COS 5-5 - CANCELLED - Nitrogen addition and temperature shifts alter the chemical composition, thermodynamics, and stoichiometry of the DOM pool in Sphagnum peat

Abstract: Boreal peatlands contain a large proportion of global soil carbon. Small, soluble compounds in peat soils that make up the dissolved organic matter (DOM) pool can be both an indicator of microbial decomposition, and an important driver of decomposition pathways. The objectives of this study were to understand the role of temperature and nutrient addition on DOM (hereafter, metabolites) and microbial nutrient use pathways in the peat system.

We investigated the effect of warming and nitrogen (N) addition on the metabolite profile of Sphagnum peat samples that were obtained adjacent to the nutrient limited SPRUCE site (Spruce and Peatland Responses Under Changing Environments, northern MN). We isolated the metabolites from the water-methanol-extractable fractions of peat following the MPLEx protocol. Samples were amended with inorganic (500 µg-N of NO₃ or NH₄) or organic (glutamate [C₅H₉NO₄]) N solutions and with an unamended control. Samples were incubated for up to 5 days at either 15 or 25 °C. Samples were collected at 3 hours (day 0), as well as days 1, 3, and 5 (n = 3 across nutrient amendments, temperatures, and time points). Metabolites were analyzed in a 21T FTICR-MS (negative mode). Peak picking was performed using a Mage File processor and NOMSI. Data were calibrated and formula assigned using Formularity. Metabolites with peaks outside the range of m/z 200–900 were removed as were metabolites that occurred in fewer than 3 samples.

We found that N amendment is the strongest driver of metabolite composition and that a greater number of metabolites could be statistically linked to unamended controls than could be linked to N amended samples. With regard to specific compounds, there was a greater richness of condensed hydrocarbons, and a lower richness of proteins, associated with 25 °C. While the averaged metabolite pool became more degraded (i.e., less thermodynamically favorable) during the duration of the experiment, there was consistently greater degradation at 25 °C across all timepoints. In addition, samples amended with organic N were even more strongly degraded, and were characterized by significantly lower H:C and N:C ratios as well as greater N:P ratios in the metabolite pool. Together, these findings demonstrate the joint role of temperature and nutrient supply changes on DOM characteristics of boreal peatland ecosystems.