Biotic and abiotic controls on microbial activity, functionality and processes in soil hotspots
by Xuechen Zhang
Date of Examination:2021-07-12
Date of issue:2021-08-23
Advisor:Prof. Dr. Michaela Dippold
Referee:Prof. Dr. Michaela Dippold
Referee:Prof. Dr. Sandra Spielvogel
Persistent Address:
http://dx.doi.org/10.53846/goediss-8786
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Abstract
English
When a carbon (C) source is added to a soil volume and causes microorganisms to thrive, a microbial hotspot is formed — a microsite with much higher process rates than the surrounding bulk soil. The formation, size, and lifetime of microbial hotspots in the vicinity of growing roots (rhizosphere) are governed by biotic and abiotic factors. Root exudation stimulates the production of extracellular enzymes and thus soil organic matter (SOM) decomposition. Nonetheless, we lack a mechanistic understanding of the enzymatic response of rhizosphere microorganisms to individual component of the root exudates. The extent and spatial distribution of rhizosphere depend on soil matrix (e.g. soil nutrient availability), plant properties (e.g. root morphology) and climate changes (e.g. warming and drought). The ongoing global change and the manipulation of exudate composition or root morphology often occur simutaneously, but the consequences of their interactive effects on microbial processes are poorly understood. For example, how the presence of root hairs regulates exudate input and microbial strategies in response to climate changes remains unknown. Therefore, this thesis coupled novel methods including high-throughout sequencing and in situ imaging approaches to demonstrate the effects of biotic or abiotic factors and their interactions on microbial localization, community structure, activity, strategy and efficiency. The objectives of this thesis are to investigate: 1) which component of root exudates plays the key role in stimulating microbial enzyme activities in the absence of living roots and how the presence of root hairs or the presence or absence of inhibitory substances within root exudates influences the formation of rhizosphere gradients; 2) the effect of soil nutrient availability on kinetic parameters in hotspots and bulk soil; 3) whether the response of enzyme activities to drought is driven by the selection of drought-resistant microorganisms; 4) how interactions of root hairs and warming or interactions of root hairs and drought affect the spatial distribution of enzyme activities and unravel strategies for plants and microorganisms to adapt to climate changes. Using artificial roots in combination with specific exudate components, we identified that the spatial distribution of microbial-derived enzyme activities is enzyme- and component-specific: 1) alanine had the overall strongest effect in this nitrogen (N)-limited soil; 2) the activities of phosphorus (P)-, N- and sulfur (S)-related enzymes showed clear gradients in the rhizosphere while the pattern for enzymes majorly involved in C-cycling was uniform and independent of the exudate composition. We also found benzoxazinoids presented in exudates suppressed β-glucosidase activity by 30% (mutant with reduced benzoxazinoid content in exudates (bx1) vs. its corresponding wild type maize), but the presence of root hairs increased exudate release and expanded the spatial extent of β-glucosidase activity around the root axis by 35% (mutant with defective root hair prolongation (rth3) vs. its corresponding wild type maize). Apart from biotic factors, abiotic factors have fundamental effects on microbial processes and microbial community structure. Effects of soil C and nutrients status on functional properties of microorganisms in soil hotspots were investigated by coupling zymography and measurements of kinetics of substrate-induced growth response and enzyme activities in two soils with contrasting soil nutrient availability. The result showed that: 1) differences in microbial growth strategy between rhizosphere hotspots and bulk soil were dependent on soil nutrient availability; 2) differences in enzyme activity and affinity were detected between hotspots and bulk soil in both soils but were enzyme-specific: the difference was significant for β-glucosidase, whereas it was insignificant for leucine aminopeptidase. Drought (another abiotic factor) only induced minor changes in bacterial community structure in rhizosphere hotspots, instead, it increased relative abundance of genera belonging to Actinobacteria capable of leucine aminopeptidase and chitinase production and thus induced a 5.0 - 17% increase in the number of gene copies encoded by Actinobacteria related to these two enzymes. This was reflected in a 35 - 70% increase in the activities of leucine aminopeptidase and chitinase under drought. This demonstrates that bacterial communities react to drought stress by increasing extracellular enzyme production and they achieve this by encoding more enzyme - genes. Root hairs proliferation and warming strongly influence exudate release, enzyme activities and microbial substrate utilization. To reveal their interactions, mutant with defective root hair prolongation (rth3) and its corresponding wild type maize were grown for 3 weeks at 20 and 30 ℃, respectively. Root hairs regulated enzyme expression, microbial growth strategies and thus substrate use efficiency, hence mediating the SOM stocks in response to warming. To clarify the relative importance of biotic and abiotic factors, we conducted an experiment considering the interactive effects of root hairs and drought. The result showed that both root hairs and soil moisture influenced spatial distribution of rhizosphere biochemical properties and processes, but soil water availability was more important than root exudates when the soil is limited by both water and C sources. In summary, both biotic and abiotic factors could influence microbial processes in hotspots. The role of root hairs was diminished when interacting with drought even though it was of great importance in regulating enzyme systems and microbial growth to adapt to climate warming. Therefore, we suggest that predicting and modeling soil C and nutrient dynamics should not only incorporate the interactive effects of biotic and abiotic factors, but also consider which factor involved in the interaction. Overall, this thesis strongly contributes to the understanding of mechanisms involved in the hotspot’s processes. This is particularly crucial for agro-ecosystems where many essential ecosystem services relevant to human well-being are linked to micro-scale processes occurring in hotspots.
Keywords: Root exudates; Maize genotypes; Warming; Drought; Imaging methods; Rhizosphere hotspots
