Microscale physicochemical- and biological configurations regulate microbial biogeochemical processes and functionality

The microscale spatiotemporal heterogeneities of physicochemical- and biological properties and their variational dynamics are key factors regulating microbial activity, soil quality and functionalities. Yet little is known about the underlying mechanisms and impacts on the functioning biogeochemical processes of the earth-surface ecosystems typically the soil-water-microbe-climate nexus. Employing microscale experiments, we illustrate how small-scale water and nutrient configurations as well as those of biological (populations) and chemical (such as O₂ and pH) gradients regulate microbial interactions and functionality, and impacts on soil carbon (C) and nitrogen (N) cycling. We firstly use pairs of fluorescently labelled bacterial strains and a hyphae-forming fungal strain that expand together across a nutrient-amended surface, and show that flagellar motility drives bacterial dispersal along the hyphal network, which counteracts the purifying effects of ecological drift at the expansion frontier and thereby increases the spatial intermixing and extent of range expansion of the bacterial strains. We further demonstrate that fungal hyphae are important regulators of bacterial diversity and promote plasmid-mediated functional novelty during range expansion in an interaction-independent manner. In addition, we employed a soil column experiment and illustrated that sufficient labile carbon from plant residues such as straw induced fast O₂ consumption with microoxic development in the straw-soil interfaces. In the meantime, the porous structure of straw materials could enhance O₂ diffusive inputs in the core area, and subsequently formed a a concentric ring-like microoxic area around the straw patch. Such enriched oxic-microoxic transient zones would induce nitrification coupled denitrification, which led to the high N₂O emissions. Additionally, the microbial degradation of straw resulted in a pulse decline of soil pH, which possibly inhibited the N₂O reductases, yielding enhanced N₂O emissions. These results contribute to a better understanding of the driving factors for microbial interactions and possible impacts of soil key element (such as C and N) cycling.