Dynamics in Phosphorus Pools in Soils and Sediments along the Land-Freshwater Continuum of Agricultural Catchments

Pronounced amounts of the precious nutrient phosphorus (P) are mobilized from arable soil due to erosion and surface run-off and are transferred into the receiving waters. If we are to mitigate P transfer from arable soil to surface waters, we must understand P pool dynamics not only in arable soil itself, but also within the interlinked P transfer continuum, including vegetated buffer strips and watercourses. We use laboratory incubation experiments based on an innovative isotopic labelling approach that uses the analysis of stable oxygen isotope ratios in phosphate to trace changes in P pools of soil and sediments under dynamic redox conditions. The outcomes will allow predicting the stability of respective storage pools in the long-term in dependence on their position within the P transfer continuum and the prevailing environmental conditions therein.

PhD: Christiane Nagel
PI: Dr. Harald Neidhardt

Funding: German Research Foundation

Subproject “Trophic-interaction, tree-diversity and soil-erosion effects on soil-plant stoichiometry“ in the research unit “Multi-Trophic Interactions in a Forest Biodiversity Experiment in China”

Nutrients are an essential link across trophic levels and networks. Tree diversity has been shown to influence nutrient cycles across trophic levels reaching from nutrient availability in soil via primary producers to consumers. Positive effects of tree diversity on the productivity of forests were attributed to a more exhaustive resource use (“nutrient overyielding”) and to more efficient recycling of diverse tree stands. In this project, we will test whether higher trophic levels, for example soil microorganisms that consume tree litter, share the same responses to tree diversity as lower trophic levels.

PhD: Haikuo Zhang
PI: Prof. Dr. Yvonne Oelmann
Co-PIs: Prof. Dr. Thomas Scholten, Dr. Steffen Seitz, University Tübingen, Germany

Cooperation partners: Prof. Dr. Naili Zhang, Beijing Forestry University, Dr. Yu Liang, Chinese Academy of Sciences, China
Funding: German Research Foundation

Effects of grassland management measures on ecosystem pools and processes within the “Offenhaltungsversuche Baden-Württemberg”

Because low-input semi-natural grasslands are threatened either by intensification or cessation of land use, conservation or restoration measures have been initiated aiming at maintaining or establishing species-rich grasslands. We assess effects of different management measures (mowing, mulching) and natural succession on nutrient cycles of grasslands across Baden-Württemberg. We focus on long-term effects enabled by the unique duration of the experiment of more than four decades.

PI: Prof. Dr. Yvonne Oelmann
Co-PIs: Prof. Dr. Peter Poschlod, University of Regensburg; Prof. Dr. Gabriele Broll, University of Osnabrück, Germany
Funding: Ministry of the Environment, Climate Protection and Energy Sector Baden-Württemberg

Biogeochemical cycling of phosphorus in floodplain aquifers

The role and fate of phosphate (PO₄) in groundwater ecosystems has only recently come into scientific focus. In contrast to common assumptions, groundwater can locally maintain considerable concentrations of PO₄. Remarkably high PO₄ concentrations were recently reported for groundwater of floodplain aquifers under the influence of reducing redox conditions. Yet sources and transformation pathways of PO₄ remain to be disentangled by means of the analysis of the stable isotope ratio of phosphate-bound oxygen (δ¹⁸O_P). We apply the δ¹⁸O_P analysis to characterize phosphate in groundwater and in different PO₄ pools within the aquifer material to improve our mechanistic understanding of the biogeochemical cycling of PO₄ in groundwater of floodplain aquifers.

PhD: Wen Shao 
PI: Prof. Dr. Yvonne Oelmann
Funding: CSC-Tübingen PhD Program

A novel method for analysing phosphorus-bound stable oxygen isotope ratios in groundwater ecosystems

The objective of this project is to develop and validate a methodological approach to reliably determine and interpret stable oxygen isotope composition of PO4 (δ¹⁸O_PO4) in solid (i.e. aquifer material) and aqueous (i.e. groundwater) phosphate (PO₄) pools in groundwater systems. This tool will be highly useful for isotopic labelling and incubation experiments, but also for the determination of natural isotopic abundances in groundwater and aquifer materials. In summary, the project will provide a novel methodological approach that will be highly useful to trace the biogeochemical cycling and fate of PO₄ in groundwater systems.

PI: PD Dr. Harald Neidhardt
Funding: German Research Foundation

Subproject “Nitrogen and phosphorus cycling“ in the research unit “Exploring mechanisms underlying the relationship between biodiversity and ecosystem functioning”

Plant diversity influences many ecosystem variables, such as productivity and nutrient cycling. However, only a few studies differentiated between initial (short-term) and longer-term effects of plant diversity. In this project, we assess the temporal course in terms of seasonality, variability, and long-term trends of the effect of plant diversity on element concentrations, fluxes, and (stoichiometric) ratios.

PostDoc: Dr. Eva Koller-France
PI: Prof. Dr. Yvonne Oelmann
Co-PI: Prof. Dr. Wolfgang Wilcke, KIT Karlsruhe, Germany

Funding: German Research Foundation

Subproject “Impacts of N/P inputs on organic P dynamics and P status in soils and plants along a nutrient gradient” in the priority program “Ecosystem nutrition: Forest strategies for limited phosphorus resources”

Atmospheric nitrogen (N) deposition has been shown to affect N cycling in forests with feedbacks on the phosphorus (P) cycle. We will determine which and to which extent soil biological processes in the forest ecosystem are affected by mineral N and P additions and how this further translates into plant nutrition. We use stable oxygen isotope ratios of phosphate as an innovative method to identify the source of forest P nutrition.

PhD student: Simon Hauenstein
PI: Prof. Dr. Yvonne Oelmann
Co-PIs: Dr. Thomas Pütz, Forschungszentrum Jülich, Germany; Prof. Dr. Emmanuel Frossard, ETH Zürich, Switzerland

Funding: German Research Foundation

Stable hydrogen isotope ratios in soil constituents: New insights into the formation of organic matter and clay minerals

Hydrogen is the most abundant element in the Universe. However, the use of the H isotopic composition (δ²H value) of soil to elucidate biogeochemical processes, apply it in environmental forensics or serve as a palaeo climate proxy is still in its infancy. Based on a steam equilibration method developed in our lab, our project will shed light on the incorporation of ambient water H into soil organic matter. These results will help to explain results observed in a preceding project namely the close relationship between δ²H values of precipitation and of soil organic matter.

PhD student: Arnim Kessler
PI: Prof. Dr. Yvonne Oelmann
Co-PI: Prof. Dr. Wolfgang Wilcke, KIT Karlsruhe, Germany

Funding: German Research Foundation

Subproject “Connecting the green and the grey world: an experimental approach to separating climate, vegetation, and geochemical effects on nutrient cycling along a climate gradient” in the priority program “EarthShape: Earth Surface Shaping by Biota”

One component why biota might shape the Earth’s surface is related to their active role as “weathering engine”. However, efficient nutrient cycling of both plants and soil microorganisms might reduce their need to access nutrients contained in bedrock and this might depend on climate decisive for the progress of weathering. We will first identify the nutrient/nutrient combination that plants and soil organisms require most (= growth-limiting nutrients) and then assess how efficiently these nutrients are used by organisms. To pinpoint causal relationships, we will combine a climate gradient and a climate manipulation experiment in Chile.

Postdoc: Rafaella Canessa Mesías
PI: Prof. Dr. Yvonne Oelmann, Dr. Harald Neidhardt
Co-PI: Prof. Dr. Katja Tielbörger, University of Tübingen, Germany

Cooperation partners: Prof. Lohengrin Cavieres, Prof. Felipe Aburto, University of Concepción, Chile
Funding: German Research Foundation