Colloidal properties and potential release of water-dispersible colloids in an agricultural soil depth profile

Abstract

The effects of soil depth on the colloidal properties and potential release of water-dispersible colloids (WDCs) in an agricultural soil were investigated in batch experiments. Nine soil samples were collected along a trench at different depths (from −7.5cm to −127.5cm) from the TERENO test site located in Selhausen (Germany). A physical gentle soil fractionation confirmed the importance of the solution chemistry in controlling the depth distribution of WDC in the Luvisol soil. A maximal fraction of total clay (fWDC) equal to approx. 45% was released at a depth of −37.5cm to −52.5cm below the surface. This was found to be related to the lowest electrical conductivity of the soil electrolyte phase where Ca2+ concentration played a major role. A rather constant content of metal oxyhydroxide (MO(OH)) accounting for about 10% in WDCs was found along the soil depth after a dithionite-citrate-bicarbonate (DCB) extraction. However, a decrease from 40% to 25% in the contribution of MO(OH) particles to SSA of WDC was calculated along the soil depth. Using photon correlation spectroscopy (PCS) and N2 gas adsorption method, variations were measured in colloidal hydrodynamic diameters (dz) and the mineral specific surface area (SSA) for WDC in water-dispersed and dried states, respectively. A function structure f(np) was defined to relate the particle size variations between the two states along the soil depth. An inverse correlation was established between the particle size variations which assumed that the number of primary particle in the water-dispersed state increases in subsoil samples. The effects of different soil parameters such as pH, organic carbon (OC) coverage at MO(OH) surfaces and humification index (HIX) of dissolved organic carbon (DOC) on the WDC structure variations are discussed. It can be hypothesized that a maximal surface OC coverage at MO(OH) surfaces of WDCs facilitates the release of small particles through electrosteric dispersion in topsoil. Lower pHs and WDC(OC) content would favor the release of large WDC particles in subsoil where an OC content of a more aliphatic character can also facilitate hydrophobic interactions.