Goethite-Bound Phosphorus in an Acidic Subsoil Is Not Available to Beech (Fagus sylvatica L.)
Anika Klotzbücher, Florian Schunck, Thimo Klotzbücher, Klaus Kaiser, Bruno Glaser, Marie Spohn, Meike Widdig, Robert Mikutta
Abstract
In forests, where supply of bioavailable P from easily weatherable primary minerals is small, plants are thought to recycle P efficiently by uptake of P released from decomposing forest floor material. Yet, a share of the P is leached into subsoil where it is strongly adsorbed onto reactive surfaces of pedogenic Fe and Al oxides. This raised the question whether P leached into subsoil is also recycled. In order to investigate the mobilization of P bound to hydrous Fe oxides, we conducted a mesocosm experiment in a greenhouse. Beech saplings were grown for 14 months in subsoil material (Bw horizon from the P-poor Lüss forest) with added goethite-P adsorption complexes, either in inorganic (orthophosphate) or in organic (phytate) form. Four types of control mesocosms were run: soil only and soil mixed with either dissolved orthophosphate or dissolved phytate or goethite. At the end of the experiment, neither total P mass in trees nor P contents in leaves differed between the treatments. According to leaf nutrient contents, plant growth was strongly limited by P in all treatments. Yet, total P mass in trees did not increase over the course of the experiment. Thus, despite of its P demand, beech was not able to acquire P from goethite surfaces within two vegetation periods. Also P added in dissolved form to the soil as well as native soil P were not available. This suggests that once inorganic and organic P is bound to pedogenic metal oxides in mineral soil, it is not recycled, which can be an explanation for field data demonstrating quantitatively significant stocks of P in subsoil of P deficient forests.