Large, sustained soil CO2 efflux but rapid recovery of CH4 oxidation in post-harvest and post-fire stands in a mixedwood boreal forest

Md Abdul Halim; Jillian M H Bieser; Sean C Thomas

doi:10.1016/j.scitotenv.2024.172666

Large, sustained soil CO₂ efflux but rapid recovery of CH₄ oxidation in post-harvest and post-fire stands in a mixedwood boreal forest

Sci Total Environ. 2024 Apr 21:930:172666. doi: 10.1016/j.scitotenv.2024.172666. Online ahead of print.

Authors

Md Abdul Halim¹, Jillian M H Bieser², Sean C Thomas²

Affiliations

¹ Institute of Forestry and Conservation, University of Toronto, 33 Willcocks Street, M5S 3B3 Toronto, Canada; Department of Forestry and Environmental Science, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh. Electronic address: abdul.halim@mail.utoronto.ca.
² Institute of Forestry and Conservation, University of Toronto, 33 Willcocks Street, M5S 3B3 Toronto, Canada.

PMID: 38653415
DOI: 10.1016/j.scitotenv.2024.172666

Abstract

The net effect of forest disturbances, such as fires and harvesting, on soil greenhouse gas fluxes is determined by their impacts on both biological and physical factors, as well as the temporal dynamics of these effects post-disturbance. Although harvesting and fire may have distinct effects on soil carbon (C) dynamics, the temporal patterns in soil CO₂ and CH₄ fluxes and the potential differences between types of disturbances, remain poorly characterized in boreal forests. In this study, we measured soil CO₂ and CH₄ fluxes using a off-axis integrated cavity output spectroscopy system in snow-free seasons over two years in post-harvest and post-fire chronosequence sites within a mixedwood boreal forest in northwestern Ontario, Canada. Soil CO₂ efflux showed a post-disturbance peak, with differing dynamics depending on the disturbance type: post-harvest stands exhibited a nearly tenfold increase (from ∼1 to ∼11 μmol CO₂^.m^-2.s^-1) from 1 to 9-10 years post-disturbance, followed by a steep decline; post-fire stands showed a more gradual increase, peaking at ∼6-7.2 μmol CO₂^.m^-2.s^-1 after ∼12-15 years. The youngest post-harvest stands were net sources of CH₄,whereas post-fire stands were never net CH₄ sources. In both disturbance types, the strength of the CH₄ sink increased with stand age, approaching ∼2.4 nmol^.m^-2.s^-1 by 15 years post-disturbance. Volumetric water content, bulk density, litter depth, and pH were significant predictors of CO₂ fluxes; for CH₄ fluxes, litter depth, pH, and the interaction of VWC and soil temperature were significant predictors in both disturbance types, with EC also showing a relationship in post-harvest stands. Our findings indicate that while soil CH₄ oxidation rapidly recovers following disturbance, both post-harvest and post-fire stands show a multi-decade release of soil CO₂ that is too large to be offset by C gains over this period.

Keywords: Boreal forest; Carbon debt; Deep carbon decomposition; Soil greenhouse gas; Soil respiration.