COS 125-5 - Effects of warming and elevated [CO₂] on N₂-fixation and its coupling to CH₄-oxidation in the Sphagnum moss microbiome

Background/Question/Methods

Sphagnum mosses represent keystone species in boreal peatlands, where they dominate ecosystem net primary productivity and facilitate the accumulation of carbon (C) in thick peat deposits. Sphagnum spp. associate with a diverse assemblage of microbial partners which directly support moss productivity and ecosystem function by regulating fluxes of C and nitrogen (N). Climate change is expected to impact both Sphagnum and their associated microbiomes, likely resulting in critical disruptions to C and N cycles in boreal peatlands. The objective of this study was to investigate the response of the Sphagnum phytobiome (plant host + constituent microbiome + environment) to experimental warming and elevated [CO₂], with a focus on diazotrophic (N₂-fixing) and methanotrophic (CH₄-oxidizing) microorganisms. We asked: (1) How do climate change perturbations (warming, CO₂, drought) influence the composition and activity of Sphagnum-associated diazotrophs and methanotrophs? (2) How will changes in the Sphagnum phytobiome influence ecosystem fluxes of C and N? We used stable-isotope incubations, next-generation sequencing, and quantitative PCR (qPCR) to quantify the effects of whole-ecosystem warming (from +0°C to +9°C) and elevated [CO₂] on the Sphagnum phytobiome in the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment, located in an ombrotrophic bog in northern Minnesota, USA.

Results/Conclusions

We found that rates of N₂-fixation in the Sphagnum phytobiome were significantly impacted by experimental warming, Sphagnum water content, and plant-available NH₄-N in surface peat. Under ambient temperatures, the addition of ¹³CH₄ to our incubations enhanced N₂-fixation by 28-73%, supporting previous evidence that diazotrophy and methanotrophy are coupled in the Sphagnum phytobiome. We observed a significant reduction in both the CH₄-induced enhancement of diazotrophy and the relative abundance of putative diazotrophic methanotrophs with increasing temperature, indicating that warming disrupts the coupling of these two processes. In contrast, we found that CH₄-oxidation rates increased significantly with warming. This observation aligns with evidence that the SPRUCE site is becoming more methanogenic with warming, creating a more favorable environment for methanotrophs to capture CH₄ released at the bog’s surface. Prior research indicates dramatic mortality of Sphagnum with warming. When coupled with impacts to diazotrophy, these changes have the potential to significantly disrupt both N and C cycling in boreal peatlands by further restricting Sphagnum productivity and limiting the supply of N to these extremely oligotrophic ecosystems. While increasing rates of CH₄-oxidation may act as a natural mechanism to offset increasing CH₄ emissions in warming peatlands, Sphagnum mortality will limit CH₄-oxidation at the ecosystem-scale.