Investigating Microbiome Contributions to Soil Organic Matter Degradation Across a Boreal Forest to Arctic Tundra Gradient

Authors

Mark McDonald¹* (mcdonaldm@anl.gov), Roser Matamala¹, Jeremy Lederhouse¹, Chien-Lu Ping², Julie Jastrow¹

Institutions

¹Argonne National Laboratory, Lemont, IL; ²University of Alaska–Fairbanks, AK

URLs

• https://ess.science.energy.gov/anl-sfa/
• http://tessfa.evs.anl.gov/

Abstract

The degradability of organic matter (OM) held in permafrost-affected soils is expected to affect the rate of carbon (C) mineralized, mobilized, and released from these soils in response to global warming. While many factors influence the degradation of OM in permafrost-region soils (e.g., temperature, C species, organomineral associations, water content, redox conditions, pH, and soil texture), warming of these soils is expected to enhance heterotrophic respiration rates and the mineralization/loss of C stocks currently held in them. Previous studies indicate that a soil’s total organic C (TOC) concentration can often reasonably predict C loss as carbon dioxide (CO₂), but unexplained variation remains. The purpose of this study is to evaluate the decomposability of permafrost and active layer soil horizons across a large latitudinal and vegetation gradient (~500 km) from Fairbanks, AK, to north of the Brooks Range. Horizon-specific samples of the full soil profiles (to a depths of 1 m) were collected, stored frozen, freeze-dried, homogenized through a 2-mm sieve, and then subsampled for analysis of the native microbial community composition and TOC. Additional aliquots of these soils are being evaluated for loss of CO₂-C when subjected to warming conditions in an aerobic manipulative bioassay-incubation (rewetting to uniform matrix potentials). To compare the decomposability of OM present in these soils with potential response predictors, researchers initiated analyses of the relationships between respiration rates during the first two months of incubation and several covariates including TOC, microbial community structure and diversity, and their combination. A clear difference was observed in respiration rates (mass basis) and time to peak respiration between organic and mineral horizons, suggesting some differences in OM degradability between these horizon types. Inclusion of microbial parameters, in addition to TOC, is hypothesized to improve predictive models of C loss from permafrost-affected soils because strong divergence is expected in microbial community structure between horizon types within different soil profiles, which might influence mineralization potentials. The data generated here will be used to: (1) inform future predictive models of soil C loss for the permafrost region and (2) guide procedures for assessing the spatiotemporal variability of both microbial and C species diversity in warming permafrost-affected soils.