Effect of polyphenol-rich litter on soil organic matter composition: Consequences of invasion by the plant Polygonum cuspidatum

Background/Question/Methods

Carbon fixed during photosynthesis forms the precursor of all organic carbon in soil and the predominant source of energy that drives soil microbial processes, hence the molecular identity of the fixed carbon could influence the formation of soil organic matter (SOM).Due to their high resource acquisition and resource use efficiencies, some invasive plant species can input disproportionately high quantities of litter that are qualitatively distinctive, and this could influence the accrual of organic carbon and overall carbon cycling in invaded habitats.  Hence we hypothesized that invasive plants with unique litter chemistry would significantly influence the overall carbon cycling in the invaded soils.  We tested this hypothesis using Japanese knotweed (Polygonum cuspidatum), which produces litter with a high content of polyphenols, a class of compounds that can significantly alter microbially mediated decomposition and re-synthesis. Soil samples were collected at 5-cm intervals of depth  inside and outside 15- to 20-year old stands of knotweed. The molecular composition of C in the soils and the degradation stage of the SOM were assessed with a biomarker approach using gas chromatography-mass spectrometry to determine the source of biomolecules (plant or microbes), and the overall SOM composition was determined using FTIR and NMR analysis. Stability of SOM fractions was assessed through oxidation with potassium permanganate for labile and with hydrogen peroxide for mineral bound fractions, followed by isotope analysis.

Results/Conclusions   

Fungal communities dominated the uppermost soils under knotweed. Spectroscopic measurements showed a higher abundance of polyphenols inside than outside stands of knotweed. Principal component analyses revealed a convergence of soils under knotweed, based on profiles of ester-bound phenolics. Biomarker analysis revealed a higher concentration of sterols in soils inside than outside stands, including fungi-derived (ergosterol) and plants-derived sterols (β-sitosterol and stigmasterol). Distributions of long-chain, n-alkanoic acids indicated greater abundance of high-cutin and suberin derived materials under knotweed than outside stands. These results indicate that knotweed can significantly influence the molecular-level characteristics of carbon accrued in soils by producing a high biomass of litter that is rich in polyphenols.