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uni'wissen 01-2015

Phosphorus is an essential nutrient for plants. It helps them to produce adenosine triphos- phate, ATP for short, during photosynthesis as a source of energy for their cells. They make it available by releasing acids that disintegrate phosphoric rock grate from their roots. But at the same time another process takes place: Phos- phorus is washed out of the soil with the rainwa- ter and flows down streams and rivers to the sea. In this way, the supply of phosphorus is depleted in the long run in many continental ecosystems across the world. The plants need to find a way to reuse the nutrient before it is lost. “We are testing the hypothesis that forest ecosystems in phosphorus-poor locations have developed an especially efficient recycling system to compen- sate for the deficiency,” says Prof. Dr. Friederike Lang. The University of Freiburg soil ecologist is director of the German Research Foundation priority program “Ecosystem Nutrition: Forest Strategies for Limited Phosphorus Resources,” which began work at the end of 2013. Leaves, Detritus, and Dead Plants Rock isn’t the only source of phosphorus for the ecosystems. The nutrient is also present in leaves, detritus, and dead plants – the layer of humus above the mineral soil. The researchers hypothesize that this organic material becomes more and more important over time as a reposi- tory for phosphorus. “Ecosystems in locations with phosphorus-rich soil evidently have the initial strategy of obtaining as much as possible from the mineral reserve. Since there is enough available, they don’t need to recycle anything, and the losses are great,” explains Lang. If the symbiotic community behaves like this over thousands of years, however, the soil ends up losing nutrients. “Then the plants begin to organize a closed nutrient cycle: They take in phosphorus from the organic material and see to it that as little as possible is washed out.” The scientists are doing research at five sites in Germany at which forest research insti- tutes have been studying nutrient flow for almost 30 years. The sites are similar in many respects: They have the same rocks and the same acidic soil, they are at roughly the same altitude, and the predominant tree species in all of them is the beech. However, the phosphorus content in the mineral soil decreases from site to site. That of the richest and the poorest differ by a factor of 40. “This approach makes the results comparable over a long period of time and allows us to draw inferences on how plants react when more or less phosphorus is available,” says Jaane Krüger, the University of Freiburg research assistant coordinating the program. Help from Microorganisms and Fungi What is clear is that the trees succeed in ob- taining enough phosphorus, even at the sites where less is available: The amount in leaves and needles is the same everywhere thanks to microorganisms and so-called mycorrhizal fungi. They decompose dead organic material, thus making the phosphorus available to the plants. The microorganisms release the nutrient into the soil, where it can be absorbed by roots. Mycor- rhizal fungi, on the other hand, cover the tips of the roots with a thick network of filaments, supply the plants directly with phosphorus, and receive sugar from them in return. So is the hypothesis on the different behaviors of forest ecosystems correct, and what precisely p antts – the layer of ll. The researchers material becomes er time as a reposi- stems in locations dently have the as much as ervve. Since they don’t the losses e symbiotic er tthousands s up losing begin to ccle: They e organic s little as research at five rest rrese earch insti-- ient flow foror aalmost r in many respects: d th idi yp of forest ecosystems correct, and what precisely Roots, humus, rocks, and particles of various sizes: Friederike Lang’s team is providing all research groups participating in the program with soil samples. Photo: Thomas Kunz 37