Soil C Burial In Eroded Agricultural Landscapes.

Estimates of soil organic carbon (SOC) change derived through modeling or direct measurements usually do not account for the variability of SOC across the landscape. Recently redistribution and burial of soil and thus SOC, has been identified as a gap in the understanding of landscape to regional scale dynamics of C. We characterized and reconstructed the history of SOC redistribution and burial over the past 60 years at five locations across Canada. Soil pits were excavated at multiple landscape positions over hillslope catenae and into adjacent riparian areas if present. Soil was sampled in 5 cm increments to beyond the depth of the A horizon. Buried profiles varied in thickness from 45 cm to over 90 cm. Utilizing ¹³⁷Cs as a marker we were able to reconstruct the history of erosion and deposition at each site. All depositional profiles contained a peak of ¹³⁷Cs towards the base of the plowing depth in ca. 1955, which was attributed to the greatest concentration of ¹³⁷Cs in the atmosphere in the early 1960’s.  Total SOC stocks varied broadly across the catenae ranging from about 30 Mg ha^-1 in an eroded position to over 300 Mg ha^-1 in multiple depositional positions. Depositional positions had high rates of soil deposition as indicated by ¹³⁷Cs fallout. In many depositional profiles SOC increased with depth towards the base of the original plow layer, and this often correlated with an increase in C:N ratio. It is speculated that this is due to black carbon increasing at the lowest portions of the buried soils presumably deposited from upslope derived from pre-settlement fires and/or initial land clearing. Implications of the broad variation of SOC stocks across the landscape are discussed with reference to C dynamics and greenhouse gas emissions from soil.