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Rhizospheric biofilms at root-microbe-mineral interfaces: A key to improve productivity, sustainability and CO2 balance in forests

Final Report Summary - RHIZO (Rhizospheric biofilms at root-microbe-mineral interfaces: A key to improve productivity, sustainability and CO2 balance in forests)

In the past decades, forest productivity was maximised by the use of large quantities of fertilisers and pesticides. The production of these chemicals consumes high levels of energy from fossil fuels, emits large amounts CO2, increases ground-and surface water pollution and soil degradation in Europe and other parts of the world. Part of the problem is a lack of fundamental understanding about mineral-derived nutrient dynamics in forests, their weathering release, storage and transport in soils and roots to maintain high, but sustainable, production of high quality wood products. The goal of this study was to investigate the physical and chemical characteristics of mineral-microbe interactions in biofilm cover on mineral particles that were either collected in the field or from growth column experiments. I hoped to gain information about understanding of the rhisospheric biofilm three dimensional physical structures, 3-D inorganic elemental composition and distribution of the biofilm, and response of rhisospheric biofilm development to cation nutrient limitations.

A controlled column growth experiment was set up in the laboratory with plants, symbiotic fungi and limited amounts of Ca, Mg and K to achieve the goals of my project. In addition, I collected mineral samples from a field experiment in the Czech Republic and from an abandoned mining site in Sweden to characterise biofilm and its interactions with minerals from forest soils that have naturally occurring cation limitations and compare the results to the column study.

My studies have shown that symbiotic fungi can promote chemical dissolution as well as applying physical forces to "manufacture" fungi diameter, channel shaped features and that they take up base cation nutrients during the process based on both field and laboratory observations. On the other hand, based on laboratory testing, abiotic processes can also contribute to similarly sized and shaped channels on biotite surfaces because mica is soft, easily scratched by sharp and hard objects such as sand grains and sampling tools. Base cation limitations under forest ecosystems did not promote dense fungal colonisation of the added base cation rich minerals in the mesh bags that were buried in the soils of base cation limited watersheds in the Czech Republic, which contradicts laboratory studies, thus further investigation is needed. We did not find differences among tree species or by depth in the soil profile either in the Czech Republic or in Sweden. The column experiment confirmed that the presence of trees decreased drainage water pH by 2 units, i. e. acidified the systems and increased weathering rates compared to controls without trees. However, the weathering rates remained low in all treatments and only one of the tested fungal species (Paxillus) promoted weathering and increased its rate by two fold compared to the other fungal species. Microscopic investigation revealed that a "biolayer" covers most of the surfaces, but a traditionally defined biofilm (with embedded bacteria) was not observed in our experiments. Bacteria seem planktonic, but they associated with fungal hyphae in all cases. Chemical analyzes of mineral cross sections indicated a slight depletion of cations below the "biolayer" but these changes were not significantly different from the analyzed controls.

Overall, my results supports that symbiotic relationship between higher plants and fungi and/or bacteria can facilitate increased uptake of limiting nutrients from mineral sources, and thus play an important role in chemical weathering processes. This implies that mineral amendments can be utilised to aid sustainable forest management if they are supplied in sufficient amounts and composition. My project also provided insight to better understand microbial attachment to mineral surfaces and their direct nutrient uptake from the solid phase without using soil solutions when nutrients present in limited amounts. This can be used in environmental and bioremediation efforts cleaning up mine fields and polluted sites by manipulating the microbial community through nutrient sources and limitations. While my research is focusing on understanding the fundamental processes that are involved in chemical weathering and nutrient acquisition, my results provide information for sustainable forest management practices that can benefit our society.