Effects of temperature and associated organic carbon on the fractionation of water-dispersible colloids from three silt loam topsoils under different land use

The release and stability of soil water-dispersible colloids (WDC) in the soil structure are critical for colloid-facilitated soil organic carbon sequestration and contaminants transport. In this study, the potential effects of temperature and associated organic carbon (OC) on the release of WDCs in three silt loam topsoils with the same clay content (~20%) under different land uses were investigated. A soil fractionation method was used for simulating the release of colloids from the soil under environmental conditions where mobilization and sedimentation processes occur sequentially. The surface loading of OC has been characterized by the analysis of organic carbon content of WDC with the measurements of the specific surface area (SSA). The effects of fractionation temperature on colloidal properties (e.g., particle size and zeta potential) were systematically investigated and the aggregation kinetics of WDC in salt electrolyte influenced by temperature was assessed by dynamic light scattering (DLS).Experimental results demonstrated that the amount of extracted WDC from three soils decreased when the fractionation temperature increased. A more rapid sedimentation of WDC at higher temperatures outweighed the effect of temperature on WDC mobilization from bulk soil in the shaking step. The sedimentation of WDC at various temperatures indicated that the temperature dependence of the water viscosity (η) was a dominate parameter and caused lower efficiency of WDC mass gained at higher temperature according to the Stoke's law. After introducing the factor of η7°C/ηT, the temperature effect only on WDC mobilization during shaking step could be described and the whole fractionation process could be successfully timely determined along the two shaking and sedimentation steps. Activation energies (Ea) of about 10kJmol−1 could be now calculated for the WDC mobilization processes from the three topsoils.The associated organic carbon contents of WDC (WDC(OC)) and the mineral surface of WDC blocked by organic carbon (SSAOC-block) after various shaking temperatures and shaking time were further determined in order to examine the WDC(OC) effect on the release of WDC from soil matrix. The results demonstrated that the escape of the mobile clay fraction (F) from soil at short shaking times is favored by the presence of effective surface loading by an OC layer (SSAOC-block), which is known to stabilize its colloidal state through electrosteric effects. The WDC(OC) surface concentration has been also used to estimate the clay-associated OC distribution in the three topsoils.In Ca2+ solution, an increase of temperature favors the colloidal stability of WDC as measured from the shift of critical coagulation concentration (CCC) to higher concentrations of Ca2+. In total, the results from this study revealed that temperature and WDC(OC) distribution are critical parameters when considering soil WDC release and stability in natural bulk soils.