Calculating heavy metal accumulation in soil: a comparison of methods illustrated by a case-study on compost application

Abstract

Accumulation of heavy metals in agricultural soil can be prevented by achieving an input that is smaller than, or equal to, the output. This is known as the ‘balance approach’. Heavy metal balance sheets can be used to determine the contribution of different input and output flows to the resulting accumulation. The change in heavy metal content in the plough layer is the result of the net difference between input and output flows per unit time. Existing balance approaches lack an analysis of the effect of soil amendments on soil composition. However, soil composition determines soil bulk density and plough layer weight and, hence, the resulting change in heavy metal concentration. It is, therefore, important to understand and account for the processes that affect soil composition. Recycling of compost produced from source-separated organic household waste as a soil amendment in agriculture is used as an example to illustrate how changes in soil composition complicate accumulation calculations. The dynamic soil composition balance approach is presented as a way to handle these complications by calculating mass balances of both heavy metals and main soil constituents. Comparing results of the dynamic soil composition balance with those of other balance approaches reveals the necessity to account for changes in soil composition and for the effect of soil-bound heavy metal flows in calculating heavy metal accumulation in agricultural soil. This new balance approach is of special relevance if organic soil amendments (e.g., manure and compost) and soil-bound heavy metal flows (e.g., erosion) are involved.