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.