The Chemical Properties of Alaskan Permafrost and Seasonally Thawed Soils

Although permafrost soils contain vast stores of carbon, we know relatively little about the chemical composition of the carbon stored in these soils. Carbon chemistry is an important predictor of decomposition rates, especially in the initial stages of decomposition, making investigations of the diversity of compounds comprising the organic carbon pool important if we are to understand storage and release of carbon from permafrost soils upon thaw. Permafrost and active layer organic and mineral soils were collected from Sagwon Hills, Alaska and the carbon chemistry was analyzed using Fourier transformed mid-infrared spectroscopy (FTIR), which vibrates and stretches chemical bonds allowing the identification of the organic functional groups in soils. Permafrost soils were separated into 5 cm increments to investigate the distribution of chemical compounds by depth. In addition to whole soil FTIR spectra, FTIR spectra of mineral soils (obtained by ashing the soils at 600°C overnight) were collected; subtraction of the mineral spectra from the whole soil spectra was performed to reveal the spectra of the organic component of the soils.

Principle components analysis (PCA) of the FTIR spectra of the whole soil shows that the organic active layer separates from the mineral active layer and permafrost layers because the mineral samples absorb at 3600 and 1870 cm-1, bands characteristic of sands and clays. Analysis of the organic component of the spectra (obtained through spectral subtraction of the ashed spectra from the whole soil FTIR) shows that the active layer and permafrost both have potentially decomposable C, but that there is less in the permafrost. In addition, the active layer and permafrost differ in the quality of the OM.  The active layer has greater absorbance at labile C bands (3400 OH/NH, 2920 aliphatic CH, and 1100 carbohydrate), and also at more resistant and processed C bands (1730, 1600, 1380). The permafrost samples also have features that are more pronounced than in the active layer and characteristic of processed carbon (1510 amide or lignin-like, 1440 C-O single bond absorbance or aliphatic CH deformation). Samples collected from the top of the permafrost (0-15 cm) have greater absorbance at the organic bands than deeper permafrost (16+ cm) at 3400, 2930, 1730, 1550, and 1225, regions that represent different forms of C, from labile 3400 to resistant 1230. Deeper permafrost samples absorb more at the mineral bands for clay and quartz overtones than shallower permafrost.

Analysis by FTIR has revealed that permafrost and active layer soils contain many of the same organic C compounds, but that there are differences between the chemistry of these soils. FTIR indicates that the active layer may have more labile organic matter than the top of the permafrost (0-15cm), which in turn may have more labile C than the deeper permafrost (16+ cm). This type of detailed chemical analysis of permafrost soils will decrease the uncertainty of the role of permafrost in the global carbon cycle as we increase our understanding of the availability of these carbon compounds to decomposition.

Related Materials and Graphics: 

PCA and loadings of FT-IR soil chemistry by depth