Increased silicon concentration in fen peat leads to a release of iron and phosphate and changes in the composition of dissolved organic matter

Prior studies have shown that silicon (Si) increases the dissolution of iron (Fe) phosphates, releasing phosphorus (P) into the soil solution of fen peat and thereby changing the composition of organic matter (OM). We formulated three questions on the occurring processes: [i] Does high Si concentration cause a mobilization of Fe phosphates? [ii] Does this mobilization result in a higher concentration of P, Fe, and dissolved organic carbon? [iii] Are there changes in the composition of organic matter? To this end, we conducted a laboratory experiment adding amorphous silica to fen peat and analyzed Fe mineralogy and carbon quality of the peat material by X-ray absorption spectroscopy (XAS). Under high Si concentration, significantly higher concentrations of P and Fe were found in soil solution. Moreover, XAS data indicated a dissolution of Fe (II) phosphates and a change in the composition of OM available to microbes with decreasing contributions of carboxylic groups and quinones. We propose to explain the effects as follows: under high Si concentration Fe (II) phosphates dissolve and OM rich in carboxylic groups are mobilized. As the nutrient concentration in the soil solution increased, the degradation of oxidized components of the OM, such as quinones and carboxylic groups is enhanced.