In order to understand the movement and bioavailability of trace elements in the critical zone, it is important to understand the chemical forms of the elements present and their changes due to environmental and biological changes in the different compartments. To achieve these advances, it is often not possible to rely on a single method. Putting together the results from a number of different methods to assess speciation and likely mechanisms for movement in biotic and abiotic parts of the system is often very fruitful. Finally, the increased mechanistic level understanding gained allows insights and interventions for managing environmental or biological activity to be designed and tested, so that more beneficial and sustainable outcomes are promoted.

This presentation gives several case studies of how such investigations can be fruitful with examples of environmental toxic or essential trace elements. Use of complementary methods for assessment is highlighted.

First is the use of LC-ICP-MS, synchrotron and NanoSIMS to follow the important threat of mobilisation and transport of arsenic in paddy rice systems. These identified 1) that arsenite species predominate under flooded conditions, 2) this is very mobile in the soil-plant interface, 3) rice plants use the same effective system for the transport for the beneficial element Si and for arsenite species, 4) inside the plant, arsenite is transformed and some is transported to the grain that people eat whereas Si is diverted mainly to the leaves and 5) although movement of inorganic As is held up to some extent at the material/seed interface, methylated organic As compounds are more effectively transported to the white endosperm of rice which people most often consume.

Second is the transformation of nanoparticles (NP) in the soil-plants system. Using a number of techniques, we can detect the transformation of ZnO, a commonly used NP, in soils and it uptake and transport in plants. From this, we know that ZnO-NPs are rapidly transformed and act like more soluble Zn salts after additions to soil. This has implications for the effective use of different forms of this essential nutrient, and for toxic risk assessment of such NPs.

Finally, Se deficiency is widespread and we have investigated the forms of Se in soils and the uptake of this essential nutrients by plants. This has revealed its speciation in soils, uptake and competition in plant sulphur uptake mechanisms, its forms transported, the location of Se in grain, and its likely bioavailability availability to human and animal consumers.