Can soil animals protect soil carbon in a warmer and drier world?


Soil is the largest stock of carbon (C) in terrestrial ecosystems; larger than the atmosphere and vegetation combined. The future of our planet’s climate thus largely depends on how this soil C stock responds to ongoing climate change. To predict this response, we need a precise understanding of the different pathways of soil C sequestration and of their response to climate change. While we have a detailed understanding of one of the dominant pathways, i.e., the enzymatic degradation of plant litter by microorganisms, the direct consumption of plant litter by soil animals, as the other dominant pathway, has remained largely unexplored.

This process is driven by detritivores (see photos), i.e., soil animals that feed on decomposing litter. In many ecosystems, detritivores process between 40% and 90% of the plant litter produced annually. Yet they only assimilate a part of it and return the largest part to the soil as faeces. Several studies have shown that by transforming plant litter into faeces, these detritivores facilitate the further decomposition of plant litter by microorganisms (e.g., Joly et al. 2020). By doing so, they may facilitate the production of microbial biomass in the soil (Angst et al. 2022), which contributes importantly to the soil C stock (Buckeridge et al. 2022). Furthermore, some of these detritivores have also been shown to be largely insensitive to changes in climatic conditions (Joly et al. 2019). Overall, these findings suggest that soil animals which feed on plant litter could facilitate soil C storage and will continue doing despite ongoing climate change. So, can soil animals protect soil carbon in a warmer and drier world?

To answer this question, we need to understand how detritivore activity responds to changes in climatic conditions, across the diversity of detritivores, covering millipedes, woodlice, snails, and earthworms. We also need to determine the factors controlling the characteristics of their faeces, and to track the fate of these faeces into the soil and their contribution to soil C storage.

The overall aim of this PhD is to determine how diverse detritivores controls soil C storage under different climatic scenarios. Specifically, through a range of integrated field sampling, experimental climate change simulations, and laboratory incubations, this PhD will answer the following questions:
1. How does detritivore activity respond to changes in temperature and rainfall?
2. How do detritivores, by converting litter and soil into faeces, facilitate soil C sequestration?
3. What are the general drivers of detritivore faeces characteristics across Britain?

Click on an image to expand

Image Captions

Detritivores include diverse soil organisms such as millipedes, woodlice, earthworms and snails, which feed on plant litter and soil, transform it into faeces that are made of a myriad of litter particles, thereby altering their decomposition and fate.



To answer these questions, the student will perform a range of integrated experimental climate change simulations, laboratory incubations, and field sampling:
– First, to determine the climatic sensitivity of detritivores, multiples species of detritivores, covering millipedes, woodlice, earthworms and snails will be incubated in field mesocosms in a common garden at the University of Stirling equipped to manipulate temperature and rainfall, and track changes in activity (litter consumption) over time.
– Second, to determine the fate of detritivore faeces into the soil, faeces from multiple species of detritivores (same range as above) will be produced under laboratory conditions, using different 13C/15N labelled plant litters as a food source. Detritivore faeces and the corresponding intact plant litters will then be incubated on top of soil microcosms under controlled conditions for one year. The proportion of 13C labelled faeces/litter respired will be quantified by regular headspace measurements of CO2 concentrations and isotope ratios. The production of microbial biomass and the sequestration of litter/faeces derived C and N within distinct soil fractions will be compared between treatments using physico-chemical fractionation with detritivore faeces or plant litter as a control.
– Last, to determine factors that determine the faeces characteristics that predict faeces fate into the soil, detritivores from multiple species will be collected from various ecosystems across Britain (including woodlands and grasslands), and the physicochemical characteristics of their faeces will be related to detritivore species, plant litter identity, soil characteristics and environmental variables.

Project Timeline

Year 1

Timeline – Year 1
– Review of literature (Question 1-3)
– Training on field, lab and analytical methods (Question 1-3)
– Set-up and monitoring of climate change experiment (Question 1)
– Set-up of faeces decomposition experiment (Question 2)
– Identification of field sites (Question 3).

Year 2

Timeline – Year 2
– Field collection of detritivore / detritivore faeces across Britain in collaboration with CASE partner Forest Research (FR) (Question 3)
– Detritivore species identification at FR, and faeces characterisation (Question 2 and 3)
– Analysis of data and paper writing (Question 1)
– Further statistical training (Question 1-3)

Year 3

Timeline – Year 3
– Harvest of faeces decomposition experiment (Question 2)
– Soil characterisation at UKCEH Lancaster (Question 2)
– Analysis of data and paper writing (Question 2-3)
– Presentation of key result at national conference (Question 1-3)

Year 3.5

Timeline – Year 3.5
– Analysis of data and paper/thesis writing (Question 1-3)
– Presentation of key result at international conference (Question 1-3

& Skills

We are committed to providing the PhD student with a supportive and inclusive working environment, and to supporting the student in gaining the skills and experience in line with their professional aspirations. In this PhD, the student will have multiple opportunities to develop a range of professionally transferable skills, including skills in project design and management, in invertebrate identification, in diverse state-of-the-art field/ experimental/ laboratory methods, in data analyses, as well as in communication, teaching and outreach. These skills will be developed through interactions with the supervisory team, within the Biological and Environment Sciences (University of Stirling) lively and supportive community, through the IAPETUS specific provision, and through external courses. The student will be able to take advantage of experimental approaches and facilities developed by the supervisors in the NERC Locked Up project (Whitaker, Elias) and to spend discrete periods of time in the Plant-Soil Interactions group at UKCEH Lancaster being trained and using a range of plant-soil biogeochemical analyses including trace gas instrumentation (IRGAs and gas chromatography), analysis of microbial abundance and activity (microbial biomass, PLFAs, extracellular enzyme activity) and use of UKCEH isotope analysis (Picarro CRDS). The student will also benefit from the CASE partnership with FR in Roslin and spend discrete periods of time being trained on soil fauna identification. Overall, the supervisory team are highly experienced in soil ecology and biogeochemistry, with complementary expertise, and with access to a breadth of facilities at the U. of Stirling, UKCEH (Lancaster), and FR (Roslin).

References & further reading

References and further reading:
Joly, F. X., Coq, S., Coulis, M., David, J. F., Hättenschwiler, S., Mueller, C. W., … & Subke, J. A. (2020). Detritivore conversion of litter into faeces accelerates organic matter turnover. Communications Biology, 3(1), 1-9.
Angst, G., Frouz, J., van Groenigen, J. W., Scheu, S., Kögel‐Knabner, I., & Eisenhauer, N. (2022). Earthworms as catalysts in the formation and stabilization of soil microbial necromass. Global Change Biology, 26(16), 4775-4782.
Buckeridge, K. M., Creamer, C., & Whitaker, J. (2022). Deconstructing the microbial necromass continuum to inform soil carbon sequestration. Functional Ecology, 36(6), 1396-1410)
Joly, F. X., Weibel, A. K., Coulis, M., & Throop, H. L. (2019). Rainfall frequency, not quantity, controls isopod effect on litter decomposition. Soil Biology and Biochemistry, 135, 154-162.

Further information
Interested applicants are strongly encouraged to make an informal enquiry about the PhD well before the final submission deadline.

For further information and informal enquires contact: Dr François-Xavier Joly, email: francois-xavier.joly1@stir.ac.uk.

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