Insight into indicators related to the humification and distribution of humic substances in Sphagnum and peat at different depths in the Qi Zimei Mountains

Abstract

Humification of Sphagnum occurs in peat bogs and locks up carbon as peat. Useful indicators and methods of humification can reflect the process of peat formation to some extent. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV/Vis spectra and excitation-emission matrix (EEM) spectra coupled with parallel factor (PARAFAC) analysis were applied to characterize the humification of Sphagnum, peat, dissolved organic matter (DOM) and chlorophyll with depth. The TEM results showed that chlorophyllous cells were almost completely degraded under the brown layer (BL), while Sphagnum hyaline cells demonstrated little degradation, and their structure could be observed even in peat. EEM spectra coupled with PARAFAC analysis is a good way to study chlorophyll in peat, and chlorophyll can be a useful indicator in humus studies. The peat had a relatively high chlorophyll content, which was negatively correlated with fulvic-like components and positively correlated with easily biodegradable organic matter. The parameters of samples from different layers fell into four groups, i.e., fulvic-like components, humification, humic-like components and/or protein-like components, easily biodegradable organic matter, and a combination or transition of the fluorescent substance of Sphagnum and humic-like components. The collapsed and defoliated layer (CDL) may be the layer in which the process of humification begins, and the humification of the grass root layer (GRL) was similar to that of peat. The degree of humification gradually increased from peat at 0–5 cm (P0-5) to peat at 10–15 cm (P10-15) and had no regular trend with depth from peat at 15–20 cm (P15-20) to peat at 25–30 cm (P25-30).