Autor:innen:
Peter Uhuegbue | Leibniz Centre for Agricultural Landscape Research, ZALF, Silicon Biogeochemistry Group, Eberswalder str. 84, 15374 Müncheberg | Germany
Prof. Dr. Karsten Kalbitz | Institute of Soil Science and Site Ecology, Technische Universität Dresden, Pienner Str. 19, 01737 Tharandt | Germany
Dr. Mathias Stein | Leibniz Centre for Agricultural Landscape Research, ZALF, Silicon Biogeochemistry Group, Eberswalder str. 84, 15374 Müncheberg | Germany
Prof. Dr. Martin Obst | Bayreuth Centre for Ecology and Environmental Research (Bayceer), Universität Bayreuth, Dr. Hans-Frisch-Straße 1-3, 95448 Bayreuth
Dr. Jörg Schaller | Leibniz Centre for Agricultural Landscape Research, ZALF, Silicon Biogeochemistry Group, Eberswalder str. 84, 15374 Müncheberg
In soils, phosphorus (P) is important for metabolic pathways and carbon (C) turnover. The availability of P in soils has a significant impact on terrestrial ecosystems' primary productivity. However, P limitation occurs in many soils as it is tightly adsorbed to mineral surfaces, and thus inaccessible to plants. Silicon (Si) can mobilize P from strong binding sites as shown for arctic and paddy soils. Furthermore, Si competes also with dissolved organic carbon (DOC) for binding. But the interdependent mechanisms are still poorly understood. This study aims to elucidate the mechanism of the interdependency of P, DOC, and Si on the binding to goethite and hematite. The experiment was conducted at pH 5 and 7, because changes in pH values cause a change in the surface charge of Fe minerals, altering the surface properties. Here, we used goethite and hematite synthesized in our labs. The minerals were preloaded with 2 mM of PO43-. For subsequent mobilization, two different forms of Si were used: 100 mg hydrophilic fumed silica and 50 mg monomeric silicic acid. The samples were left to incubate for 30 days. Sampling was conducted at five different dates (after 0.2, 1, 3, and 30 days). Scanning transmission X-ray microscopy (STXM), an approach that employs spatially resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to analyze directly and indirectly the binding mechanisms of Si, P, and DOC to the Fe-oxides and their interdependency. The result of the adsorption experiment revealed that about 0.23 mg L-1 and 0.25 mg L-1 phosphate were adsorbed after 24 hours at pH 5 and 7 on goethite, respectively. After 30 days of incubation, the amorphous Si released 12 mg L-1 and 99 mg L-1 Si (Si < 200 nm) into the solution at pH 5 and 7, respectively. This resulted in the mobilization of about 0.02 mg L-1 and 0.06 mg L-1 P which is more than the amount released in the control (< 0.001 mg L-1 P) at both pH values from the goethite surface. Compared to the reference (where no Si was added to P suspension), a release of 0.03 mg P was observed at pH 7. At pH 5 however, a higher P release (0.1 mg L-1 P) was observed. In conclusion, it should be noted that silicon has a significant role in controlling P mobility in Fe minerals, which is important for the management of P availability in soils.