Root-induced cycling of lead in salt marsh sediments

A gold-mercury amalgam microelectrode was used in situ to measure Pb(II) by anodic stripping voltammetry and O2, Fe(II), Mn(II), and HS- by square-wave voltammetry in sediment pore water in a Haliomione portulacoides stand in a Tagus estuary salt marsh. The measurements were made in spring, summer, and fall, and were supplemented with analysis of Pb in solid phases and stable isotope analysis of Pb. In spring, the pore water was anoxic, Fe(II) reached concentrations as high as 1700 micromol/L, and Pb(II) was undetectable (<0.1 micromol/L). However, in summer, the pore water was oxic, Fe(II) was undetectable, and Pb(II) was present throughout the 20 cm deep root zone in concentrations reaching 6 micromol/L. In fall, low levels of O2 and Pb(II) were detected in the upper half of the root zone, and low concentrations of Fe(II) were detected in the lower half. The annual cycle of Pb is controlled by the growth and decay of roots. Roots deliver oxygen, which oxidizes lead-bearing solid phases and releases Pb(II) to the sediment pore water. Iron oxides, which form in the rhizosphere when Fe(II) is oxidized, are apparently not efficient sorbents for Pb(II) under the organic-rich conditions in this sediment. This allows Pb(II) to remain soluble and available for uptake by the roots. In fall and winter,when roots decay and the oxygen flux to the sediment stops, Pb is released from the decaying roots and returned to and precipitated in the anoxic sediment, likely as a sulfide. On an annual basis more than 20% of the total mass of Pb in the root zone cycles between root tissue and inorganic sediment phases. Depending on location, anthropogenic Pb constitutes 30-90% of total Pb in Tagus Estuary salt marshes.