Convergence of Organic Carbon of Soils with Diverse Qualities after a 21-Year Field Bioassay.

Soil organic carbon (SOC) is transient and can be lost if environmental or management conditions favour enhanced decomposition. How quickly does SOC change in response to a shift in these conditions? This question is usually answered by measuring OC change in different soils located in different regions, landscapes, or landscape positions, but such measurements may be confounded by differences in environment. To address this shortcoming, a long-term field bioassay was conducted where soils with diverse qualities were transplanted onto a common subsoil and monitored for yield and OC change over time, thus removing the landscape quality factor. Such an experiment was initiated in 1990 at Lethbridge, Alberta; 36 diverse soils were transplanted to the experimental area, where topsoil had been removed, and randomly assigned in three replicates to 108 plots. Plots were yearly seeded to spring wheat under no-till. Among other chemical and physical soil properties, OC was analyzed every seven years: in 1990, 1997, 2004, and 2011. The OC of the initial soils ranged from 8.6 to 74 mg g-1. Management histories included native grassland, long-term cropping, summer fallow, manured soil, and sub-soils. Generally, soils under grasslands and pastures had highest OC contents followed by continuous cropping and the lowest were the sub-soils. The results after 21 years show a direct relationship between OC change and initial OC content: soils initially high in OC lost the most, and soils initially low gained OC. The largest percentage of OC loss occurred in soils where management changed from native to arable. Soil clay content had no discernible effect. Light fraction carbon, a measure of ‘labile’ SOC, is converging more quickly than total SOC. The change in OC can be described by an exponential function which suggests that soils may only converge after more than another century.