Element-pool balances in soils containing rock fragments

Abstract

The quantification of element pools and their changes in time is an important part of the analysis of ecosystems. This quantification is more difficult in soils with rock fragments because of their heterogeneity and because of the difficult analysis of the rock fraction for the pool substance. Repeated sampling and geostatistical analysis were used to quantify spatial heterogeneity in rock fragment content. 137Cs measurements were used to quantify fine-earth contamination of dry-sieved and washed rock fragments because they are nondestructive and allowed for repeated analysis of the same sample after different treatments. 239+240Pu, C and N were determined for comparison. They are typical for substances that are difficult to be determined directly in rock fragments and they also occur mainly in topsoils. Small distance heterogeneity may cause an error in rock fragment content of 10% with similar errors biasing the determination of other pools. After dry-sieving, up to 10% of the 137Cs pool may still adhere to the rock fragments. Washing could remove a further 9% but 1% still remained on the rock fragments. These fractions will normally be ignored in analyses, which cannot be applied to the rock fragment fraction. Pu, C and N were distributed similarly to 137Cs between the fine earth from dry-sieving and the wash-off. The 137Cs technique therefore allows an easy and less laborious estimation of the part of other element pools associated with rock fragments than direct measurement, which in many cases is difficult. Where the 137Cs technique is used to estimate past erosion, since the beginning of nuclear bomb testing, errors of several 100 t ha −1 soil loss may result if heterogeneity and Cs contamination of rock fragments are ignored.