Knowledge of soil organic carbon (SOC) changes that occur under different agricultural practices is important for policy development, carbon (C) marketing, and sustainable land management. Our objective was to quantify agricultural impacts on SOC sequestration for dryland cropping systems in different agroclimatic zones (ACZs) of the Pacific Northwest (PNW). Data from 131 SOC studies were analyzed to assess land management-induced changes in SOC, including the conversion of native ecosystems to agricultural crops, conversion from conventional tillage (CT) to no-tillage (NT), and alternative crop rotations and management practices. Cumulative probabilities of SOC change were developed for assessing uncertainties inherent in SOC studies and for informing SOC markets. These analyses showed that 75% of converted native land lost at least 0.14 to 0.70 Mg C ha(-1) y(-1) (0.06 to 0.31 tn C ac(-1) yr(-1)) over an average of 55 to 74 years depending on ACZ. Converting from CT to NT was predicted to increase SOC at least 0.12 to 0.21 Mg C ha(-1) y(-1) (0.05 to 0.09 tn C ac(-1) yr(-1)) over 10 to 12 years in 75% of studies analyzed and was also ACZ specific. Compared to annual cropping, mixed perennial-annual systems would be expected to gain at least 0.69 Mg C ha(-1) y(-1) (0.31 tn C ac(-1) ha(-1)) over 12 years in 75% of ACZ 2 sites. Other conclusions were that (1) SOC databases are lacking for low precipitation areas of the PNW; such as the dryland wheat-fallow region; (2) baseline sampling of SOC prior to management change is largely nonexistent for PNW databases except for a few notable cases; (3) soil erosion processes have likely impacted SOC and contributed to large variability among studies; (4) sampling methodologies and analyses for SOC have been inconsistent, thereby contributing to SOC variability; and (5) a validated C model for the PNW would aid evaluation of SOC changes due to management, particularly for specific farms and sites with unique SOC history and circumstances.
