Seasonal dynamics of dissolved silicon in a rice cropping system after straw incorporation

Rice is an important staple for over half the world's population, and silicon (Si) is a vital nutrient that helps to improve yields through its role in alleviating biotic and abiotic stresses. Despite Si being abundant in crustal materials, it is only slowly released to soil pore-waters through chemical weathering. However, biocycling of Si through plant material (i.e. phytoliths) and back into soil is comparatively rapid, and thus may exert a dominant control on the biogeochemical cycling of Si within soils and near-surface sediments in some environments. Despite the potential importance of this pathway, Si cycling is poorly resolved in cultivated systems, such as rice cropping. Here, we monitored seasonal trends of Si in pore-water, plants, and soil over a two-year period in a California rice cropping system where straw is incorporated into the soil during the fallow season. There was a clear seasonal trend of high pore-water Si concentrations during the winter fallow that approached predicted equilibrium with amorphous Si, followed by low concentrations during the growing season within the top 20 cm of the profile. The seasonal change in Ge/Si ratios from low values during the winter fallow to high values-up to 36 μmol mol−1-during the growing season was due to a greater change in Si concentrations rather than Ge concentrations. These data indicate a low-[Ge], high-[Si] source of Si during the winter fallow, which may be due to incorporation of rice straw (a low-[Ge], high-[Si] source) and subsequent phytolith dissolution. Our results suggest that incorporation of high-[Si] plant material (e.g. straw) releases additional Si to soil pore-waters that is available for plant-uptake during the growing season.