Phosphate sorption-desorption behavior in volcanic topsoils along a chronosequence (1.5 ka – 1070 ka) in the humid zone, Galápagos Islands

Soil phosphorus (P) is one of the main factors affecting ecosystem productivity. With progressing soil weathering, P can be increasingly immobilized and become the limiting nutrient in ecosystems. Volcanic soils are known for their exceptionally high phosphate (PO₄) retention capacity. However, the changes in PO₄ sorption behavior as their mineralogy evolves during pedogenic development, are still not fully understood. Short-term and longer-term PO₄ sorption-desorption behavior was studied in six volcanic topsoils (0 – 10 cm) from four Galápagos Islands along an age gradient (chronosequence, 1.5 – 1070 ka) under humid climate. Labile P (Mehlich-3 P, resin P), PO₄ sorption kinetics (4 h – 62 days), PO₄ sorption capacity (sorption isotherms, equilibration time = 72 h) and longer-term desorption (resin P after 1 and 6 month incubation, respectively) were analyzed. Soils developed very high PO₄ sorption capacity within 4.3 ka of soil weathering (Langmuir Q_max = 18.2 g P kg^-1) due to the development of amorphous soil constituents. As the colloidal fraction changed to 2:1-type crystalline clays after 26 ka of soil weathering, PO₄ sorption capacity declined rapidly while the labile P fraction reached a maximum. In older soils (≥ 165 ka), acidification and prevalence of Al und Fe (hydr)oxides led to increased P sorption again. Overall, soil PO₄ retention capacity was closely related to amorphous Si, Al and Fe phases; however, it did not predict P availability.