (P<0.01). No effect of tillage on SOC was detected in wheat-fallow systems at Bozeman after 4 yr. We are currently analyzing SOC from the 6th year of these long-term studies (2008), intending to present these results at the 2009 annual ASA-CSSA-SSSA conference. These long-term studies promise to increase our understanding of SOC sequestration rates for semiarid cropping systems in Montana. This project has been supported by CASMGS, NRCS-Conservation Innovation Grants Program, DOE Big Sky Regional Carbon Sequestration Partnership, and Montana Wheat and Barley Committee.