Stand Scale Variability in Humification Processes of Boreal Forest Soils.

In soils of the boreal forest, much of the organic matter accumulates in the forest floor overlaying the mineral horizons. The initial stages of litter decomposition lead to the development of an F horizon, which consists of partially decomposed plant and fungal materials. Further degradation can lead to the formation of the H horizon, where the origins of the residues become indiscernible. The objective of this study was to examine how the variability in F layers at the stand level may lead to differences in humification processes. We collected and described forest floor materials from a trembling aspen-dominated stand, where F and H materials were separated for further chemical analysis.

In order to compare the composition of F and H horizons, we used ¹³C solid state Cross Polarization-Single-Pulse (CPSP) and Total Suppression of Spinning Sidebands Non-Quaternary Suppression (TOSSNQS) Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy. The former technique allowed us to semi-quantitatively compare the chemical signature of each horizon, while the latter enabled us to further resolve the aromatic region and compare changes in lignin and tannin contents. We also used Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) on the base hydrolysis extracts as a complimentary technique. This allowed us to monitor changes in the biopolymers cutin, suberin, and lignin along the decomposition gradient. Forest floor genesis is crucial to our understanding of carbon cycling in the boreal ecosystem, and further adapting these techniques to the study of soil organic matter will greatly increase our knowledge of soil organic carbon dynamics at large.