Soil silicon: Plant-availability and biotic solubilisation

While many plants strongly accumulate silicon (Si) in their biomass, Si is no essential nutrient for most plants, but is beneficial in alleviating environmental stresses (plant diseases, herbivory, drought, metal toxicity). After Si uptake and translocation to the plant shoot, it is concentrated and precipitated in the leaves due to transpiration, forming amorphous SiO2 bodies (phytoliths). These phytoliths are returned to the soil in plant litter and harvest residues, and are, together with amorphous SiO2 precipitated from the soil porewater, the main bioavailable Si fraction in soil. By exporting phytoliths from the field to- gether with the crop and harvest residues (straw), and by increased erosion rates, agriculture has a strong impact on plant-available soil Si and the terrestrial Si cycle. Although there is currently a strong research focus on soil Si dynamics (e.g. global biogeochemical Si cycling, agricultural impact on bioavailable soil Si), many aspects of plant-soil Si interactions are not well understood. This applies particularly to the Si biogeochemistry of soils of the temperate region, and to the impact of biota, such as plant roots and earthworms, on labile soil Si fractions and soil Si solubilisation. There is a strong need for closing these knowledge gaps in order to avoid decreases in crop yields and quality, to enhance the sustainability of agricultural production in future, and to close the terrestrial and global Si cycles. Therefore this project aims (1) at using innovative infinite sink extraction methods combined with elemental and isotopic fingerprinting for quantifying plant-available Si pools and their mineral origin, (2) and at studying the effects of root exudates and (3) earthworms on soil Si solubilisation and plant-availability. 1

Silicon (Si) is no essential plant nutrient, but is beneficial in alleviating environmental stresses (e.g. diseases, drought). After Si uptake and translocation to the plant shoot, it is concentrated and precipitated in the leaves, forming amorphous SiO2 bodies, 'phytoliths'. These phytoliths are returned to the soil in plant litter and harvest residues. Together with soil-borne amorphous SiO2, they form the main plant-available soil Si fraction. By exporting Si from the field together with the harvest residues (straw), and by increasing the risk of erosion, agriculture has a strong impact on plant-available soil Si. Although there is currently a strong research focus on soil Si dynamics, many aspects of plant-soil Si interactions are not well understood. This applies particularly to the Si biogeochemistry of soils of the temperate region, and to the impact of soil organisms such as plant roots and earthworms, on labile soil Si fractions and Si plant availability. Therefore, this project aimed at (1) better understanding impact of land use and soil management on the plant available soil Si fraction and at studying the effects of (2) root exudates (substances released by roots into soil) and (3) earthworm activity on soil Si solubilisation and plant-availability. The results of this study show that there is considerable variability of the plant available Si content in field soils. Silicon availability is affected by many factors such as lime, clay and amorphous Si content, land use (grassland/cropland) and soil management activities such as phosphorus fertilization and crop residue management. Furthermore, soil organism seem to increase soil Si availability. Several common root exudates substances substantially increased the Si solubility form soil and soil minerals. However, we did not observe that roots released exudates in soils with very low Si availability, indicating that plants do not actively forage for Si, as they do for some essential nutrients. Ingestion of soil by earthworms, which is a common feeding behaviour of these animals, increases the solubility of Si in soils. While this did not increase plant Si uptake in our experiments, continuous earthworm activity in natural ecosystems with high earthworm density may contribute to Si availability to plants. The activity of soil organisms therefore apparently plays a role in Si cycling in soils. Silicon fertilization is being increasingly discussed as a measure to enhance crop stress tolerance, especially in a warming climate. Subsequent to this project, the project team and collaborating scientist and farmers have already started basic research on the physiological role of Si in plant metabolism as well as practise-oriented development and testing of Si fertilizers and fertilization systems. The results of this project provide a valuable knowledge base for these and other follow-up projects.