The importance of nutrients for microbial priming in a bog rhizosphere

Wetlands host anaerobic microbes which convert organic carbon into methane (CH₄), a powerful greenhouse gas. Wetland plants can influence which carbon compounds are available for microbial processing by exuding freshly fixed carbon from their roots. Exudation of carbon from plant roots can trigger microbial priming: the process of new carbon stimulating the microbial community into processing more soil carbon than they otherwise would have. This study utilized high resolution Fourier transform ion cyclotron mass spectrometry (FT-ICR-MS) analysis to probe the composition of soil organic compounds from the rhizosphere of Carex aquatillis, a common wetland sedge, which is known to have stimulated microbial priming within peat soil. The goal was to identify what types of molecules were created or lost during microbial priming in the wetland rhizosphere and thus advance mechanistic understanding of the process. FT-ICR-MS analysis demonstrated that more microbial transformations of carbon occurred among water-soluble compounds than among hydrophobic compounds, but that some hydrophobic compounds were processed. Crucially for understanding microbial priming, the root exudates triggered increased processing of high molecular weight molecules regardless of nutrient content but processed low molecular weight compounds only if they contained nitrogen or sulfur, essential nutrients for plant growth. The importance of sulfur in determining molecular utilization is noteworthy because priming literature typically focuses on nitrogen-mining. The fact that some molecules were processed and others were not is evidence for a selective priming effect in which some carbon compounds with specific properties are used at an increased rate, while others are left unaltered.