Quantifying carbon and sulphate loss in drained acid sulphate soils

Abstract

SummaryApproximately 25 years before they were sampled, one of a pair of adjacent and initially similar acid sulphate soils was drained. During the ensuing 25 years, the surface of the drained soil declined approximately 1.2 m relative to its undrained neighbour. The losses of carbon (C) and sulphur (S) from the drained soil were estimated as the difference between the amounts remaining in the drained soil and its undrained neighbour. I made these comparisons in two ways. Firstly, I determined the amounts of C and S by summing their volumetric concentrations to a common depth (relative to the Australian Height Datum) where these concentrations became equal. Secondly, I made comparisons using mass‐based concentrations and a material coordinate defined by integrating the bulk density profile down from the soil surface. In this latter convention, the distance is the cumulative mass of soil solids per unit area of cross‐section, measured to any depth from the soil surface. The former approach yields estimates of C loss almost 30% greater than the latter. The S loss based on volumetric calculations was approximately 25% greater than that based on mass‐based coordinates. These overestimates of loss of C and S arise because the depth‐based method fails to take account of the vertical movement of the soil solid accompanying drainage in these soils. As a result, chemical reactions and their products, which are uniquely identified with parcels of soil solids, have no unique depth in swelling soils over time. The material calculations are therefore correct, but I identify conditions where a conventional volume‐based estimate might be used.