Institut für Bodenkunde und Standortslehre

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    Unraveling the interplay of microorganisms, organic matter, and secondary minerals in the mineralosphere of grasslands and forests
    (2024) Brandt, Luise; Kandeler, Ellen
    Mineral surfaces within soil create a distinct microhabitat for microorganisms, known as the “mineralosphere”. In this habitat, the physicochemical properties of minerals play a crucial role for the establishment of mineral-associated microbial communities and their functionality. Secondary minerals, such as iron oxides and clay minerals, possess large surface areas and charge densities, allowing them to retain the majority of the soil’s carbon (C) as mineral-associated organic matter (MAOM). However, these two types of secondary minerals exhibit contrasting surface properties, which greatly influence their interactions with organic matter (OM). Consequently, specific interactions between mineral surfaces, microorganisms, and MAOM are likely to occur, resulting in variations in MAOM formation and microbial turnover processes across different minerals. However, our understanding of how specific secondary minerals impact the biomass, composition, and functions of microbial communities colonizing their surfaces, as well as their interactions with adsorbed MAOM, remains elusive. Moreover, the colonization of mineral surfaces by microorganisms is not solely determined by mineral properties but is further influenced by environmental factors. Different land uses and management practices in grasslands and forests can significantly alter the input of OM into the soil, thereby affecting the formation of MAOM, but possibly also the composition and functions of microbial communities within the mineralosphere. Knowledge of these complex interactions is limited but crucial for understanding the role of secondary mineral surfaces to serve as microbial habitats and stabilize OM in forest and grassland ecosystems. The objective of this thesis was therefore to unravel the complex interplay between microorganisms, organic matter, and secondary minerals within the mineralosphere under the influence of common anthropogenic land use practices in grasslands and forests. The first three studies of this dissertation aimed to move beyond laboratory-based investigations and consider the complex and dynamic nature of soils under field conditions. Over a five-year period, mineral containers containing either goethite (an iron oxide) or illite (a clay mineral) were buried at a depth of 5 cm in soil at 150 grassland and 150 forest sites across three regions of the Biodiversity Exploratories in Germany. This in situ approach enabled us to experimentally isolate the mineralosphere microhabitat within the soil and to examine its unique characteristics. Moreover, we were able to examine how mineral type as well as land use and management influence mineralosphere characteristics by comparing patterns of MAOM accumulation and microbial parameters between goethite and illite and between the different grassland and forest Management practices. Collectively, these three studies underscore the relevance of iron oxides and clay minerals as distinct microhabitats. The accumulation of MAOM played a pivotal role for microorganisms on both goethite and illite, as evidenced by the strong correlation between microbial biomass and MAOM-C contents in the mineralosphere. Microbial communitiesassociated with the minerals differed from those in the bulk soil, consistently showing relative enrichment in fungi and Gram-negative bacteria across grasslands and forests. Furthermore, the stoichiometry of enzyme activities involved in C, nitrogen (N), and phosphorus (P) cycling indicated an increased relative acquisition of nutrients (N and P) than C compared to the surrounding bulk soil. Mycorrhizal fungi likely played a pivotal role in the increased relative nutrient acquisition in the mineralosphere, as they are independent of the C availability in the microhabitat due to the C supply via their host plant. In contrast, saprotrophic fungi in particular showed limited competitiveness compared to other fungal taxa, as they are poorly adapted to the low-C conditions in the mineralosphere. Within the mineralosphere, the mineral type played a crucial role in shaping its characteristics. The consistently higher MAOM accumulation on the iron oxide goethite compared to the clay mineral illite emphasized the significance of the mineral type for the extent of MAOM formation. Goethite, with its higher OM sorption capacity and stronger bonds formed with OM via ligand exchange, contrasted with illite, which bound less OM through weaker cation bridges. These differences in sorption behavior also had implications for the accessibility of OM and enzymes, and ultimately, the microbial communities inhabiting the two minerals. The higher C-specific microbial biomass on illite compared to goethite suggested a greater availability of illite-associated OM for microbial utilization. Microorganisms on goethite likely needed to enhance enzyme production at the cost of biomass synthesis to counteract a reduced substrate availability. Conversely, microorganisms on illite could utilize acquired OM to synthesize relatively more biomass than on goethite. These mineral-specific interactions between mineral surfaces, OM, enzymes, and microbes likely modify microbial MAOM formation and turnover pathways on different mineral types. The consequences of the strong association between OM and certain minerals on enzymatic substrate turnover were also evident in a separate laboratory experiment in the fourth study of this thesis. By adding β-glucosidase to co-precipitates of litter- or soil-derived dissolved organic matter (DOM) and specific aluminosilicates, the amount of mineral-associated substrates available for enzymatic degradation was determined. In this experiment, the enzymatic degradation of mineral-associated OM was reduced compared to free OM in solution. This emphasizes how strong interactions between OM and mineral surfaces have the potential to protect MAOM from enzymatic degradation, reducing its bioavailability to microbes. Not only effects of the mineral type but, to a lower degree, also effects of land use and management on the interplay of microbes and MAOM were evident. Fertilization in grasslands as well as tree species selection, thinning, and harvesting in forests altered the quantity and quality of nutrient and OM inputs into the soil. Moreover, land use had an impact on soil pH, consequently modifying the sorption capacity of the minerals. These effects were also reflected in various microbial parameters within the mineralosphere, which in turn, likely affected microbially mediated pathways of MAOM formation and turnover. Overall, this dissertation reveals the intricate relationship between microorganisms, OM, and different secondary minerals within the mineralosphere microhabitat and the mineral-specific involvement of microorganisms in MAOM turnover. The gained insights enrich our understanding of the ability of soils with different mineral compositions and under different land use and management practices to stabilize C in the MAOM pool.