EGU22-12825, updated on 30 Sep 2022
https://doi.org/10.5194/egusphere-egu22-12825
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Temperature and mineralogical effects on decadally cycling mineral associated soil organic matter

Jeffrey Beem Miller1, Craig Rasmussen2, Alison Hoyt3, Marion Schrumpf1, Georg Guggenberger4, and Susan Trumbore1
Jeffrey Beem Miller et al.
  • 1Department of Biogeochemical Processes, Max Planck Institute for Biogeochemistry, Jena, Germany
  • 2Department of Soil, Water, and Environmental Science, The University of Arizona, Tucson, AZ 85721, USA
  • 3School of Earth, Energy, & Environmental Sciences, Stanford University, Stanford, CA, USA
  • 4Institute for Soil Science, Leibniz Universität Hannover, Hannover, Germany

Climate and parent material interact to form mineral assemblages that contribute to soil organic matter persistence across a range of time scales. Mineral associated soil organic matter (MAOM, the heavy soil component separated by density fractionation) generally contains more C and persists in soils longer than free or occluded light material. Yet while some MAOM persists for centuries, other forms of MAOM turnover on annual to decadal timescales. In order to predict the response of soil C pools to changes in inputs and decomposition rates under climate change we must be able to distinguish the relatively labile component of this mineral-associated soil C pool from the relatively passive component.

We collected samples in 2001, 2009, and 2019 from 9 sites along a combined gradient of parent material (granite, andesite, basalt) and mean annual temperature (MAT) (6.7°C, 9.1°C, 13.6°C). Mean annual precipitation was similar across all sites (80-130 mm yr-1). We measured the radiocarbon (14C) values of bulk soils, respired CO2, density fractions, and thermal fractions. We used selective dissolution and quantitative x-ray diffraction to determine mineral assemblages. We modeled turnover rates for bulk soil and MAOM using SoilR with C stocks and 14C data as constraints.

Using the difference between respired 14C and bulk 14C as a proxy for soil C protection, we observed a strong negative correlation with poorly crystalline mineral content at all time points, suggesting these secondary minerals play a key role in protecting soil C from decomposition. Poorly crystalline mineral content was greatest in the andesite soils, followed by basalt, then granite soils. Temperature also affected poorly crystalline mineral content, with greater abundances in sites with MAT of 9.1°C than in warmer or colder sites across lithologies.

Mineral assemblages were also related to the change in bulk 14C over time. Between 2001 and 2019, bulk 14C declined 4-5‰ yr-1 faster (p < 0.05) in granite and basalt versus andesite soils at the 9.1°C MAT sites. Within the andesite soils, bulk 14C declined 6‰ yr-1 faster at 13.6°C than 9.1°C (p < 0.05). Overall, slower rates of bulk 14C change were correlated with older mean C ages in the models. When compared within each MAT regime, our models revealed andesite soils to have older mean soil C ages than the basalt or granite soils. Respiration fluxes from these soils were more enriched in 14C than the fluxes from the basaltic or granitic soils, and were also enriched relative to the atmosphere. This indicates active decomposition of older decadally cycling soil C derived from mid-20th century nuclear weapons testing in the andesitic soils but not in the basalt or granite soils.

Measurements of 14C in MAOM and associated thermal fractions (currently underway) will enable us to quantify the relative amounts of MAOM cycling at time scales relevant for improving near-term C budgets not only at our sites, but with implications for improving future models of soil C cycling at broader scales as well. 

How to cite: Beem Miller, J., Rasmussen, C., Hoyt, A., Schrumpf, M., Guggenberger, G., and Trumbore, S.: Temperature and mineralogical effects on decadally cycling mineral associated soil organic matter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12825, https://doi.org/10.5194/egusphere-egu22-12825, 2022.

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