Soil microorganisms are the main players that link formation, degradation and cycling of soil organic matter to the performance of plants. This link is not well explored and therefore the role of rhizospheric microorganisms for ecosystem functioning is currently highly debated in the literature. In the past years we used compound specific isotope techniques to investigate the flow of carbon from plant through microorganisms to soil organic matter and traced the flow of nutrients back to the plants using nitrogen isotopes. We manipulated plant diversity and climate in short term isotopic pulse experiments and long term vegetation change experiments in order to understand controls on the microbial community using bulk isotopic (CFE, RNA, DNA) and compound specific isotope techniques (PLFA) and combined this information with changes in the community structure (PLFA; 16s RNA).
Results/Conclusions
Our results demonstrated that soil microorganisms are unexpectedly the major force for the formation of soil organic matter even though they have a fundamental role in the decomposition of soil organic matter and the nutrient return to the plants. We could show that rhizospheric microoganisms are more important for this process then litter degrading microorganisms in the soil. However, the spatial distance of bacterial groups to the root was more important than their community structure. In contrast were soil fungi stronger involved in the carbon flow than bacteria, they were independent from spatial effects and plants benefited strongly from this carbon investment in extreme events. Extreme drought led to a decoupling of the plant carbon flow to soil microorganism and soil bacteria benefited in the recovery phase form built carbon stock returning nutrients for plant regrowth. Most interestingly we found that the soil microbial community - even though being overall controlled by soil type - is mainly bottom up controlled by the available substrate whereas the soil microbial functioning, i.e. nutrient return to plants, remains the same. Our results highlight that the understanding of soil microbial functioning in contrast to soil microbial identity is of fundamental importance for prediction of future ecosystem functioning.