Characterizing organic matter lability in Alaskan tundra soils using mid-infrared spectroscopy

Abstract:

Soils in permafrost regions contain large amounts of soil organic carbon (SOC) that is preserved in a relatively undecomposed state due to cold and often wet conditions, yet the potential lability of these SOC stocks is still largely unknown. Traditional methods of assessing SOC lability (e.g., laboratory incubation studies) are labor intensive and time consuming. Fourier-transform mid-infrared spectroscopy (MidIR) provides a means to quickly estimate SOC quantity and quality based on the wealth of spectral information. In this study, we explored the possibility of linking MidIR spectra with SOC lability in Arctic tundra soils. Soils from four sites on the North Slope of Alaska were used in this study: a wet non-acidic tundra site in the coastal plain (CP), two moist acidic tundra sites between the northern foothills and the coastal plain (HC and SH), and another moist acidic tundra site in the northern foothills (HV). Active-layer organic and mineral soils and upper permafrost soils from the four sites were incubated for 60 days at -1, 1, 4, 8 and 16 °C. Thawed soils were allowed to drain to field capacity. Carbon dioxide (CO₂) production was measured throughout the study. The chemical composition (e.g., total organic carbon and nitrogen) and MidIR spectra of soil samples were obtained before and after the incubations. CO₂ production varied among soils and temperatures. CO₂ production was greatest at 16 °C for CP and SH organic layers and for HC and HV permafrost layers. These trends among soil layers and sites remained similar at all temperatures. We found a good correlation between MidIR and cumulative 60-day CO₂ production across different soils and temperatures. Characteristic MidIR bands and band ratios previously identified in the literature were also correlated with total CO₂ production. For example, several band ratios (such as the ratio of aliphatics to clay or the ratio of lignin or phenolics to minerals) in the mineral active layer were highly correlated with respired CO₂, suggesting such ratios might serve as useful lability indicators. Further investigation of characteristic MidIR bands and band ratios for additional soils and for longer term incubations are needed to fully assess their utility as indicators of the relative degradation state and potential decomposability of permafrost-region soils.