Goethite-bound phosphorus in an acidic subsoil is not available to beech (Fagus sylvatica L.)

Abstract

In forests, where the supply of bioavailable phosphorus (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 the subsoil, where it is strongly adsorbed onto the reactive surfaces of pedogenic Fe and Al oxides. This raises the question of whether P leached into subsoil is also recycled. 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 beech forest) with added goethite-P adsorption complexes, in either inorganic (orthophosphate) or 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 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 before transplanting 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 or hardly recycled, which can be an explanation for field data demonstrating quantitatively significant stocks of P in the subsoil of P-deficient forests.