uni'wissen 01-2015

stimulates them to change their strategy? A total of 25 groups from soil science, forest science, microbiology, plant science, hydrology, and earth sciences are working together in the program to find out – six of them at the Faculty of Environ- ment and Natural Resources of the University of Freiburg. Lang’s team is studying the soil structure: Particles in the soil accumulate to form aggre- gates. Roots play a major role in this process. How this benefits the plant is not yet clear. “We believe it is important how phosphorus is distrib- uted in these aggregates,” says Lang. If the nutrient is on the surface the roots have an easier time absorbing it, but if it is inside it is better protected from being washed away. The team is planning on studying the distribu- tion of phosphorus in the aggregates later on. Up to now they have concentrated their efforts on measuring the size of the balls of particles at the five sites. It has become evident that the aggre- gates at the phosphorus-poor sites are smaller and more stable. A possible explanation for this is that nutrients from the humus layer released by microorganisms seep through pores that form between the aggregates. If fine roots enclose an aggregate, the plant can easily access the nutri- ents. “The flow of nutrients is greater in some of the pores, lower in others – just like expressways have more traffic than country roads,” says Krüger. Thus, it is possible that the plants direct the flow of nutrients to their own advantage. “But these are new ideas that will have to be proven.” The team is also responsible for preparing the soil samples for the 25 research groups and collecting data that is important for everyone involved in the project. This includes the soil’s pH, the amount and distribution of the roots, the proportion of rocks and soil per square meter, and the thickness of the humus layer. “Everyone receives comparable samples that are repre- sentative of the individual sites,” Krüger stresses. The team will write an interim report after three years and the final report after six – that is also part of their project coordination duties. This will involve filtering the most important findings out of all of the experiments conducted by the groups and combining them into a coherent whole. Insight for Forestry and Agriculture According to the Freiburg scientists, most of the findings from the first year of the project support the central hypothesis. One finding they consider important is that the organic material plays a key role at the sites with a low supply of phosphorus: The less phosphorus the mineral soil of a site contains, the stronger is the humus cover and the thicker the upper layer of roots in the soil below it. “A thick tangle of roots allows the plants to quickly reabsorb the phosphorus released from the humus before any of it is lost,” says Krüger. Another finding from the microbiology subprojects took them by surprise: “The researchers found completely different compositions of species of microorganisms at the rich and poor sites,” reports Lang. “Now they want to investigate what function the individual species have for the eco- system.” In the end, the program could lead to new in- sight for forestry management – for instance with regard to the question of how much biomass can be removed from the forest. “It used to be that only trunks and large branches were interesting, while a large part of the treetop, which contains a lot of phosphorus, was allowed to rot in the forest,” says Krüger. Today, by contrast, it has become common – for hauling reasons and to obtain more burnable biomass – to take entire trees, a practice that robs the forest of a lot of phosphorus. Scattering the ashes of the biomass “We want to determine which components and interactions of an ecosystem enable a closed phosphorus cycle.” depletion of phosphorus Acquiring Systems Recycling Systems The main hypothesis of the research program: As long as the soil contains a lot of phosphorus, forest ecosystems obtain most their supply from this mineral reserve. Most of the phosphorus from leaves, detritus, and dead plants is not reused and is therefore lost (left). In the long term, the supply of the nutrient in the soil is depleted. Then ecosystems change their strategy and recycle phosphorus from dead organic material. This leads to a closed cycle. Illustration: Department of Soil Ecology 38

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