| Penguin colonies from permanently cold environments have a strong impact on their surrounding ecosystem because their excrements provide ample nutrients to the soils and sediments. The high phosphate content of the penguin guano directly affects primary productivity. However, phosphate solubility is dependent on the presence of iron and other metals, which can form stable PO4-rich minerals. Phosphate can also be sorbed onto minerals, including iron oxides. The present study investigated the biogeochemistry of phosphorus in a 42 cm-deep soil profile on Gardiner Island in Antarctica in order to assess the effect of penguin excrements on P partitioning in the solid and aqueous phases. The results indicate that the porewaters were slightly acidic (pH 5-6) and contained extremely high levels of dissolved organic carbon (DOC; 120 mM), PO4 (120 mM), SO4 (27 mM), NO3 (18 mM), Cl (320 mM), F (2 mM), Sr (0.10 mM), Ca (18 mM) and Mg (150 mM) at the top of the soil profile. Dissolved iron concentrations were generally low (< 0.04 mM) and increased at a depth of 15-20 cm and at the bottom of the profile. Chemical extraction revealed the presence of two zones of reactive phosphorus (P-ascorbate extractable fraction) in the soil profile, i.e., at the surface and between 16 and 20 cm. Enriched reactive and crystalline iron fractions were also present at a depth of 16-20 cm, but fluctuated throughout the profile. The Fe(II)/Fe(III) molar ratio of the soil was greater than 1 at the surface of the profile and declined with depth. X-ray diffraction analysis showed that the soil likely contained berlinite, strengite and vivianite, along with silicates and quartz. Saturation index calculations also indicated that Ca and Mg-rich phosphate minerals were likely present in the soil. Based on the above results, the presence of penguin colonies on Gardiner Island strongly impacted the geochemical and mineralogical composition of the soil, as observed in other studies on bird guano impacted (ornithogenic) soils. In addition, the presence of both Fe(II) and Fe(III) points to the fact that the soil undergos redox changes, likely as a result of seasonal water table fluctuations. Microcosm experiments with selected samples from the soil profile and an iron-reducing bacterium indeed showed that iron and phosphorus were released into solution as a result of microbial iron reduction. However, abiotic systems also showed a release of phosphorus indicating that non Fe-rich phosphate minerals are soluble under the conditions prevailing in the growth medium. |
