Dry ash, a better reference base than dry matter for heavy metals and other persistent pollutants

Abstract

Concentrations of pollutants in biotic materials are usually related to dry matter. This work demonstrates with the use of heavy metal-contaminated poplar leaves (Populus nigra 'Italica'), that ash of dry matter (dry ash) obviously yields a better reference, as it eliminates the natural variations among organic substance contents.Values of heavy-metal concentrations in dry ash of leaves were generally more homogeneous and better correlated with corresponding values in air and soil than those in dry matter. It is further shown that concentrations of non-biodecomposable and non-volatile pollutants in the mineral fraction (the same as ash) of, for instance, plants and their residues (autumn leaves, mulch compost) can directly be compared with those in soil (nearly 100% mineral), and vice versa. This facilitates the setting of maximally permissible values for pollutant concentrations in plant dry matter or their residues, and in soil, which can be used in the protection of soils against pollutants accumulated in fresh or fallen leaves, mulch or compost. In this connection, the heavy-metal and lindane margins in West German regulations for issuing environmental certificates for compost products are discussed.Concentrations of pollutants in biotic materials are usually related to dry matter, rarely to fresh material. Within the framework of regional bioindication of lead-and cadmium immissions by means of leaves of the pyramidal poplar (Populus nigra 'ltalica') it was demonstrated that ashes of dry matter (dry ash) can also be used as a reference base (Claussen, 1987a;b). The leaves were sampled in the vicinity of 43 continuous measuring stations of The Institute for Protection against Immissions of North-Rhine Westfalia, which made its data available for this work. Heavy metals were determined both in ash and in dry matter.In contrast to dry substance, dry ash used as a reference medium for pollutants cannot be influence by the natural variabilities in water content and in organic substance content. One should bear in mind that the content of organic substance varies with age of the tissue and with circumstances during the growing season. Therefore these investigations in the Rhine-Ruhr region show that the measured values of lead and cadmium contents in dry ash are predominantly more homogeneous than those in dry matter. For instance, whereas the dry-matter values of these metals had coefficients of variation of 62.8% and 49.5%, respectively, those in dry ash were smaller with 57.9% and 48.1%, respectively (Claussen, 1987a).The correlations between lead and cadmium contents in dry ash and those in air or soil were

Should foliar cadmium concentrations be expressed on a dry weight or dry ash weight basis?

“…Due to the physiological background-the loss of organic compounds-of DAW, the reduction in RSD when using DAW instead of DW is a common trend. Claussen 31 reported it for Cd in the leaves of poplar (Populus nigra 'Italica' L.), and Ovington and Madgwick 9 found a reduction in the variability of the Na, K and P concentrations in tree boles when the concentrations were expressed as percentages of the DAW.…”

“…Thus, the dry ash concentration is not influenced by the natural variation in the content of water and organic compounds. 8,31 We determined the dry ash concentration for the same 292 locations as for the Cd concentration, and successfully used this parameter as a correction factor for the variability caused by growth dilution. The variability of the Cd concentrations decreased when expressed on a DAW basis (Table 1); the RSD of the Cd concentration decreased from 26% for DW to 17% for DAW.…”

“…Elevated heavy metal concentrations can have serious effects on forest ecosystems. [33][34][35][36] Threshold values expressed on a DW basis should be time dependent; 31 the leaves that meet a threshold value based on DW at the onset of leaf litter decomposition will no longer do so at a later date due to a reduction in the organic matter concentration of the leaves. The decomposition of organic compounds leads to a decrease in the DW, thus resulting in higher heavy metal DW concentrations in the decomposing leaf litter.…”

The Need to Improve Riparian Forests Management in Uranium Mining Areas Based on Assessment of Heavy Metal and Uranium Contamination

“…DA is noted as the best source material for evaluating the pollution status of forests, especially in large-scale surveys [26]. The advantages are: (1) heavy metal contents in DA of leaves are generally more homogeneous and better correlated with corresponding values in soil than those in dry weight (DW); (2) heavy metal content in DA of leaves is less dependent on sampling methodology; and (3) DA is a better reference base to evaluate pollutant contents in litter and thus preferred in forest management and protection of soils [26][27][28]. DA to DW ratio of leaves was determined after ashing [27] for result comparison with similar studies, because DW contents are usually used as a reference base.…”

Plants as Indicators of Mineralisation and Pollution

“…Usually metal concentrations are reported in dry plant material. However, it has been suggested (Claussen, 1990) that the concentrations of pollutants are better related to the dry ash of a plant, than to the dry weight, since it eliminates the natural variations of organic matter. Heavy metal concentrations in ash of Populus nigra 'Italica' were reported to correlate better with values in air and soil than those in dry plant material.…”