Characterizing Soil Organic Matter Degradation Levels in Permafrost-affected Soils using Infrared Spectroscopy

Abstract:

Diffuse-reflectance Fourier-transform mid-infrared spectroscopy (MidIR) was used to (1) investigate soil quality along a latitudinal gradient of Alaskan soils, and in combination with soil incubations, (2) to assess the relative lability of soil organic matter in the active layer and upper permafrost for some of those soils. Twenty nine sites were sampled along a latitudinal gradient (78.79 N to 55.35 N deg). The sites included 8 different vegetation types (moss/lichen, non-acidic and acidic tundra, shrub areas, deciduous forests, mixed forests, coniferous forests, and grassland). At each site, soils were separated by soil horizons and analyzed for pH, cation exchange capacity (CEC), organic and inorganic C, and total N. Samples were also scanned to obtain MidIR spectra, and ratios of characteristic bands previously suggested as indicators of organic matter quality or degradation level were calculated. Principal component analysis showed that axis 1 explained 70% of the variation and was correlated with the general Organic:Mineral ratio, soil organic C, total N, and CEC, but not with vegetation type. Axis 2 explained 25% of the variation and was correlated with most of the band ratios, with negative values for the condensation index (ratio of aromatic to aliphatic organic matter) and positive values for all humification ratios (HU1: ratio of aliphatic to polysaccharides; HU2: ratio of aromatics to polysaccharides; and HU3 ratio of lignin/phenols to polysaccharides) suggesting that axis 2 variations were related to differences in level of soil organic matter degradation. Active organic, active mineral and permafrost layers from selected tundra sites were incubated for two months at -1, 1, 4, 8 and 16 ⁰C. The same band ratios were correlated with total CO₂ mineralized during the incubations. Data from 4⁰C showed that the cumulative respired CO₂ from the active organic layer across all sites was negatively correlated with the HU1 humification ratio, suggesting that HU1 might be a good indicator of lability for comparing active layer organic soils. We will explore correlations at the other incubation temperatures and further evaluate the utility of MidIR band ratios for predicting the potential decomposability of organic matter in permafrost-region soils.