Chemical status of zinc in plant phytoliths: Impact of burning and (paleo)environmental implications

Fire effects on phytolith carbon sequestration

Phytolith have been recognized as an important soil bioavailable Si source for plants, as well as a sink of C and heavy metals in soils. Though the impacts of fire and heat on phytolith sequestration of some nutrients (phosphorus, potassium) and heavy metals have been addressed, little attention has been paid to fire's effects on phytolith carbon sequestration. In this study, the carbon and dissolved Si content of phytoliths extracted from 6 common grass species and their burned ashes, as well as phytoliths collected from different areas (burned, transitional, and unburned) of a pine forest, were compared to characterize the effects of open fire on phytolith carbon content, solubility, and carbon sequestration. The carbon content and Si dissolution of ashed phytoliths varied between plant species, and differed with phytoliths from modern plants. The topsoil phytoliths had increased carbon content, and generally decreased solubility across the gradient of unburned, transitional, and burned pine forest. We therefore conclude that open fire can cause changes in phytolith related carbon content and solubility, as well as its preservation in soils. This study provides new perspective on the effects of open fire on phytolith carbon sequestration and its estimation.

Elemental composition of grass phytoliths: Environmental control and effect on dissolution

Plant phytoliths, which represent the main pool of silica (Si) in the form of hydrous Si oxide, are capable of providing valuable information on different aspect of environmental issues including paleo-environmental reconstruction and agricultural sustainability. Phytoliths may have different chemical composition, which, in turn, affects their preservation in soils ad impacts terrestrial cycle of the occluded elements including micro-nutrients and environmental toxicants. Yet, in contrast to sizable work devoted to phytoliths formation, dissolution and physico-chemical properties, the mechanisms that control total (major and trace) elemental composition and the impact that various elements exert on phytolith reactivity and preservation in soils remains poorly known. In order to fil this gap in knowledge, here we combined two different approaches - analytical trace element geochemistry and experimental physical chemistry. First, we assessed full elemental composition of phytoliths from different plants via measuring major and trace elements in 9 samples of grasses collected in northern Eurasia during different seasons, 18 grasses from Siberian regions, and 4 typical Si-concentrating plants (horsetail, larch, elm and tree fern). We further assessed the dissolution rates of phytoliths exhibiting drastically different concentrations of trace metals. In the European grasses, the variations of phytolith chemical composition among species were highly superior to the variations across vegetative season. Compared to European samples, Siberian grass phytoliths were impoverished in Ca and Sr, exhibited similar concentrations of Li, B, Na, Mg, K, V, Zn, Ni, Mo, As, Ba, and U, and were strongly enriched (x 100-1000) in lithogenic elements (trivalent and tetravalent hydrolysates), P, Mn, Fe and divalent metals. Overall, the variations in elemental composition between different species of the same region were lower compared to variations of the same species from distant regions. The main factors controlling phytoliths elemental composition are the far-range atmospheric (dust) transfer, climatic conditions (humidity), and, in a lesser degree, local lithology and anthropogenic pollution. Despite significant, up to 3 orders of magnitude, difference in TE composition of grass and other plant phytoliths, the dissolution rates of grass phytoliths measured in this study were similar, within the experimental uncertainty, to those of other plants studied in former works. Therefore, elemental composition of phytoliths has relatively minor impact on their preservation in soils.