Soil food webs change with land use: basal resources, functional diversity and trophic niche breadth
by Zheng Zhou
Date of Examination:2024-08-22
Date of issue:2024-09-06
Advisor:Prof. Dr. Stefan Scheu
Referee:Prof. Dr. Stefan Scheu
Referee:Prof. Dr. Catrin Westphal
Persistent Address:
http://dx.doi.org/10.53846/goediss-10705
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Abstract
English
The diversity and interactions of organisms, along with ecosystem functions and stability, are intricately linked to food webs. Among these, soil food webs have only been investigated relatively recently due to the small size and obscure feeding habits of soil animals, although they play crucial roles for ecosystem functions. Land use changes, particularly in the tropics, alter the structure of ecological communities and are associated with losses in multiple ecosystem functions. Understanding how land use affects soil food webs can enhance biodiversity conservation and ecosystem service management. In this thesis, I provide new insight into how tropical land use impacts soil food webs through comprehensive assessment of changes in basal resources and functional diversity in rainforests and plantations in Sumatra, Indonesia. Additionally, by integrating data from tropical study sites with a global dataset of stable isotopes of soil animals, I offer novel evidence on the trophic niche breadth of soil animals across different functional groups, land-use types and climatic regions. In Chapter 2 and 3, I investigated the role of basal resources—leaf litter and root-derived resources—on soil microbial and faunal communities through resource exclusion experiments (root trenching and litter removal) in rainforests and plantations of rubber and oil palm. For microbes, I found that the linkage between living roots and microbes was stronger in litter than in soil. Trenching reduced litter microbial biomass in rainforests and plantations, with stronger effects in plantations than in rainforests. While trenching did not reduce total microbial biomass in soil, it specifically reduced fungal biomass. For soil fauna, results highlighted the importance of living root resources as an alternative resource pathway, particularly in oil palm plantations where root-based resources were more critical than litter-based ones. Root-derived resources influenced the body size structure of soil animal communities by favouring small, abundant taxa, while litter-derived resources affected the vertical distribution of soil animals. Further, building on the role of basal resources in soil food webs, I examined how the functional diversity of soil food webs changes with tropical land use by combining community-based stable isotope analyses and size-based metabolic measures for the first time in Chapter 4. Most soil animal groups shifted towards using freshly fixed plant carbon instead of older carbon, as indicated by lower δ13C values, except earthworms, which monopolized the slow detrital channel after land use. By using novel methods to estimate food web properties from both community (unweighted by metabolism) and energetic (weighted by metabolism) perspectives, we found that while land-use effects on soil food webs from a community perspective were partially buffered by earthworms, from an energetic perspective, the resilience of plantations to future changes in climate and land use may be compromised. To identify the relationship between changing food webs and environmental factors, I used structural equation modelling, which showed that community food webs were directly influenced by land use (tree properties and litter amount), whereas energetic food webs were indirectly influenced via pH-induced changes in earthworm abundance. To generalize our understanding of land use effects on soil food webs across different locations and climate types, and to gain more detailed insights into how each taxon changes, I assessed changes in the trophic niche breadth of soil animals on a global scale in Chapter 5. By compiling stable isotope data from tropical study sites with those from temperate sites, I provided a comprehensive assessment of trophic niche breadth across different functional groups, land-use types, and climatic regions. Microbivores and omnivores exhibited higher trophic diversity than detritivores and predators. Additionally, trophic diversity was 36% larger in agricultural ecosystems than in woodlands, and 47% larger in tropical than in temperate regions, indicating soil animals' ability to exploit a wider range of resources and alternative prey under resource limitation, potentially buffering the impacts of global changes on ecosystem functions and stability. Based on these findings, I synthesize the changes in soil food webs with land use, and the differences between temperate and tropical regions in Chapter 6. I advocate for a revisioning of soil food web studies by incorporating trophic position information into metabolism-based food web models, aligning soil food web studies with aboveground and aquatic food webs, and highlighting the functions provided by soil food webs.
Keywords: food web; land use; soil fauna; tropics; roots
