On-Farm Soil Carbon Monitoring in Western Nebraska and Eastern Colorado: Effects of Irrigation, Cropping System and Residue Management.

Since the 1970’s, large areas of dryland winter wheat-fallow croplands of western Nebraska and eastern Colorado have been converted to pivot-irrigated row crops.  These irrigated croplands are highly productive, with corn grain yields reaching 220-250 bu/ac (approx. 11-13 T ha^-1) and are expected to increase soil organic carbon (SOC) stocks through higher inputs, relative to the previous dryland wheat-fallow systems.  Farmers’ fields were sampled near Imperial, NE and Otis, CO to determine the influence of irrigation, cropping system and residue management on SOC change over time.  A precision sampling strategy was employed that utilized a stratified random design with permanent markers so sampling locations could be relocated within a few centimeters of the original samples.  Initial samples were collected in 2004 and 2005 on 3 farms near Imperial (10 fields total) and one farm near Otis (3 fields).  Soil samples were taken from inside the pivot (irrigated) and from the field corners (non-irrigated), as well as from a native grassland pasture on one of the Imperial farms.  All fields were sampled again in 2012 and management information was recorded from the farmers over the whole time series.   In the initial sampling effort, the native grassland site had the highest SOC stocks, with 95 Mg C ha^-1  over the whole profile (0-75 cm).  SOC stocks in irrigated fields ranged from 64-86 Mg C ha^-1, while non-irrigated corners ranged from 46-74 Mg C ha^-1.  Variation of effects of irrigation on SOC stocks corresponded to differing irrigation histories, with some fields having been under irrigation for 3-4 decades, while others have only been irrigated for 1-2 decades.  Preliminary results from the 2012 sampling show small increases in soil organic carbon (SOC) in the surface layers and very little SOC change in subsurface depths over 8 years.  We will present SOC stocks and changes for all fields and evaluate the influence of changing management practices, varying land use histories, climate and other factors on SOC change.