Sensitivity of the Century Model to Scale- Related Soil Texture Variability

Sequestering C in agricultural soils presents an immediate viable option to reduce atmospheric CO2 to help mitigate global warming. Agricultural land managers who adopt practices that sequester C might market the sequestered (i.e., stored) C as a C credit to industrial CO2 emitters who wish to reduce their net CO2 emissions. Land managers or landowners will need to verify changes in soil organic carbon (SOC) related to a change in management practice to facilitate C credit trading. The objective of this study was to assess the accuracy of Century model predictions of SOC change due to the adoption of no-tillage using site specific data and data from existing soil databases. We hypothesized that (i) using site-specific soil data would result in the most accurate Century estimates and (ii) Century estimates are sensitive to soil clay percentage. Five paired tillage/no-tillage farm sites in north-central Montana were used to test model predictions. Sites were chosen such that soil, landscape, climatic conditions, and historical cropping systems were similar within each tillage/no-tillage pair. The Century model overestimated SOC content using site-specific soils data by an average of 10%. Century was sensitive to the effects of clay content when predicting the total amount of SOC in a particular field. There was insufficient evidence to suggest that a linear association exists between clay content and Century-estimated C change due to no-tillage. Results suggest that (i) the effect of clay percentage on the rate of C change is not well understood and (ii) the Century model is an acceptable predictor of soil C for C trading. Further examination of the relationship between soil clay content and the rate of C storage in agricultural systems is needed to determine if adjustments to the Century model are required.