Phytolith solubility in paddy soil
Final Report Abstract
Phytoliths serve as an important source of plant-available Si because of their high solubility. However, some phytoliths can persist in soils for hundreds of years, which is evidenced by the fact that archaeologists use phytolith morphology to reconstruct past vegetation. Previous work suggests that phytolith solubility decreases during aging in soil, however, the factors controlling solubility are hardly understood. We hypothesized that (i) redox oscillations promote the accumulation of Fe and Al oxides and organic matter at phytolith surfaces and (ii) these surface alterations reduce the solubility of phytoliths over time. To test these hypotheses, we conducted laboratory and field studies to analyze the alterations of phytolith surface properties during exposure to soil solutions and natural soil environments differing in redox conditions. In a laboratory experiment we repeatedly exposed phytoliths to soil solutions with oscillating redox potentials versus continuously oxic soil solutions. Exposed phytoliths were analyzed for morphology, surface charge, chemical composition, and solubility. Iron oxides and organic matter accumulated rapidly at phytolith surfaces exposed to oscillating redox conditions. Other phytolith properties including Al content, specific surface area, and zeta potential showed only minor changes after the exposure experiments. Phytolith dissolution rates decreased with the number of exposure steps in both continuously oxic and oscillating redox potentials, but the decrease was more pronounced in phytoliths exposed to redox oscillations (factor 3.2) compared to consecutive oxic exposures (factor 2.1). The Si release rate was negatively related to the total phytolith Fe content in phytoliths exposed to redox oscillations. In a subsequent field experiment at different sites in Vietnam and the Philippines, phytoliths were buried in topsoil environments in paddy fields and nearby non-paddy fields for up to 550 days. Phytoliths were recovered from soils at different time steps and analyzed for mass balance, morphology, surface charge and chemical composition as well as solubility. Phytoliths dissolved more than three times faster in paddy than in non-paddy soils. Despite significant dissolution of up to 29 % of the material the changes in phytolith properties were surprisingly little. Specific surface area, median particle size, and zeta potential in both paddy and non-paddy soils were similar to fresh phytoliths. Phytoliths rapidly accumulated small amounts of Fe (between 1.0–3.5 g kg–^1) and Al (between 1.0–4.5 g kg^–1) within 50 days, but there was no significant further accumulation of both Fe and Al during the remaining exposure time in soil. Most surprisingly, accumulation of Fe was unrelated to management type, i.e. paddy and non-paddy soils. We therefore conclude that Fe coatings on phytoliths are not stable against reductive dissolution under submerged conditions. Dissolution rates measured in the laboratory decreased with time in the field, again the decrease was unrelated to the management type. Laboratory dissolution rates were an order of magnitude greater than those derived from the mass balance in the field, highlighting the need for field studies to better understand the Si cycle in paddy fields.
Publications
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Effect of different extraction methods on the morphological and chemical properties of phytoliths. EGU General Assembly, Vienna, April 2019.
Nicolai Koebernick; Thimo Klotzbücher; Maximilian Wolff; Klaus Kaiser; Anika Klotzbücher & Robert Mikutta
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Redox-dependent surface passivation reduces phytolith solubility. Geoderma, 428, 116158.
Koebernick, Nicolai; Mikutta, Robert; Kaiser, Klaus; Klotzbücher, Anika & Klotzbücher, Thimo
