Elucidating cm-scale heterogeneity in soil biogeochemistry with a 13C pulse-chase assay
- 1Institute for Atmospheric and Earth System Research (INAR)/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
- 2University of Helsinki, Faculty of Agricltural Sciences, Department of Agricultural Sciences, Helsinki, Finland
- 3Department of Environmental and Biological Sciences, Faculty of Science, Forestry and Technology, University of Eastern Finland, Kuopio, Finland
- 4School of Forest Sciences, Faculty of Science, Forestry and Technology, University of Eastern Finland, Joensuu, Finland
- 5Department of Forest Sciences, University of Helsinki,Helsinki, Finland
- 6Institute for Atmospheric and Earth System Research (INAR)/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, Helsinki, Finland
Spatial heterogeneity in the soil pore network is commonly understood to lead to spatially distinct biogeochemical transformations like the production of methane in anaerobic pockets in unsaturated soils. Yet, demonstrations of this heterogeneity and its linkage to soil structure (e.g., the spatial position in the soil pore network architecture) remains elusive.
We therefore developed an assay to elucidate centimeter-scale differences in biogeochemical reactions within and between peat soil cores. For this, we injects a isotope-labeled substrate (sodium 13C2-acetate) at different locations in intact peat samples (10 cm diameter x 10 cm height) and followed its conversion to 13CO2 and 13CH4 over 5 days time in an automated measurement system using a Picarro G2201-i trace gas analyser. We analyse the ratio of 13CH4 and 13CO2 produced from the amended substrate, the fraction of substrate converted to 13CH4/13CO2, and the time course of 13CH4/13CO2 release.
To test this approach, we collected seven pairs of peat core samples (15-25cm depths, 10 cm diameter, >30m between apart) at a drained forested peatland in Southern Finland. As one of the goals was to evaluate the effects of water retention hysteresis, half of the samples were set to -15 hPa water potential after draining to -30 hPa water potential, while the other half was set to same water potential after water-saturating the samples. In three experiments per core, we injected 10 nmol sodium acetate in 1mL water at 2, 5, and 8cm depth. We find both fixed effects (of core, injection depth, water treatment) and random effects that might be governed by the position of the injection with the peat core.
We find, for example, that while a subset of the peat cores emitted (natural abundance) CH4, these cores showed highly heterogeneous conversions of the injected label into CH4 and CO2 that could not be explained by the fixed effects, demonstrating the spatial heterogeneity of methanogenesis and heterotrophic respiration within the peat core.
In our future work, we will explore if pore networks models extracted from microtomographic images can explain these contrasting results.
How to cite: Kohl, L., Kiuru, P., Palviainen, M., Raivonen, M., Koskinen, M., Matala, L., Pihlatie, M., and Laurén, A.: Elucidating cm-scale heterogeneity in soil biogeochemistry with a 13C pulse-chase assay, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11593, https://doi.org/10.5194/egusphere-egu23-11593, 2023.
