How silicon affects phosphate availability and carbon turnover in soils

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Phosphorus (P) is a key element for life's structure and metabolic pathways. Phosphorus is one of the most limiting elements for primary production and yield of crop plants, resulting in the use of large amounts of P fertilizers in agriculture at the global scale. Whereas the total P content of soils is not particularly low, a large fraction is stored in plant unavailable form like organic phosphorus, or is bound/adsorbed to Al-Fe or Ca minerals depending on soil pH. Due to limited resources, reducing P fertilization at constant yield is one of the main challenges for agriculture in future.Recent studies indicate that the highest P availability coincides with highest share of silicate bound P. Thus, we aim to disentangle the importance of available silicon (Si) in soils for mobilization of P from the binding sites of Al and Fe (hydr)oxides and Ca minerals for a large range of minerals and soils with different binding forms of P in the proposed project. Si availability varies over at least two orders of magnitude in soils and sediments, ranging from 0.01 to 2.0 mM. Recent research also suggests the potential of Si affecting P availability in soils and respiration in soils. Our preliminary results indicate a potentially large effect of Si availability in peat on carbon turnover rates by a mechanism wherein Si mobilizes P from Fe binding sites. This indicates a new but nearly unstudied link between Si and C-cycling that is potentially important for global C turnover. Most importantly for the proposed project: first results revealed a significant increase of CO2 and CH4 concentrations in peat soils under increased Si availability. We also found a strong increase of P and dissolved organic carbon (DOC) mobilization to pore water by increased Si availability. Further analysis indicated that Si interferes with the Fe-phases present in the soil and possibly influences phosphate binding by these Fe-phases. The aim of the proposed study is to disentangle whether and to which extend (i) increased Si availability will lead to mobilization of P from mineral binding sites and therefore increases P availability, (ii) Si has higher affinity for binding sites of soil mineral as compared to P and C, (iii) higher Si availability will enhance the mineralization of organic matter due to increasing P availability resulting in mobilization of C (i.e. increased DOC) from the binding sites, and (iv) Si will interfere with P availability at the soil pH range from 4 to 8. Understanding how Si interferes with P availability is a potential tool to reduce the agricultural need for P fertilizer for optimal P supply.