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The leak in the phosphorus cycle . exploring the mechanisms and controls of phos-phorus leaching in soils of acquiring and recycling forest ecosystems

English title The leak in the phosphorus cycle . exploring the mechanisms and controls of phos-phorus leaching in soils of acquiring and recycling forest ecosystems
Applicant Hagedorn Frank
Number 171171
Funding scheme Project funding (Div. I-III)
Research institution Swiss Federal Research Inst. WSL Direktion
Institution of higher education Swiss Federal Institute for Forest, Snow and Landscape Research - WSL
Main discipline Pedology
Start/End 01.06.2017 - 31.05.2021
Approved amount 268'611.00
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All Disciplines (2)

Discipline
Pedology
Geochemistry

Keywords (6)

forest; leaching; nutrient cycling; soil fertility; isotope; phosphorus

Lay Summary (German)

Lead
Phosphor (P) ist ein essentieller Nährstoff für Pflanzen. In Waldböden ist jedoch nur ein kleiner Anteil, das freie gelöste Phosphat, direkt pflanzenverfügbar. Der grösste Teil liegt in schlecht verfügbaren Formen vor, entweder ist es an Minerale gebunden oder es liegt als organischer Phosphor vor. Ein kleiner Anteil dieses organischen Phosphors wird im Bodenprofil verlagert und ausgewaschen, entweder als Phosphat oder als „dissolved organic phosphorus“ (DOP). Obwohl der P-Fluss gering ist, kann er langfristig zur P-Verarmung von Böden führen. Allerdings ist bisher noch wenig über quantitative Bedeutung und steuernden Faktoren der P-Flüsse in Böden bekannt.
Lay summary

Inhalt und Ziel des Forschungsprojekts

Das Forschungsvorhaben hat zum Ziel, zu klären, welche Mechanismen und Faktoren die Mobilisierung und Auswaschung von P in Waldökosystemen steuern. Im Rahmen des Schwerpunktprograms der Deutschen Forschungsgemeinschaft ‚Ökosystemernährung‘ werden in diesem Projekt (1) in einem Düngungsexperiment mit Phosphor und Stickstoff im Wald, die gelösten P-Formen sowie deren Bioverfügbarkeit gemessen, (2) in Mikrokosmenversuchen mit organischen Substanzen unterschiedlicher Qualität die steuernden biotischen und abiotischen Faktoren identifiziert und (3) mittels 13C-markierter Komponenten der Umsatz von niedermolekularem DOP bestimmt.

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts

Dieses Projekt soll die Bedeutung von Phosphorflüssen im Nährstoffkreislaufs von Waldökosystemen ermitteln. Da viele Ökosysteme zunehmend durch Phosphor limitiert sind, können die Erkenntnisse über ein verbessertes Prozessverständnis zu einem nachhaltigen und zukunftsorientierten Waldmanagement beitragen.

Direct link to Lay Summary Last update: 01.06.2017

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Abstract

Phosphorus (P) is a major nutrient for plants. There are strong indications for an increasing P scarcity in forest ecosystems. During ecosystem development, forest ecosystems gradually change from the acquisition of rock-derived P to the recycling of organic P. Much of the cycling takes place in the organic forest floor, thereby releasing orthophosphate and dissolved organic P (DOP), which partly becomes leached into the mineral soil. Our results obtained during phase 1 of SPP 1685 indicate decreasing leaching of orthophosphate from organic layers with increasing recycling tendency of the forest ecosystems. In comparison, DOP does not decrease much. Most DOP is not enzymatic hydrolysable, and consequently bioavailability is little. Orthophosphate decreases with depth in the mineral soil; the decline in DOP is less strong. Likely, DOP contributes substantially to the steady P loss from all forest ecosystems, but also orthophosphate is not retained completely in the mineral soil. Yet, the factors controlling the differential production of dissolved P in organic layers of acquiring and recycling forest ecosystems are not resolved. Also, the causes for the steady leaching of dissolved P are not fully understood.The proposed work aims at exploring the mechanisms and controls of the mobilization and leaching of P in forest ecosystems.Phosphorus mobilization in the organic layer will be assessed by measuring the release of P forms and DOP bioavailability in the SPP-fertilization experiment and in microcosms with organic matter types of different quality. In addition, we will trace the fate of 13C-labelled compounds to determine DOP turnover and the rate limiting steps in the mobilization process. By measuring P leaching with and without nutrient fertilization and from substrates with differing stoichiometry (C:P:N ratios), we can identify if DOP production is primarily driven by P availability or if it is rather a byproduct of soil organic matter cycling. In climate chambers, we will estimate the temperature dependency of P mobilization processes, which is a key factor in the P cycle.One possible cause for the mobility of dissolved P is colloidal transport. We, therefore, will analyze soil solutions for colloids, using ultracentrifugation. Binding to colloids could be also an explanation for the poor enzymatic hydrolyzability of DOP. The possible release of DOP from soil matrix-bound organic P compounds in subsoils will be tested by separating DOP in P-rich and P-poor fractions, combined with 14C analyzes. Characterization of dissolved P will be carried out with 31-P-NMR, which is, however, often limited by the small amounts of P in subsoil solutions. Therefore, we will employ XPS with Ag anode excitation to determine P binding forms in small samples. Part of the project will be dedicated to the development of this new method.In order to generalize the obtained results, we will determine dissolved P in soil waters sampled sporadically at a number of additional forest sites. Finally, we will contribute to the synthesis on transport- and flux-related results within SPP 1685, which is a prerequisite for the modeling of the P cycle in forest ecosystems.
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