Microbial storage compounds: A neglected dimension of soil C cycling

• Applicant: Dr. Callum Banfield
• Subject Area: Soil Sciences
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 434040903

Microbial transformation of organic matter in soil is of fundamental importance to soil-climate feedbacks, plant nutrition, and ecosystem health. It is generally presumed that carbon (C) taken up by microbes is either respired as CO2 or used for growth, with this growth envisioned as cell replication. This concept is central to current models of soil C cycling and plays a pivotal role in theories of ecological stoichiometry, connecting microbial C transformation to the supply of key nutrients. However, evidence from pure culture microbiological studies demonstrates that a diversity of archaea, bacteria and fungi accumulate intracellular storage compounds, up to 50% of their dry cell biomass. The ability to store C for future growth or energy has profound implications for soil microbial activity, stoichiometry, and responses to environmental variability. Existing evidence confirms that storage compounds occur naturally in soil and are accumulated under high C supply, but suppressed under high nutrient supply, which likely favours replicative growth. Seasonal fluctuations have been reported, suggesting that storage may support microbial activity over winter. However, a consistent and systematic investigation of the quantitative and functional significance of microbial storage in soil is still lacking.This project will examine four important storage compound classes (triacylglycerides, polyhydroxyalkanoates, glycogen and trehalose) in a pasture soil by combining compound-specific analysis and 13C stable isotope probing (13C-SIP) with complementary measures of microbial biomass, activity and community structure. The research is divided into five work packages (WPs): in WP 1, methods of glycogen and trehalose quantification will be adapted for soils, extended to 13C-SIP and used to test for storage compound presence and synthesis in the pasture soil; WP 2 will quantify C allocation to each of the four storage compounds under differing nutrient availability, determine storage compound turnover rates, and reveal how storage influences the microbial response to subsequent inputs; WP 3 will extend these insights to a plant-soil system, using 13CO2 labelling to demonstrate and quantify belowground storage compound synthesis from photosynthates; WP 4 will elucidate seasonal variation in soil storage compound levels and implications for microbial activity e.g. during winter. Finally, the outcomes of all work packages will be synthesized by modelling in WP 5, in which the existing SMMARTS microbial C model will be adapted to include storage and used to explore the implications for microbial dynamics. The proposed research will consider key storage compounds across a range of scales (laboratory to field, and modelling). The project will thereby determine the importance of microbial storage for soil C and N cycling, and provide a basis of methods and new knowledge for the integration of microbial storage into soil ecology and biogeochemistry.

• DFG Programme: Research Grants