Is higher silicon availability increasing the mineralization of organic matter in fen peatlands?

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The Si cycle is a globally important biogeochemical cycle that directly interacts with major long-term global C sinks. Si availability in ecosystems differs, depending on ecosystem type, bedrock, vegetation and other factors. Recent research has shown that Si availability is also a crucial factor determining organic matter nutrient content, nutrient stoichiometry and litter recalcitrance, especially in grass vegetation. Moreover, Si can affect organic matter decomposition rates by changing the microbial decomposer community in grass systems. Lower nutrient (N, P) content and higher stoichiometric nutrient to phosphorus ratios are well-known to have negative effects on litter decomposition. However, in recent experiments, higher Si content was able to override this effect, with litter from plants exposed to high Si availability degrading faster than controls, even for nutrient-poor litter. The same effect was already confirmed in a field experiment, in which Si decoupled litter decomposition and fungal biomass: While decomposition rates increased, fungal biomass decreased. Consequently, Si exerts an important control on C turnover and on the microbial decomposer community in grass dominated systems. Fen peatlands, providing important carbon stocks and being relevant in global greenhouse gas emissions, are such grass dominated systems with potential impact of Si on C turnover. A preliminary study examining the effect of Si on C-turnover in a fen ecosystem showed enhanced C-turnover and a doubling of soil methane concentration in Si fertilized plots. However, along with the increase in C-turnover a mobilization of P was found. Increasing P availability is well known to also increase C-turnover. The aim of the proposed study is to disentangle the direct effects of Si on C-turnover from the indirect effects via increasing P availability. Our hypothesis are (i) higher Si availability will enhance the mineralization of organic matter (fen peat) to both CO2 and CH4, via (ii) direct Si effects and indirect Si effects by increasing P availability, (iii) the higher mineralization rate of organic matter to both CO2 and CH4 due to increased availability of Si will be achieved rather by bacteria than by fungi, and (iv) increased Si availability will increase P and N availability and uptake into plant biomass and thus lead to more labile litter. Understanding the mechanism how Si is affecting C-turnover in fen systems will contribute significantly to the general understanding of the terrestrial C-turnover.