Localized flux maxima of arsenic, lead, and iron around root apices in flooded lowland rice.

Paul N Williams,Ronnie N Glud,Eva Oburger,Walter Wenzel,Jakob Santner,William Davison,Hao Zhang,Niklas J Lehto,Morten Larsen

doi:10.1021/es501127k

Abstract

In wetland-adapted plants, such as rice, it is typically root apexes, sites of rapid entry for water/nutrients, where radial oxygen losses (ROLs) are highest. Nutrient/toxic metal uptake therefore largely occurs through oxidized zones and pH microgradients. However, the processes controlling the acquisition of trace elements in rice have been difficult to explore experimentally because of a lack of techniques for simultaneously measuring labile trace elements and O2/pH. Here, we use new diffusive gradients in thin films (DGT)/planar optode sandwich sensors deployed in situ on rice roots to demonstrate a new geochemical niche of greatly enhanced As, Pb, and Fe(II) mobilization into solution immediately adjacent to the root tips characterized by O2 enrichment and low pH. Fe(II) mobilization was congruent to that of the peripheral edge of the aerobic root zone, demonstrating that the Fe(II) mobilization maximum only developed in a narrow O2 range as the oxidation front penetrates the reducing soil. The Fe flux to the DGT resin at the root apexes was 3-fold higher than the anaerobic bulk soil and 27 times greater than the aerobic rooting zone. These results provide new evidence for the importance of coupled diffusion and oxidation of Fe in modulating trace metal solubilization, dispersion, and plant uptake.

Highlights

Rice contains ∼10 times more arsenic than other cereal staples[1] and is the dominant food source of inorganic arsenic exposure to the world’s population.[2,3] The prevalence of lead-enriched rice is less than that of arsenic but is a particular concern in regions where waste recycling and base metal mining coexist with farming.[4]
We use new diffusive gradients in thin films (DGT)/planar optode sandwich sensors deployed in situ on rice roots to demonstrate a new geochemical niche of greatly enhanced As, Pb, and Fe(II) mobilization into solution immediately adjacent to the root tips characterized by O2 enrichment and low pH
The Fe flux to the DGT resin at the root apexes was 3-fold higher than the anaerobic bulk soil and 27 times greater than the aerobic rooting zone. These results provide new evidence for the importance of coupled diffusion and oxidation of Fe in modulating trace metal solubilization, dispersion, and plant uptake

Summary

Introduction

Rice contains ∼10 times more arsenic than other cereal staples[1] and is the dominant food source of inorganic arsenic exposure to the world’s population.[2,3] The prevalence of lead-enriched rice is less than that of arsenic but is a particular concern in regions where waste recycling and base metal mining coexist with farming.[4]. Lead, and iron is influenced strongly by soil processes and root activities, but our understanding of related mobilization/immobilization of those elements in rice rhizospheres remains incomplete because of the lack of satisfactory biogeochemical in situ mapping technologies

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