Soil Aggregates: a Scale to Investigate the Densities of Metal and Proton Reactive Sites of Organic Matter and Clay Phases in Soil

SummaryDetermining site density of reactive sites of metals in the main soil phases remains a challenging task. This study aimed to show that densities of reactive sites in soil may be assessed by a fractionation procedure based on soil being spatially organized as aggregates. The method is described with copper as a model trace element and a common silty loam soil after applying a low energy fractionation method to maintain the integrity of soil aggregates. The reactivity of five soil size fractions (> 250, 250–63, 63–20, 20–2 and < 2 μm) to protons and copper was quantified by acid–base titrations. The total proton sorption capacities were assigned to the total concentration of copper reactive sites and fitted to a linear combination of the relevant reactivity data of each phase, namely the total contents of organic carbon, copper and acid‐extractable aluminium. Acid–base reactivity was linearly related to the distribution of copper, and differences between fractions were used to reconstruct the distribution of acid–base and copper‐complexing sites among the clay, organic and weakly reactive residual phases. In accordance with our hypothesis that key reactive phases are mainly organic materials and clays, we used this procedure to determine the site densities of (i) two size classes of particulate organic matter, (ii) strongly reactive organic matter (e.g. soil humic and fulvic acids) and (iii) clay. The site densities and the distributions of copper obtained were used to validate our conceptual model for predicting global soil reactivity to metals.Highlights Precise site densities of key soil reactive phases are often lacking in transfer modelling We applied specific experimental methods of aggregate analysis and partial leaching of Al phases Site densities of the various types of organic matter were assessed using variably amended soils Soil aggregate analysis is powerful for determining base parameters in element transfer modelling