Topsoil aggregate breakdown under water saturated conditions

When the aggregate stability of soils is evaluated under water-saturated conditions by conventional methods of wet-sieving at fixed times (5 to 20 min), the dynamic features of aggregate breakdown are often lost or ignored. An exponential equation, y(t) = a[1 - exp(-t/c)] + b, is proposed to describe the dynamic features of aggregate breakdown as a function of wet-sieving time. Parameter a is the maximum estimated abrasion loss of aggregates, b is the incipient failure of the aggregates when saturated in water, and c is a parameter that links the rate of aggregate breakdown to wet-sieving time. The equation was validated with experimental data using 100 samples of a wide range of Italian topsoils developed on Tertiary deposits. With R2 values close to unity for fitting data from all 100 soil samples, the equation described successfully the dynamics of aggregate breakdown, and the parameters accounted for variations among the soils based on soil type, parent material, and land use. We also scale the exponential equations for the 100 soils with scaling factors to define a scale mean curve for the aggregate breakdown of all soils. The scale factors were obtained by minimizing an objective function containing measured values of aggregate loss and the scale mean curve. The magnitude of the scaling factor gives a quantitative estimate of aggregate stability (the larger its value, the greater the aggregate stability) for each soil, whereas the particular values of parameters a, b, and c describe the kinetics of aggregate breakdown for each soil. A multivariate analysis showed that the value of the scale factors depends significantly on the amounts of amorphous iron oxides and organic carbon in the topsoil as well as on the topographical elevation of the soil surface.