Dr. Christina Kaiser
Soil as a complex system
Soil is a highly structured and complex environment, both in physical as well as in biological terms. A chemically heterogeneous and dynamic system of pores and aggregates is populated by a highly diverse community of soil microbes and fauna, who interact with each other, continuously shaping their own community structure and their environment. As known from mathematics and theoretical ecology, spatial structure significantly affects the system’s behaviour by enabling synergistic dynamics, facilitating diversity, and leading to emergent phenomena such as self-organisation and self-regulation.
Only little is known about mechanisms of microbial C and N turnover in the soil, making it still difficult to predict the response of soil to climate change. Still, fine-scale spatial structure and the relevance and preservation of a functioning microbial interaction network are rarely considered in soil biogeochemical research. Analyses of pools and processes in soils are usually based on sieved, homogenised bulk soil samples, and most soil models represent, if at all, microbial biomass and their substrates in distinct, aggregated pools to calculate fluxes. In both cases, dynamics caused by small scale interactions in a spatially structured environment can’t be captured. What, however, if mechanisms of soil C and N turnover depend precisely on this small-scale structure, and phenomena emerging from the interaction network it preserves? (picture on the right: soil bacteria colonising a root surface, made within the ROMY project at CIUS/University of Vienna)
In my group, we aim at developing tools, both theoretical and experimental, for investigating the soil system from the point of view of a “soil systems ecology”. Our approach recognizes the need to understand the interactions between individual agents in order to be able to predict the behaviour of the system, following the directive “the whole is more than the sum of its parts” (Durkheim, 1982). Our aim is to understand if and how mechanisms of microbial C and N cycling in the soil may depend on emergent properties driven by spatiotemporal dynamics of microbial communities and their substrates based on microbial microscale interactions. Phenomena which may be of potential importance in regulating C and N cycling, but are not yet well understood are, for example: the Rhizosphere Priming Effect, the Birch Effect or the long-term storage of C and N, especially in the deep soil.
Mycorrhizal fungi and soil organic matter decomposition
Understanding soil as a system, requires to consider all possibly relevant components and interactions. One major interaction, which is still often neglected in soil research, is Mycorrhiza. Almost all land plants form this symbiosis, which is one of the oldest on earth (more than 400 Mio years). Mycorrhizal fungi help plants to scavenge for nutrients in the soil and are in turn provided with C by the plant. Due to their unique connection to plants, mycorrhizal fungi represent a major route for the input of plant-derived C to the soil. In addition, they may considerably contribute to microbial decomposition of organic matter, not only by excreting extracellular enzymes that target plant-derived organic polymers, but also by interacting with soil microbes (f.e. transferring plant C to soil bacteria, triggering a “priming effect”). Still, mycorrhizal fungi are not often considered in soil C and N turnover studies, particularly their interactions with soil bacteria have not received much attention so far. My group focuses on the mechanisms of C and N transfer between plant and mycorrhizal fungi, as well as between mycorrhizal fungi and soil bacteria and the effects of this tripartite partnership on the overall C and N turnover rates.
We use an individual-based, spatially-explicit microscale microbial C and N turnover model, which I have developed, tested and refined over the last years, to simulate competitive and synergistic interactions between functionally different microbes in a spatially structured micro-scale environment. Based on this ‘bottom-up’ approach, we explore emergent behaviours of the decomposer system based on individual microbial traits.
Investigating C and N flow through the plant-soil system with stable isotope analysis
My group uses stable-isotope labeling (based on EA-IRMS, GC-IRMS and GC-MS analyses), to trace the flow of plant-assimilated C, soil organic matter C, and soil-borne or amended N through the plant - soil - microbe system. Additionally, we use nano-scale secondary ion mass spectrometry (NanoSIMS) to visualize 13C and 15N flows directly across the plant-fungus interface in mycorrhizal roots, and – in collaboration with Dagmar Woebken’s group (DOME) - across the fungus-bacterial interface on mycorrhizal hyphae.
Exploring C and N turnover at the microscale
Investigating soil processes at the small scale in undisturbed soil samples is a challenge, but we are aiming towards that goal. We aim to develop methods to be able to analyse individual soil aggregates by using micro-sensors, adapting fluorimetric measurements of enzyme activities, using ultra sensitive HPLC methods, and NanoSIMS. In addition, we plan to extend our existing experience with measuring stable-isotopes (13C) in phospho-lipid fatty acids and microbial biomass, to other lipids and amino sugars, which are known biomarkers for fungi and for microbial necromass in the soil.
The microbial community response to litter stoichiometry. Organic matter decomposition and nutrient cycling rates are thought to be driven by the imbalance between substrate and microbial biomass stoichiometry. Using an individual-based model, we challenged this view by showing that adaptations of the microbial community to initial litter C:N ratios can alleviate microbial N limitation during litter decomposition, allowing microbial decomposers to overcome large imbalances between resource and biomass stoichiometry (Kaiser et al, 2014)
The effect of social interactions among microbial decomposers on biogeochemical cycles. A self-regulating mechanism emerges in our model when microbial decomposer communities include “cheaters” (opportunistic microbes which benefit from the catalytic activities of others), which ‘buffers’ the effect of changing environmental conditions, such as temperature or nutrient availability, on decay rates. Our results further indicate that the ubiquitous presence of microbial “cheaters” in decomposer communities facilitates ecosystem N retention and the long-term build-up of terrestrial C and N stocks. (Kaiser et al, 2015)
Drivers of CO2 flux under fluctuating rainfall patterns. Large rainfall events after long dry periods result in a flux of CO2 from soil that is larger than current models predict (the “Birch” effect). In a collaboration within IIASA’s Young Scientists Summer Program (YSSP) (Link) we examined the biological and physical dynamics leading to a CO2 pulse after drying-rewetting. (Evans et al, 2016)
Exploring the transfer of recent plant photosynthates to soil microbes. Plants release photoassimilated carbon to soil microbes both, via root exudations and via mycorrhizal fungi at rapid timescales. In collaboration with the University of Western Australia (UWA) we used nanoscale secondary ion mass spectrometry (NanoSIMS) imaging to visualize plant-assimilated 13C, and fungal-delivered 15N at the plant-fungus interface in roots of 13CO2 exposed wheat plants. In addition, we traced the distribution of plant-derived 13C in root- and hyphae associated microbial communities using 13C phosphor- and neutral lipid fatty acids. We showed that a significant proportion of plant photosynthates were delivered through the arbuscular mycorrhizal pathway and utilised by different microbial groups compared to C released by roots. (Kaiser et al, 2015)
CURRENT RESEARCH PROJECTS
- Modelling emergent phenomena of complex microbial communities
- Carbon and Nitrogen flow through the Plant-Mycorrhiza-Soil continuum
- Constraining uncertainties in the Permafrost Carbon Feedback
- Kaiser C, Franklin O, Richter A, Dieckmann U. (2015) Social dynamics within decomposer communities lead to nitrogen retention and organic matter build-up in soils. Nature communications
- Evans S, Dieckmann U, Franklin O, Kaiser C. (2016) Synergistic effects of diffusion and microbial physiology reproduce the Birch effect in a micro-scale model. Soil Biology and Biochemistry
- Kaiser C, Kilburn MR, Clode PL, Fuchslueger L, Koranda M, Cliff JB, Solaiman ZM, Murphy D V. (2015) Exploring the transfer of recent plant photosynthates to soil microbes: mycorrhizal pathway versus direct root exudation. New Phytologist 205(4): 1537-1551.
- Kaiser C., Franklin O., Dieckmann, U., Richter A., (2014) Microbial community dynamics alleviate stoichiometric constraints during litter decay. Ecology Letters, 17: 680-690.
- Kaiser C., Fuchslueger L., Koranda M., Kitzler B., Gorfer M., Stange F., Rasche F., Strauss J., Zechmeister-Boltenstern S., Sessitsch A., Richter A. (2011). Plants control the seasonal dynamic of microbial N cycling in a beech forest soil by belowground allocation of recently fixed photosynthates, Ecology, 92 (5): 1036-1051.
- Kaiser C., Koranda M., Kitzler B., Fuchslueger L., Schnecker J., Schweiger P., Rasche F., Zechmeister-Boltenstern S., Sessitsch A., Richter A. (2010) Belowground carbon allocation by trees drives seasonal patterns of extracellular enzyme activities by altering microbial community composition in a beech forest soil, New Phytologist 187: 843-858.