Controls on phytolith stability upon exposure in paddy soils

Phytoliths are an important component in the cycling of silicon (Si) in rice cultivation, yet little is known about their medium to long-term stability. While it is commonly accepted that phytolith solubility in soil decreases with time, the mechanisms that cause this decrease remain unclear. Most studies on phytolith aging to date have been conducted under laboratory conditions and field studies are rare. Here, we present a comprehensive field study of phytolith aging in paddy and non-paddy fields in the Philippines and Vietnam. Phytoliths extracted from rice straw were placed in mesh bags and exposed to ambient soil conditions for up to 550 days. Following exposure, total mass loss and changes in physical (particle size, specific surface area, ζ potential) and chemical (surface chemical composition by acid digestion and X-ray photoelectron spectroscopy) properties as well as dissolution kinetics (56 days in 0.1 M CaCl2 at pH 6.5) were determined. Phytolith dissolution in the field was rapid (up to 30 % mass loss) and three times faster in paddy than in non-paddy soils. Differences in phytolith properties were unexpectedly little between paddy and non-paddy soils. Laboratory and field-derived dissolution rates decreased with exposure time but were one order of magnitude lower in the field. While laboratory dissolution rates were negatively related to surface Al contents this was not observed for field-exposed phytoliths. Our findings suggest that under natural field conditions, phytolith dissolution is overwhelmingly governed by water regime, while the influence of protective surface coatings is only minor. We conclude that reliable estimates of phytolith dissolution can be best obtained in field experiments.