Soil Management and Carbon Calculation Methods Influence Changes in Soil Carbon Estimation.

Throughout the years, many studies have evaluated changes in soil organic carbon (SOC) mass on a fixed-depth (FD) basis without considering changes in soil mass caused by changing in bulk density (ρ_b).  In two study sites, we investigated the effect of different management practices on soil organic carbon (SOC) changes calculated as a (i) concentration, (ii) on a fixed-depth (FD), (ii) on an equivalent soil mass (ESM), and on a minimum equivalent soil mass (ESM_min).  The long-term (39 yr) treatments sampled from the first site were: conventional tillage (CT); moldboard plow (MP); no-tillage (NT); and reduced tillage (RT) with winter wheat (Triticum aestivum L.) summer fallow (WF) system.  The short-term (7 yr) treatments sampled from the second site were from NT and chisel plow (CP) with multiple crop rotation and continuous corn. Soil samples were collected from 0- to 30-cm depths in both sites. The changes in soils bulk densities were also evaluated. For the first site and on a fixed depth basis, SOC was 21% greater in NT and RT than CT and MP.  However, on ESM basis, SOC was 11% greater with NT, MP, and RT compared with CT.  At the second site, the changes in SOC concentration was 19.7% greater after seven years of managements compared with initial.  Standardizing the soil masses on an ESM showed an average gain in SOC of 5.8 Mg C ha^-1 after seven years compared with initial. Estimating SOC levels on an ESM_min showed that SOC stock was not influenced by tillage practices. In general, the ESM calculation scenario appears to be more effective in evaluating SOC stock compared with other scenarios.