Methane fluxes but not respiratory carbon dioxide fluxes altered under Si amendment during drying – rewetting cycles in fen peat mesocosms

Abstract Peatlands are globally important sinks and sources for greenhouse gasses. Their carbon (C) balances are dependent on input of biomass, its quality and decomposition. Silicon (Si) is known to influence the C quality, especially of graminoids, and it influences decomposition under oxic conditions. Drying – rewetting events (D/W), which may influence C mineralization and change nutrient availability, have been predicted to increase in frequency. This raises the question if effects of Si on decomposition are modified by such D/W events. To elucidate this question, we conducted a study with mesocosms extracted from a fen peatland. We hypothesized, that (I, II) there would be higher gas and solute concentrations after Si addition compared to the control after D/W. Further we expected (III) that higher respiratory fluxes of CO2 and CH4 would occur after Si addition and D/W and that (IV) there would be a higher gross primary production (GPP) under Si addition due to more viable biomass that is less prone to drought effects. Finally we hypothesized that (V) the chemical composition of Carex rostrata Stokes leaves grown under Si fertilization after a D/W cycle would be less recalcitrant compared to of C. rostrata leaves from the controls. To test these hypotheses, a control and a Si treatment group of mesocosms were subjected to a controlled D/W cycle. We measured soil gas (DIC and CH4), and solute concentrations (P, Si, Fe and TOC) and gas fluxes of CO2 and CH4. Furthermore, we characterized the graminoid biomass for major element contents and by infrared spectroscopy (FTIR) for organic matter quality. Higher CH4 fluxes were observed in the Si fertilized mesocosms. There was no difference between the treatments in the ER CO2 fluxes, whereas the NEE and GPP fluxes indicated higher CO2 uptake in the Si treated mesocosms. Solute concentrations showed no differences, neither for dissolved gasses , nor for solutes. C. rostrata leaves had lower shares of lignin, waxes, fats, and phenols in plants from the Si fertilized mesocosms. We concluded, that during D/W, plants from Si fertilized mesocosms accumulated more Si than plants from the control mesocosms. These plants were more viable, produced potentially more exudates and had more readily decomposable litter than controls, due to their lower share in recalcitrant moieties. This may have stimulated CH4 production and emission of CH4 from the soil into the atmosphere through abundant aerenchyma roots, bypassing oxic, methanotrophic peat layers.