Organic Carbon Influences on Soil Particle Density and Rheological Properties

Soil particle density (rs) is not routinely measured and is assumed to range between 2.60 and 2.70Mgm or to be a constant (2.65Mgm) when estimating essential properties such as porosity, and volumetric water and air relations. Values of rs for the same soil may, however, differ significantly from the standard range due to managementinduced changes in soil organic carbon (SOC) concentrations.Wequantified the rs andAtterberg limits of a Rayne silt loam for five long-term (.22yr)moldboard-plowed continuous corn (ZeamaysL.;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 rs and the Atterberg limits and the impact of rs on soil porosity. Mean rs across NT, NTm, and pasture (2.35 Mg m) was |7% lower than that for MP in the 0to 10-cm soil depth (2.52 Mg m, P , 0.01). Forest had the lowest rs of all soils (1.79 Mg m). The NTm caused a greater reduction in rs and a greater increase in SOCconcentration, liquid limit (LL), plastic limit (PL), andplasticity index (PI) than NT. Surface soils under MP had the highest rs and rb and the lowest SOC concentration, LL, PL, and PI. The SOC concentration was correlated negatively with rs (r 2 5 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 rs overestimated the ‘‘true’’ porosity by 12% relative to that using the measured rs. ARELIABLE MEASURE of rs, the mass per unit volume of the inorganic and organic soil solids, is required to calculate total porosity, rates of particle sedimentation, heat capacity, thermal conductivity, and water and air relations on a volumetric basis (Hillel, 1998). These rs-based parameters are essential to understanding and modeling several processes including water, air, and heat flow as well as chemical transport through the soil. Such numerous and important applications of rs necessitate an accurate measurement of this soil physical property. The rs is not, however, routinely measured during soil characterization because it is often assumed to range between 2.60 and 2.70 Mg m or equal a constant value of 2.65 Mg m for most quartz-dominated soils (Hillel, 1998). Thus, derivation of soil properties requiring rs as input is normally based on the assumed constant value. While a rs of 2.65 Mg m 23 can be accurate for some soils, variations in the composition of soil solids, such as an increase in SOC concentration, can significantly lower the rs value since the organic fraction is an important component of soil solids. Tillage and cropping systems are not expected to considerably affect the rs, but changes in SOC concentration resulting from these management systems may significantly modify rs. For example, conversion of traditional tillage to reduced and no-till systems increases SOC pools (Allmaras et al., 2004; Lal, 2004; Hooker et al., 2005). Yet, the effects of such SOC increases on rs have not been well documented, and the general assumption is that any change in rs would be negligible. Singh et al. (1994) reported that the rs for tilled soils was slightly higher than that for NT soils due to lower SOC concentration in a plowed clay loam soil. Studies comparing rs values as a result of changes in SOC concentration under long-term tillage and cropping management systems are not widely documented. Total soil porosity, an important property estimated directly from rs, is widely reported when assessing soil physical quality under different tillage and cropping systems, but it is often computed with the assumed standard value of rs (2.65Mgm ), thus overlooking possible management-induced changes (e.g.,Radcliffe et al., 1988; Eynard et al., 2004; Shukla et al., 2004). The reliability of data thus obtained is questionable if rs within the same soil varies significantly as a function of soil management. Thus, characterization of rs across contrasting management systems with differential C input for the same soil is a research priority. Implications of any changes in rs due to soil management for the calculation of rs-based soil properties have not been quantified. Another important soil physical parameter that can be affected by changes in SOC concentration is the Atterberg limits, which describes the limits of soil consistency such as LL, PL, and PI. Measurement of these limits during routine analyses of agricultural soils is, however, very uncommon (Mapfumo and Chanasyk, 1998). Knowledge of soil water contents at LL and plastic limit PL is fundamental to predicting the tillage and wheel traffic effects on soil consistency. For example, plastic limit is the highest water content at which a soil can be tilled without negatively affecting its structure (Dexter and Bird, 2001; Arvidsson et al., 2004). Data on Atterberg limits are needed to assess long-term land use and tillage impacts on soil mechanical and rheological behavior (Terzaghi et al., 1988). The LL and PL of the soil depend primarily on clay and SOC concentrations (De Jong et al., 1990). Themagnitude of dependence of these properties on SOC can be variable and influenced by soil texture, organic matter (OM) source, and soil–crop management. The LL and PL can be strongly, weakly, or H. Blanco-Canqui and R. Lal, Carbon Management and Sequestration Center, FAES/OARDC, School of Natural Resources, The Ohio State Univ., 210 Kottman Hall, 2021 Coffey Rd., Columbus, OH 43210-1085; W.M. Post, Environmental Science Div., Oak Ridge National Lab., Oak Ridge, TN 37831; R.C. Izaurrralde, Joint Global Change Research Inst., 8400 Baltimore Ave., Suite 201, College Park, MD 20740-2496; and M.J. Shipitalo, USDA-ARS, North Appalachian Experimental Watershed, P.O. Box 488, Coshocton, OH 43812-0488. Received 23 Oct. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1407–1414 (2006). Soil & Water Management & Conservation doi:10.2136/sssaj2005.0355 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: LL, liquid limit; MP, moldboard plow; NT, no-till with no beef cattle manure; NTm, no-till with beef cattle manure; OM, organic matter; PI, plasticity index; PL, plastic limit; rb, bulk density; rs, particle density; SOC, soil organic carbon. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1407 Published online June 21, 2006