Trophic and non-trophic interactions in the heterogeneous and opaque soil matrix

• INT36 • Trophic interactions in soils: overview of current methods and recent advance > M. Melanie POLLIERER
Content : Melanie Pollierer1
1J.F. Blumenbach Institute of Zoology and Anthropology, University of Goettingen, Untere Karspüle 2, 37073, Goettingen,
Germany, Melanie.Pollierer@biologie.uni-goettingen.de

Soils harbour a great diversity of organisms, ranging from micro- to macrofauna. However, trophic interactions among these organisms are difficult to disentangle due to the complex and opaque nature of this habitat. In addition, animal consumers can feed on a great diversity of resources, including dead organic matter, living roots or their exudates, and microorganisms such as fungi and bacteria. To understand the food web structure and the channelling of energy and nutrients from different basal resources to higher trophic levels, indirect methods are needed. Bulk stable isotope analyses (δ15N and δ13C) and fatty acid analyses are canonically used methods to disentangle trophic interactions in soils. However, recent methodological advances utilize compound-specific isotopes in fatty acids and in amino acids to gain more detailed insights. In particular, δ15N and δ13C isotopes of specific amino acids are a promising new tool to simultaneously identify trophic positions and basal resources of
consumers. They offer several advantages, such as including the isotopic baseline signature in so-called ‘source’ amino acids, and allowing to treat microorganisms as trophic analogues of animals, facilitating their integration into the food web. Here, I will give an overview of current methods and highlight recent advances for the analysis of soil food webs, shedding light on advantages and drawbacks. In particular, I will focus on compound-specific amino acid analyses and how they can be used to further the understanding of trophic interactions in soils. • INT37 • Networks of soil organisms: methods and interpretation > E. Elly MORRIEN
Content : Elly Morriën1,4, S.E. Hannula2,4, L.B. Snoek3, W.H. van der Putten4,5
1 Department of Ecosystem and Landscape Dynamics, Institute of Biodiversity and Ecosystem Dynamics (IBED-ELD), University
of Amsterdam, P.O. Box 94240, 1090 GE Amsterdam, The Netherlands.
2Department of Environmental Biology, Institute of Environmental Sciences (CML), Leiden University, Einsteinweg 2, 2333 CC
Leiden, The Netherlands
3 Evolutionary Genomics and Integrative Bioinformatics, Utrecht Bioinformatics Centre (UBC), Padualaan 8, 3584 CH, Utrecht,
Utrecht University
4 Department of Terrestrial Ecology, Netherlands Institute of Ecology, PO Box 50, 6700 AB, Wageningen, The Netherlands.
5 Laboratory of Nematology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands.
*Corresponding author: W.E.Morrien@uva.nl


Soil organisms have an important role in aboveground community dynamics and ecosystem functioning. However, most studies have considered soil biota as a black box or focussed on specific groups, whereas little is known about entire soil networks. Here we present methods and interpretation on how to study the soil food web belowground. In our previous research we discovered that during the course of nature restoration on abandoned arable land a compositional shift in soil biota, preceded by tightening of the belowground networks, corresponds with enhanced efficiency of carbon uptake. We also discovered that although fungi represent a relative small amount of the total microbial biomass half the amount of carbon that flows from plants into soil is taken up by the soil fungi in early stages of succession. After 30 years, that share has risen to three quarters of the plant-
derived carbon stored in the soil. By labelling the carbon atoms, we were able to follow the carbon flow into the soil food web. In this way, we could link the organisms to their corresponding functions in the community. We also use stable isotope tracing in the DNA fraction with illumina-sequencing and the fatty acid (PLFA/NLFA) fraction for coarse level analyses to find out who is driving who during land use change. We have also actively manipulated soil cores from the field by transplanting from natural grasslands into agricultural fields with soil as well as microbes and fauna. We assessed the effect on the carbon flow and responses on the plant community. We will be able to show preliminary data on active photosynthesized carbon that travels via root exudates through the rhizosphere community under contrasting land uses, as well as give a prospect on the wider applications for this in terms of carbon sequestration potential in these grasslands. • INT38 • How to couple soil food web and ecosystem engineers? > S. Sébastien BAROT
Content : Sébastien Barot1
1IEES-Paris, IRD, France, sebastien.barot@ird.fr

In ecology, there is a rich literature on food webs, their functioning and their dynamics. There is also an important literature on the impacts of ecological engineers on ecosystem functioning and feedbacks between these ecosystem engineers and ecosystem physico-chemical properties. However, studies tend to tackle these two types pf processes separately, while they are obviously interacting: (1) ecosystem engineers are directly involved in food webs (they eat and are eaten), (2) when an ecosystem engineer modifies its environment it potentially impacts the growth, the survival and the capacity to feed of all organisms of the food web, therefore impacting its whole functioning. It leads to a bias in our understanding of the functioning of all types of ecosystems, especially in soils. Most organisms living in soils necessarily modify soil physico-chemical properties as they move within the soil and transform its organic matter. In the same vein, within soils, non-trophic (i.e. ecosystem engineering activities) and trophic activities are tightly intermingled. For example, when an endogeic earthworm moves within the soil, it creates galleries and aggregates and consumes organic matter. Likely, the modified soil structure influences the chances of encounter between prey and predators, which should in turn modify the functioning of the whole soil food web. In addition, trophic and non- trophic interactions lead to the mineralization of organic matter, which modifies the chemical properties of the soil through the release of mineral nutrients that serve as a resource for primary producers and microorganisms. This means that there is a feedback from all trophic levels to the bottom of the food web, which contrasts with the traditional view of food webs as pyramids. This broad rationale will be exemplified with some modelling results. • INT39 • How the interaction of root and soil structure affects the fate of carbon > M. Maik LUCAS
Content : Maik Lucas1,2, Andrey Guber1, Alexander Kravchenko1
1DOE Great Lakes Bioenergy Research Center, Department of Plant, Soil and Microbial Sciences, Michigan State University,
lucasmai@msu.edu

2Department of Soil System Sciences, Helmholtz Centre for Environmental Research
Carbon input into the soil occurs mainly through rhizodeposition and root decay. Pore structure determines the spatial inaccessibility of organic matter for organisms, and is thus a key factor in stabilizing carbon in soils. This structure is in turn influenced by roots, which explore the soil by rearranging existing soil particles and can thus compact the rhizosphere, especially if the soil does not contain a well-connected macropore system. Here we conducted a split-root-experiment to determine how plant roots grow into soil depending on the structure they encounter and how this affects the fate and distribution of SOM. Soil containers were planted with Switchgrass and Black-eyed Susan, plants with contrasting root characteristics. These containers enclosed ingrowth cores, with four different structures, either intact or destroyed by sieving, from monoculture Switchgrass and prairie systems were incorporated into. By combining 14C plant labelling with X-ray CT we identified root-soil contact as a previously unrecognized influence on C flow and storage. When roots predominately grew into the soil matrix root- soil contact increased. This led to higher rhizodeposition together with a greater decomposition upon root’s death and created a zone of high microbial activity and optimal pore structure for long-term carbon storage. When roots elongated into large macro- and biopores they relied more on symbiotic fungal networks. The results further demonstrated that the high plasticity of the root system of Black-eyed Susan allows high root-soil contact and thus increased C input in all treatments.

The study is funded in part by the NSF-DEB Program (Award#1904267) and GLBRC
(Award#DESC0018409). • INT40 • Do climate and land-use type affect the relationship between soil structure and nematode communities > X. Xiaoli YANG
Content : Xiaoli Yang1; Steffen Schlüter¹; Nico Eisenhauer2; Martin Schädler1
1Helmholtz-Centre for Environmental Research – UFZ, Halle, Germany, xiaoli.yang@ufz.de
2German Centre for Indiversity Research (iDiv) Halle-Jena-Leipzig, Germany

Soil structure is an important driver of soil food web structure and trophic interactions in belowground communities. This is because access to food resources by soil fauna is limited by the size and connectivity of soil porosity, especially for those who are unable to create pores themselves such as nematodes. However, global climate change and agroecosystem management may alter the critical role of soil physical structure in belowground biodiversity, and their interactions make it difficult to predict shifts in ecosystem functions. Here, in the framework of Global Change Experimental Facility (GCEF) in Germany, we used a field experiment to study the interacting effects between simulated climate changes and two types of land use consisting of conventional farmland and extensively grassland on nematode assemblage structure and community-weighted mean (CWM) biomass. We quantified soil microstructure of individual soil aggregates by X-ray computed tomography (X-ray CT). We currently revealed that climate change and land use type induced shifts in the community composition of nematodes. Results are also expected to assess the correlations between soil microstructure, nematode abundance, biomass, diversity, and trophic structure using structural equation modeling (SEM). • INT41 • Direct investigation of the influence of soil pore space structure and top-down predators on bacterial and fungal functions > E. Edith HAMMER
Content : Edith Hammer1
1 Lund University, Sweden, edith.hammer@biol.lu.se

Soil organisms live and interact in the intricate soil pore space labyrinth, but interactions with their habitat and realistic biotic interactions are difficult to study because of the opaqueness of the soil. We recently developed microfluidic model systems that simulate the spatial microstructure of soil microbial habitats in a transparent material, which we call Soil Chips. They allow us to study the impact of soil physical microstructures on microbes, microbial behavior and realistic microbial interactions, live and at the scale of their cells. Using microbial model strains, we could show the influence of the pore space geometry on bacterial and fungal growth, substrate usage and carbon use efficiency. Sharper and repeated angles in channel-shaped pores consistently diminished microbial biomass and degradation activity, but reduced connectivity in maze like structures increased bacterial biomass and even more so their enzymatic activity. Inoculating the chips with soil, we get a large proportion of the natural microbial community into our chips and can study natural communities of soil bacteria, fungi and smaller protists and nematodes in their food webs and in different spatial habitats. The colonization succession pattern of the chips showed predator-prey oscillations, with periodically high levels of different protists, followed by retreat or encystation. In chips that were containing initially dry pore spaces, colonization success of protists was strongly increased by the presence of fungal hyphae, which paved the way for protists by wetting pore spaces. The soil chips enable us to study the influence of trophic interactions such as the presence of predators on bacterial and fungal nutrient cycling, such as the effect of disturbances that stronger influence protists than bacteria on bacterial population sizes and degradation activities. Beyond the scientific potential, the chips can also bring soils closer to people and hopefully increase engagement in soil health conservation.