Greater regulation of permafrost organic matter composition by enzymes and redox than temperature

Abstract

Accelerating permafrost thaw across high-latitude ecosystems increases organic matter (OM) availability to soil microorganisms. The chemical composition of permafrost-derived OM, which influences microbial metabolic efficiency, may determine whether the Arctic remains a critical carbon sink in the future or shifts to net a source of greenhouse gases. During a 90-day, simulated thaw experiment, we measured shifts in the chemistry of soil and dissolved organic matter pools sourced from surface (0–10 cm) and subsurface (16–25 cm) permafrost layers. We manipulated temperature (1 versus 15 °C) and redox (drained versus saturated) to mimic field-relevant conditions and tested whether variability in OM chemistry could be explained by the activities of six hydrolytic enzymes that catalyze carbon, nitrogen, and phosphorus mineralization. We found that permafrost depth and enzyme activities were significantly correlated with shifts in soil OM (SOM) functional group chemistries, as measured by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Greater variability in dissolved OM (DOM) chemistries were driven by redox and pH. Our results suggest depth-resolved analyses of permafrost vulnerability to enzymatic hydrolysis may help explain distinct patterns in SOM versus DOM cycling that can be used to constrain modeled projections of Arctic carbon storage during global change.