Field-Scale Heterogeneity and Geochemical Regulation of Arsenic, Iron, Lead, and Sulphur Bioavailability in Paddy Soil

A method using miniaturized arrayed DGT-probes (PADDI) for high-frequency in-situ sampling with LA-ICPMS and CID analysis was developed to measure the field-scale heterogeneity of trace-element bioavailability. Robust calibrations (R2>0.99) combined with high-sensitivity (LOD=0.35 ng cm-2), multi-elemental detection, and short measurement times were achieved using a new LA-ICPMS microDGT analysis. In the studied paddy-site (size:~2500m2), total element concentrations across the field were approximately uniform (R.S.D.<10%), but bioavailability was shown to vary significantly as determined from 864 microgel measurements housed within 72 PADDI arrays. Porewater As measurements were unable to differentiate the top/rhizosphere and bulk/deeper-soil layers. However, dynamic sampling with DGT revealed significant differences. Heterogeneity behaviors varied greatly between the different elements. Arsenic bioavailability was stable laterally across the field, but varied with depth, which was in contrast to the trends for Pb. Fe/S(-II) change was bi-directional, differing horizontally and vertically throughout the field. The heterogeneity in Pb bioavailability due the high frequency of hotspot maxima, which were discretely dispersed across the paddy proved the most difficult to simulate, requiring the greatest number of probe deployments to determine a reliable field-average. The DGT-PADDI system provides a new characterization of infield trends for improved trace-inorganics’ management in agricultural wetlands.