The unexplored role of secondary metabolites in root priming: phenolic root exudates cause striking changes in soil organic matter decomposition and microbial community structure

Dr. Marie Zwetsloot1, Juana Munoz ucros2, Dr. Kyle Wickings2, Prof. dr. Andre Kessler2, Prof. dr. Jed Sparks2, Dr. Roland Wilhelm2, Prof. dr.  Taryn Bauerle2

1Wageningen University, Wageningen, Netherlands, 2Cornell University, Ithaca, USA

Root-soil interactions fundamentally affect soil carbon cycling and thereby ecosystems feedbacks to climate change. This study investigated the microbial processes by which secondary metabolism of temperate forest tree species influence soil organic matter decomposition. We hypothesized that root phenolic compounds (secondary metabolites) have diverse functions in soils including food source, toxin, enzyme inhibitor and signaling. Therefore, we expected phenolics to cause contrasting trends in soil organic matter decomposition and lead to greater shifts in microbial community composition (MCC) than glucose (primary metabolite). Using a modified root exudate collection method and high-performance liquid chromatography (HPLC), we found that the root phenolic profiles are highly tree species-specific. As a preliminary screening test, we performed a five-day incubation study to assess the effect of a wide range of phenolic compounds representative of the biosynthetic classes identified by HPLC on soil microbial respiration. Here, our results show that root phenolic compounds can both increase, decrease or have no effect on microbial respiration, which is probably driven by specific functional groups. As a follow-up, we performed another soil-exudate incubation lasting 38 days aimed at understanding the microbial mechanisms behind phenolic-driven shifts in soil organic matter decomposition using stable isotope-labeled compounds, interval gas sampling, microbial sequencing and soil enzyme analyses. Our findings show that the degradation of phenolics varied by compound over time, while glucose was respired within the first two days of the experiment. Benzoic acid significantly increased soil organic matter decomposition, which was amplified in glucose-amended soils. Consistent with trends in SOM decomposition, benzoic acid caused the most striking shifts in MCC. Even the minimally-degraded phenolics often led to greater shifts in MCC than glucose. In conclusion, we demonstrate the drastic effects that root phenolics can have on soil organic matter dynamics and advocate for the inclusion of secondary metabolites in future priming studies.


Marie Zwetsloot is a postdoctoral researcher in the Soil Biology Group at Wageningen University. She completed her graduate research at Cornell University, specializing in soil and rhizosphere ecology. In specific, she is interested in how plant-soil interactions drive soil biogeochemical cycles from the microbial to field scale in both managed and natural ecosystems. Among others, she has published on how phenolic root exudates influence soil organic matter dynamics and the implications of belowground tree species interactions for carbon cycling in forests. She is currently also involved in the LANDMARK project, which aims to quantify soil functions across Europe.


7th International Symposium
Soil Organic Matter

6 – 11 October 2019

Hilton Adelaide

Adelaide, South Australia


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