Chemical composition of soil organic carbon from mixed aspen‐conifer forests characterized with Fourier transform infrared spectroscopy

Abstract

AbstractForest species control the quantity and chemistry of organic matter input, which in interaction with the soil physicochemical properties, environmental conditions and microbial community associated with a given ecosystem may result in specific patterns of soil organic carbon (SOC) stabilization and chemistry. The objectives of this study were: (a) to characterize the chemistry of soil organic matter and SOC fractions across the gradient from pure aspen (Populus tremuloides Michx.) to pure conifer (Abies lasiocarpa (Hook.) Nutt. and Pseudotsuga menziesii (Mirbel) Franco) stands in semi‐arid montane forests, and (b) to determine whether the effect of overstory composition on SOC chemistry was patent beyond the influence of site conditions and microbial decomposer community. We used Fourier transform infrared spectroscopy to analyse the chemistry of bulk soil (BS), light fraction (LF) and mineral‐associated SOC (MoM) from mineral soils (0–15 cm) sampled across the natural gradient of aspen and mixed conifer stands from northern and southern Utah. Vegetation overstory had a subtle effect on the MoM fraction, indicating higher proportion of aliphatic C with aspen dominance, whereas there were no differences in LF chemistry between vegetation types. Independently of the vegetation cover type, the MoM fraction was enriched in aliphatic C compared to the LF, although the proportion of polysaccharides and C‐O groups increased in the MoM fraction for plot samples. Differentiation between spectra from soils developed on sedimentary rock and soils developed on basalt, quartzite and limestone, highlighted the influence of parent material and mineralogy on MoM chemistry. The patterns in SOC fractions' chemistry do not allow an affirmation that greater SOC storage under aspen is due to the accumulation of recalcitrant compounds (i.e., aliphatic C) and controlled by litter chemistry. Rather, they suggest that the ensemble of litter chemistry, microbial community and soil properties in aspen stands enhances SOC storage.Highlights Vegetation overstory and site characteristics (e.g., parent material) influence SOC chemistry and stabilization patterns. Light fraction SOC spectra did not differ between forest species in the aspen‐conifer ecotone. The proportion of aliphatic C in mineral‐associated organic carbon (MoM) increased with aspen dominance. The effect of overstory composition on MoM chemistry was patent beyond the influence of site conditions and microbial decomposer community.