Extracellular polymeric substances and aggregate stability - how microorganisms affect soil erosion by water

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Soil erosion determines the shape and surface of many landscapes and the stability of soil aggregates is considered to be a crucial factor regulating soil erosion. Microbial processes have not been discussed as important controls of soil erosion although they should directly be related to aggregate stability by the production of extracellular polymeric substances (EPS). It is unknown whether the composition of the bacterial community as influenced by vegetation affects EPS produced in situ. Changes in amount and composition of EPS and their effects on the stability of soil aggregates and thus on soil erosion by water are largely unknown. Therefore, the main objective of this proposal is to study the complex interactions between vegetation, the composition and capability of the soil bacterial community to produce EPS, and the effects of these EPS on the stability of formed aggregates and subsequently on soils susceptibility to erosion. In our project we will bridge different temporal and spatial scales and enhance our knowledge about biotic modulation of erosion and sediment routing depending on climate. We will use two gradients, three study areas along a precipitation gradient in Southern Spain (Almería Province) and a gradient in vegetation coverage within each study area. The project will start with an explorative field survey to study relationships between vegetation, bacterial community composition, and amount and properties of EPS and how aggregate stability is affected by these relationships. Then, joint greenhouse and rhizobox experiments will be used to study in detail how EPS formed by soil bacteria influence aggregate stability in presence or absence of plant roots. The rhizobox experiment will deliver deep insights in spatial aspects of bacterial EPS production and how it is related to roots and aggregation. Rainfall simulation experiments in the field will be the ultimate proof whether changes in aggregate stability controlled by EPS will be responsible for in situ changes in soil erosion by water. In our project we will establish novel methods to extract and characterize EPS from soils and to determine their spatial distribution patterns. In all experimental approaches we will use the same soil samples and combine advanced methods spanning a broad range from molecular microbial ecology (e.g., 16S rRNA amplicon sequencing, quantitative PCR, PCR-Southern blot hybridization), microscopy (e.g., CLSM, CARD-FISH, gold-FISH, ESEM), soil science (e.g., aggregate size fractionation, stability of aggregates), spectroscopy (fluorescence, pyrolysis GC/MS, high resolution MS) and geomorphology (rainfall simulations). Here we present an innovative and interdisciplinary approach enabling unprecedented insights how soil erosion by water is controlled by bacterial communities via their production of EPS and their effects on aggregate stability.