Phosphate sorption-desorption properties in volcanic topsoils along a chronosequence and a climatic gradient on the Galápagos Islands

Abstract

Background: Phosphorus (P) retention behavior in the soil is one of the main factors driving ecosystem productivity and changes strongly during soil weathering. Volcanic soils are known for their high phosphate (PO4) sorption capacity (PSC). However, the changes of PO4 buffering properties and fixation processes in the course of their pedogenic development are not fully understood. Aim: We investigated the short-term and longer-term PO4 sorption-desorption behavior in volcanic topsoils from the Galápagos Islands along a precipitation gradient (elevation sequence, 47–866 m asl) and a soil age gradient (chronosequence, 1.45–1070 ka) under humid climate. Methods: Labile P (Mehlich-3, resin-P), P desorption (resin-desorbable P after 1 and 6 months incubation), PSC (sorption isotherms, equilibration time = 72 h) and sorption kinetics (4 h–62 days) were analyzed and compared against a broad range of topsoil parameters. Results: Soils developed very high PSC within 4.3 ka of soil weathering (Langmuir Qmax = 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 under humid climate, PSC declined rapidly, while labile P and resin-desorbable P reached a maximum. In older topsoils (≥ 165 ka), acidification and prevalence of aluminium (Al) and iron (Fe) (hydr)oxides led to increased P sorption again. Along the elevation sequence, increased precipitation favored the formation of amorphous constituents, resulting in a steep increase of P sorption capacity (from 0.9 to 19.9 g P kg–1) from the arid lowlands to the very humid highlands. Conclusion: Amorphous Fe and Al phases as well as Fe and Al bound to organic matter seemed to be the main driving factors for P sorption along the studied pedogenic gradients.