Plant Exudate-Soil Interactions Linked to Delayed Mineralization of Subsurface C Stores After 3 Years of Experimental Warming.

Abstract:

Following one year of experimental deep peat warming at the Spruce and Peatland Response Under Changing Environments (SPRUCE) project within the Marcell Experimental Forest (Minnesota, USA), we observed changes in shallow subsurface methane production, but no response of CO₂. The shift towards increasingly methanogenic conditions was itself concerning because methane is a much more potent greenhouse gas than CO₂ and could accelerate climate-peatland feedbacks to warming. However, at that time, we found little evidence that the vast majority of stored C in the peatland was vulnerable to rising temperatures. Three years after introduction of whole ecosystem warming (WEW), however, we find radiocarbon evidence that enhanced production of both CH₄ and CO₂ with warming is fueled by mobilization of the ancient buried peat that had previously been shown to be stable. Tandem radiocarbon (¹⁴C) and stable isotope (¹³C) mass balance modeling indicates that up to 30% of current DIC production originates from the mineralization of peat up to 2m depth. Using a combination of complementary ultra-high resolution geochemical and microbial analyses, we find evidence that increasing plant root exudates to the subsurface may be providing the necessary substrates to stimulate this decomposition of buried peat (“priming”). Although acetoclasty and hydrogentrophy have traditionally been considered the dominant pathways for methane production in peatlands, using our complementary approach we find multiple lines of evidence for methylotrophic methanogenesis including (1) correlations between peat temperature and porewater methanol concentrations, (2) all of the necessary enzymes involved in the methylotrophic methanogenesis pathway, and (3) SSU rRNA gene amplicon and metagenomic evidence for known methylotrophic methanogens in the higher temperature treatments suggesting that, as the peat warms, new pathways of methane production become increasingly favorable with the potential to further lower CO₂:CH₄ production ratios. Accumulation of plant-derived lignin decomposition products, including methanol, has increased with warming providing a compelling link between above-ground production and changing soil dynamics.