Hypotheses

SoilSystems developed three main hypotheses on the premises that soils are highly complex, open thermodynamic systems and that the soil ecosystem structure, function and stability are controlled by energy discharge and consumption. It may even be argued that soil microbial biomass as well as SOM can be understood as dissipative structures emerging from the energy and matter fluxes (Fig. 1).

Hypothesis A:

"The microbiome modulates energy dissipation and matter turnover along various energy use channels."  The microbial carbon turnover activity (‘carbon pump’) is part of the energy-use-channel and the dominant `contributor´ to SOM via carbon use and recycling and necromass stabilisation.

Research under this hypothesis will target microbes as energy, organic matter, and nutrient consumers, mass contributors, and `shapers´ of the soil system within the overall frame set by the boundary conditions (see Hypothesis C). Focus will be laid on trophic networks within the microbiome, i.e. the linkage between microorganisms and fauna that are organized by spatial and temporal arrangement within the soil microhabitat architecture thereby modulating SOM turnover. Integrating soil food webs is thus important to understand and manage the processes of SOM regulation, which has not yet been systematically analysed based on a systems ecology view. Studies under this hypothesis will link the energy content and the input and stoichiometry of plant/detritus-derived substrates to the amount and type of degrader biomass formed as well as degradation kinetics under the respective boundary conditions of a given soil, which was not in the focus of previous research.

Projects shall employ thermodynamic theories of systems ecology on the levels of molecules, organisms, and habitats, resulting in extensive interdisciplinary research. Basic thermodynamic approaches that consider the sum of heat fluxes, enthalpies, and Gibbs reaction energy changes available for work will be analysed in relation to turnover and fractions of SOM. Modern ecological methods such as metabolic footprints, combining biomass and activity of biota as functional trait (Ferris, 2010b; Mulder and Maas, 2017) will be applied. Determined parameters can be used for quantitative thermodynamic predictions of biological growth and turnover (Heijnen, 2013).

Hypothesis B:

"Energy and matter input, discharge, and consumption in the soil system affect biological complexity", i.e. the structural and functional diversity, trophic networks and organization of the soil microbiome.

Research under this hypothesis will address microbial diversity and complexity in soils that are not random or accidental but a result of various factors. Also, the potential energy yield from substrates entering a (micro)habitat determines the functional diversity of soil biota involved in metabolic transformation. Yet, predictions on how microbial complexity and network structures are shaped by, and how this feeds-back on SOM composition and storage, are highly limited and require more systematic understanding of these factors in relation to microhabitat conditions (e.g. presence of electron acceptors, nutrients, activity of water). In line with the maximum power principle (Odum and Odum, 1981), we hypothesize that syntrophic microbial groups, entire communities or trophic networks able to exploit the highest amount of energy from a particular carbon source for growth and respiration will become dominant in comparison to less efficient competitors. The potential energetic yield from a compound presumably gives the link to functional diversity – in the sense that a function is redundantly provided by diverse communities. In case of redundancy energy yield would be apparently independent from community composition. This has implications for resilience research to be tested against disturbance and changes of boundary conditions (Ludwig et al., 2018). Integrating soil food web interactions will enable to understand and also manage processes of SOM regulation and energy cycling. This, however, still awaits systematic research (Fierer, 2017).

Projects shall address the question how the provided substrate with its energy content, the microbial community composition with their functional traits, and their faunal grazers are interlinked and whether trigger values and tipping points exist, beyond which community and/or pathways and fluxes are sustainably altered.

Hypothesis C:

"The boundary conditions and mineral composition shape the channel for energy and matter use."  They constrain the non-equilibrium steady states of living and non-living organic matter in soil.

Research under this hypothesis will focus on the soil mineral composition (parent rock material, secondary minerals) and boundary conditions shaping the energy use channel in soil. Boundary conditions encompass (i) factors of soil formation, such as pedoclimate, (ii) nutrients, (iii) structures (e.g. aggregates) developed upon pedogenesis, and (iv) present physicochemical properties such as pH, redox potential and electron acceptor availability as well as water activity (Mikutta et al., 2009; Turner et al., 2017). These conditions are determinants of the energy channel that can be exploited by the microbiome. Linking functional traits of microbes to mineral composition and boundary conditions and properties of macro- and micro-aggregates (e.g. connectivity, tortuosity and heterogeneity of the 3D pore space) is needed to understand SOM turnover and energy use in the sphere of energy and matter consumption. The resulting composition and spatial arrangement of microhabitats along with the basic principle that self-organization is a feature also of soil systems need to be analysed (Addiscott, 2010; Prigogine and Stengers, 1984).

Studies will investigate the link of functional traits of microbes to SOM turnover and energy use at the scales of macro- and micro-aggregates. Research will give answers to the question in how far boundary conditions are shaping or even driving the energy use channel in soil.