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 Luss 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.
Highlights
Recent research interest in phosphorus (P) nutrition of European beech (Fagus sylvatica L.) forests is triggered by continued decreases in foliar P concentrations over the last decades (Jonard et al, 2015; Talkner et al, 2015)
The three main findings supporting rejection of the hypothesis are (i) addition of goethiteP adsorption complexes did not affect P uptake by plants leaf P contents were far below a deficiency level, (ii) addition of dissolved P did not enhance P uptake by beech as it was rapidly immobilized in soil by adsorption to minerals, and (iii) native P from the subsoil was not available to beech as, despite P limitation, beech leaves were only supplied with P by P redistribution within the plant tissue
In the conducted mesocosm experiment, we have shown that beech was not able to take up organic or inorganic P from a subsoil in which P was bound to pedogenic Fe and Al oxides, which provided free adsorption sites
Summary
Recent research interest in phosphorus (P) nutrition of European beech (Fagus sylvatica L.) forests is triggered by continued decreases in foliar P concentrations over the last decades (Jonard et al, 2015; Talkner et al, 2015). Their work suggests that, in forests with young soils, plants and soil organisms mobilize P mainly from primary minerals, and P losses are high (“acquiring strategy.” open P cycles), and in forests with mature soils, roots and microorganisms sustain their P demand mainly from the forest floor and soil horizons rich in organic matter, and P losses are low (“recycling strategy,” tight P cycles) Their large data set supports the hypothesis of Odum (1969) on the nutrition strategies of vegetation, which presumes that P cycles “tighten” during succession, meaning that P losses due to exports become lower due to an efficient uptake and recycling of P (Odum, 1969). This raises the question of whether P in subsoil is recycled or if it is unavailable for beech
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