Changing environmental correlates of peatland C fluxes in a thawing landscape: do transitional thaw stages matter?

Abstract:

Soils of the northern permafrost regions store 50% (1672 Pg) of the world’s belowground organic carbon (C). Peatlands within this region store 277 Pg as soil C and often occur as a mosaic of wetland types, each with variable sensitivity to climate change. Permafrost thaw further increases landscape heterogeneity in peatland structure and C function. While end-member stages such as fully intact or fully thawed permafrost are well studied in peatlands, C fluxes are not well constrained in transitional thaw stages that also cover a significant area of these ecosystems. Moreover, changes in the environmental correlates of C fluxes, due to thaw, are not well described and are vital to modeling future changes to C storage of permafrost peatlands. We used 10 thaw stages in a sub-arctic peatland as a space for time substitution to measure changes in CH₄ and CO₂ fluxes and their correlates. Growing season mean CH₄ fluxes showed a large range across the thaw gradient, increasing with thaw, from -1.1 to 370.2 mg CH₄ m^-2 d^-1. CO₂ fluxes were highly variable along the thaw gradient with GP_MAX (maximum gross photosynthetic CO₂ capture at maximum photosynthetically active radiation; PAR) ranging from 2.4 to 8.2 μmol CO₂ m^-2 s^-1. Across the site, we found significant relationships of C fluxes with the correlates: water table depth, thaw depth, temperature, PAR, vascular green area and water chemistry parameters. Within individual thaw stages, strengths of bivariate environment-C flux correlations changed inconsistently as the thaw progressed. A notable exception, temperature sensitivity of CH₄ fluxes, increased linearly with thaw, suggesting a shift from substrate to temperature limitation. However, C fluxes have multiple and interactive controls that are not adequately described by bivariate analyses. We found that interactive effects on C fluxes changed variably along the thaw gradient, showing that transitional stage dynamics differ unpredictably from adjacent or end-member stages. Our results suggest that transitional stages contribute to the large variability of C fluxes in a thawing landscape and may require variable parameterization in models estimating C fluxes from abiotic and biotic controls.