Evaluating carbon dynamics and microbial activity in arctic soils under warmer temperatures

Abstract

A large portion of carbon (C) is stored in the world’s soils, including those of peatlands, wetlands and permafrost. However, there is disagreement regarding the effects of climate change on the rate of organic matter decomposition in permafrost soils of the arctic. In this study it was hypothesized that soil exposed to a higher ambient temperature would have a greater flux of CO2 as well as a change in the metabolic diversity of culturable soil microorganisms. To evaluate this hypothesis we determined soil C dynamics, soil microbial respiration and activity, and 13C and 15N fractionation in laboratory incubations (at 14 and 21°C) for an organic-rich soil (Mesic Organic Cryosol) and a mineral soil (Turbic Cryosol) collected at the Daring Lake Research Station in Canada’s Northwest Territories. Soil organic C (SOC) and nitrogen (N) stocks (g m-2) and concentration (%) were significantly different (P < 0.05) between soil horizons for both soil types. Stable isotope analysis showed a significant enrichment in δ13C and δ15N with depth and a depletion in δ13C and δ15N with increasing SOC and N concentration. In laboratory incubations, microbial respiration showed three distinct phases of decomposition: a phase with a rapidly increasing rate of respiration (phase 1), a phase in which respiration reached a peak midway through the incubation (phase 2), and a phase in the latter part of the incubation in which respiration stabilized at a lower flux than that of the first phase (phase 3). Fluxes of CO2 were significantly greater at 21°C than at 14°C. The δ13C of the evolved CO2 became significantly enriched with time with the greatest enrichment occurring in phase 2 of the incubation. Soil microbial activity, as measured using Biolog EcoplatesTM, showed a significantly greater average well color development, richness, and Shannon index at 21°C; again the greatest change occurred in phase 2 of the incubation. Principal component analysis (PCA) of the Biolog data also showed a change in the distinct clustering of the soil microbial activity, showing that C sources from the soil were metabolized differently with time at 21 than at 14°C, and between soil horizons. Our results show that Canadian arctic soils contain large stores of C, which readily decompose, and that substantial increases in CO2 emissions and changes in the metabolic diversity of culturable soil microorganisms may occur when ambient temperatures increase from 14 to 21°C. Key words: CO2 flux, C fractionation, global warming, soil organic C and N, stable isotopes