Microorganisms and the turnover of soil aggregates: The importance of extracellular polymeric substances

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Soil aggregation is critical for carbon sequestration, plant growth, and soil fertility. Microbial processes have been recognised as an important control of aggregate turnover (formation, stability, and destruction). However, how microorganisms contribute to these processes is still a matter of debate. One crucial mechanism determining aggregate turnover may be the excretion of extracellular polymeric substances (EPS) as microbial glue, but the role of the amount and composition of EPS on macroaggregation is largely unknown. Moreover, interdependencies between important aggregation factors like the amount of fine-sized particles (clay), the decomposability of organic matter and microorganisms (amount and composition, as well as the excretion of EPS) are still poorly understood. Thus, the main objective of this proposal is to study the complex interactions between these factors and their role in aggregate turnover. The overarching goal is to evaluate and identify the mechanisms defining the temporal patterns of macroaggregate turnover with respect to the role of microorganisms. Thereby, the project focuses on the role of EPS for macroaggregate formation and stabilisation. The basis of this project is an incubation experiment which will be conducted across a gradient of clay content and substrate decomposability as main drivers of the microbial activity. We hypothesise that an increase in microbial activity, induced by the input of organic substrates, will stimulate EPS production and therefore the formation and stability of aggregates. Furthermore, we expect that EPS-proteins will increase aggregate stability more than EPS-polysaccharides. A combination of aggregate separation and stability tests will be applied. The results will be examined with respect to the obtained microbial parameters (CO2 respiration, microbial biomass, phospholipid fatty acid, amount and composition of EPS), to disentangle the mechanisms and factors controlling macroaggregate turnover affected by soil microorganisms. We will directly evaluate if EPS-proteins or EPS-polysaccharides contribute more to macroaggregate stability. Water-stable macroaggregates will be obtained, and the amount and composition of EPS will be measured therein. In this regard, high resolution mass spectrometry will offer a very detailed view into the molecular composition of EPS. This analysis is expected to provide conclusive results on the role of EPS on the stability of macroaggregates. Here we present an innovative and interdisciplinary approach to disentangling how macroaggregate turnover is controlled by microorganisms, in particular by examining their role in the production and composition of EPS. This project will provide an improved understanding of the underlying processes of macroaggregate turnover in soils, which is necessary for the implementation of better soil management practices, to improve soil structure, reduce susceptibility to topsoil erosion, and enhance C sequestration.