Organic carbon influences on soil particle density and rheological properties

Soil particle density (rho(s)) is not routinely measured and is assumed to range between 2.60 and 2.70 Mg m(-3) or to be a constant (2.65 Mg m(-3)) when estimating essential properties such as porosity, and volumetric water and air relations. Values of rho(s) for the same soil may, however, differ significantly from the standard range due to management-induced changes in soil organiccarbon (SOC) concentrations. We quantified the rho(s) and Atterberg limits of a Rayne silt loam for five long-term (> 22 yr) moldboard-plowed continuous corn (Zea mays L.; MP), no-till continuous corn (NT), no-till continuous corn with beef cattle manure (NTm), pasture, and forest systems. We also assessed the relationships of SOC concentration with p, and the Atterberg limits and the impact of rho(s) on soil porosity. Mean rho(s) across NT, NTm, and pasture (2.35 Mg m(-3)) was -7% lower than that for MP in the 0- to 10-cm soil depth (2.52 Mg m(-3), p < 0.01). Forest had the lowest rho(s) of all soils (1.79 Mg m(-3)). The NTm caused a greater reduction in rho(s) and a greater increase in SOC concentration, liquid limit (LL), plastic limit (PL), and plasticity index (PI) than NT. Surface soils under MP had the highest rho(s) and rho(b) and the lowest SOC concentration, LL, PL, and PI. The SOC concentration was correlated negatively with rho(s) (r(2) = 0.75) and positively with Atterberg limits (r(2) > 0.64) at > 20-cm depth. Estimates of soil porosity for NT, NTm, and pasture using the constant p. overestimated the "true" porosity by 12% relative to that using the measured rho(s).