Impact of Populus trees on the composition of organic matter and the soil microbial community in Orthic Gray Luvisols in Saskatchewan (Canada)

Abstract

The conversion of agriculture land to short rotation coppice (SRC) systems with hybrid poplar (Populus spp.) is becoming an important practice to store carbon and produce biomass for the fiber industry. This land-use change is likely to affect the microbial community structure, in part through the introduction of an ectomycorrhizal host, but also by changing organic matter inputs, ultimately shifting the molecular composition of soil organic matter (SOM) because these properties are crop-specific. The objective of this study was to examine the effect of different aged poplar stands growing on Orthic Gray Luvisols on these soil properties compared to an adjacent perennial crop. The site consisted of four treatments: hybrid poplar (Populus deltoides × Populus petroskyana) in SRC of 7 and 14 years age (Pop7 and Pop14), a 40-year-old aspen (Populus tremuloides) forest (Aspen), and an adjacent arable site with alfalfa (Medicago sativa). The molecular composition of SOM was determined using pyrolysis-field ionization mass spectrometry (Py-FIMS) and synchrotron-based X-ray absorption near edge spectroscopy at the nitrogen (N) K-edge (N-XANES). Phospholipid fatty acid (PLFA) profiles revealed higher concentrations of fungal but not bacterial biomarkers in poplar plantations than under alfalfa. Fungal:bacterial ratio was lower under alfalfa than under poplar trees. Overall microbial community structure also changed in response to vegetation type and the duration of woody biomass. Along the chronosequence of poplar, Py-FIMS revealed significant increases in the proportions of carbohydrates with pentose and hexose subunits, phenols and lignin monomers, alkylaromatics and peptides and significant decreases in lipids, alkanes, alkenes, bound fatty acids and alkylmonoesters and free fatty acids with increasing poplar stand age. Thermal stability of sterols decreased along the chronosequence, suggesting that poplar plantations are not involved in the build-up of these substances. Differences in the ordination were well reflected by N K-edge XANES spectra. Increased carbohydrate portions indicated a significant impact of the ectomycorrhizal host poplar on the molecular composition of SOM in arable soils. Our results demonstrate that a change in land-use from agricultural cropping to woody biomass caused shifts in microbial community structure that were lagged by a shift in SOM chemistry. These shifts impact soil functioning and should be considered in future land management strategies when SRC are harvested.