ROOT SURFACES OF AQUATIC PLANTS ARE AN OVERLOOKED SITE OF IRON AND SULFUR IMMOBILIZATION

In freshwater systems, root surfaces are a dynamic site of iron and sulfur cycling. Aquatic plants rooted in anoxic sediment release oxygen from their roots (radial oxygen loss), converting soluble ferrous iron to insoluble iron (hydr)oxides. This mechanism precipitates iron (hydr)oxides onto root surfaces and controls phosphorus and sulfur mobility and reactivity near roots. When sulfate is elevated in freshwater ecosystems, plants sometimes develop iron sulfide plaques on root surfaces. Using experimental pots containing a single wild rice plant each, we quantified temporal patterns of iron oxide and iron sulfide on root surfaces in high (300 mg/L) and low (~10 mg/L) sulfate conditions. The distribution and speciation of iron and sulfur changed throughout the growing season and especially during life stage transitions of the plant. We propose a mechanism by which iron oxides are reduced and mobilized from root surfaces during life-stage transitions that initiates an accumulation of sulfur and ferrous iron on root surfaces in sulfur-impacted freshwater sediment. More than half of the ferric iron on root surfaces was lost during the start of seed production regardless of high or low sulfate conditions, suggesting that plant mechanisms control both immobilization and mobilization of iron in the rhizosphere. To provide context for the quantities of sulfur and iron amassed at the root surface, we compared the rhizosphere mass balance of sulfur in high and low sulfate conditions. In high sulfate conditions, root surface sulfide is ~1 to 8 % of the overall sulfur budget in amended sediment but is <1 % in low sulfate conditions. Near the end of the growing season, sulfide was ~50x more concentrated on root surfaces compared to adjacent sediment in high sulfate conditions (normalized by mass). Porewater sulfide was a small component of the mass balance, and the remainder of the sulfur was split between porewater sulfate and sediment AVS, although the proportion of each varied throughout the plant’s life cycle. We suggest that in vegetated aquatic ecosystems, it may be worthwhile to include a root surface component of iron and sulfur budgets and to consider transient pools on time scales corresponding to plant development.