Mechanisms that link climate, soil properties and microbes to macroscale soil organic carbon dynamics: Insights from a geoclimatic gradient
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Author
Date
2024Type
- Doctoral Thesis
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Abstract
Adequate representation of the soil organic carbon (SOC) cycle at the macroscale (km to global) is important in order to understand and predict the implications of SOC dynamics for the climate and our planet. Such adequate representation is challenging, because mechanistic knowledge about soils is derived from the much smaller mesoscale (mm to m). However, the dominant mechanisms and controls of the SOC cycle vary across scales. Therefore, specific information is needed about which mesoscale processes affect SOC dynamics at the macroscale. This thesis uses a geoclimatic gradient of temperate grassland soils to investigate whether three well established SOC cycle mechanisms at the mesoscale translate to the macroscale, and therefore require scaling. The mechanisms of interest are (1) stable microaggregates as an SOC reservoir, (2) competitive exclusion and substrate specialization as determinants of bacterial community composition, and (3) microbial community composition as a driver of biomass-specific carbon (C) metabolism.
In Chapter 1, I first describe the role of SOC in the terrestrial C cycle and provide an overview of the three fundamental spatial scales at which the SOC cycle can be studied. I explain why the mechanisms that are relevant for the SOC cycle can differ across these scales and introduce the concept of “scaling” to bridge from meso- to macroscale. I then summarize the current consensus view of SOC dynamics at the macroscale and identify the knowledge gaps that this thesis will address. I provide a brief overview of methods which can be used to inform scaling and explain why gradient studies are particularly well suited for this purpose. Lastly, I introduce the geoclimatic gradient with which I worked and present the detailed research questions of this thesis.
In Chapter 2, together with coauthors I investigated whether stable microaggregates – a mechanism of spatial organization within soil – constitute a quantitatively relevant SOC fraction at the macroscale. For this, we applied a fractionation scheme which separates SOC into particulate organic matter, silt- and clay-sized particles and stable microaggregates. We found that stable microaggregates contained a large fraction of SOC, with environmental drivers and chemical characteristics that were distinct from particulate organic matter and silt- and clay-sized fractions. We concluded that stable microaggregates merit scaling to the macroscale.
In Chapter 3, we investigated whether competitive exclusion and bacterial substrate specialization –mechanisms of microbial community assembly – provide scalable links between soil bacterial community composition and the quantity as well as qualitative characteristics of SOC. For this, we measured qualitative characteristics of SOC and characterized bacterial community composition with sequencing of the 16S rRNA gene. We found patterns in line with the mechanism of competitive exclusion but could not conclusively rule out other potentially underlying mechanisms. In addition, we did not find evidence that bacterial substrate specialization translated directly to the macroscale as a mechanism of bacterial community assembly.
In Chapter 4, we investigated how soil microbial traits and functions – which are products of microbial C metabolism – relate to the environment and to soil microbial community composition at the macroscale. For this, we measured microbial traits and functions important for the SOC cycle and characterized climatic and soil physicochemical conditions as well as bacterial and fungal community composition. We found that respiration and growth normalized for microbial biomass (i.e., biomass-specific) were related to different features of microbial community composition, which resulted in strong effects on microbial C use efficiency. We concluded that biomass-specific C metabolism is a mechanism that merits scaling to the macroscale.
In Chapter 5, I compare the SOC dynamics of two contrasting systems along the geoclimatic gradient of temperate grassland soils in order to summarize the findings of Chapters 2 to 4, and to highlight interdisciplinary links among them. I then discuss the general conclusions from each of these Chapters in the context of the current state of the art. I further elaborate the implications of the findings for the respective research fields and propose future research directions that build on the insights of this thesis. Following these perspectives, I briefly provide guidance on how to place the findings of this thesis in a global context. Lastly, I conclude with a general outlook about future research with this dataset. Show more
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https://doi.org/10.3929/ethz-b-000672416Publication status
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Contributors
Examiner: Doetterl, Sebastian![cc](/themes/Mirage2//images/orcid_icon.png)
Examiner: Boeckx, Pascal
Examiner: Schnecker, Jörg
Examiner: Weintraub-Leff, Samantha
Publisher
ETH ZurichSubject
soil organic carbon; geoclimatic gradient; carbon fractions; soil microbial community; soil microbial functionsOrganisational unit
09646 - Dötterl, Sebastian / Dötterl, Sebastian
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