PS 22-12 - Landscape-scale soil carbon and nitrogen dynamics in sagebrush-dominated ecosystems of the Intermountain West

Background/Question/Methods

Big sagebrush (Artemisia tridentata) landscapes cover approximately 485,000 km² of North America and are characterized by variable spatial patterns of soil carbon (C) storage and slow C accumulation and decomposition. Ecosystems that represent a small proportion of the landscape may store more and cycle C and nitrogen (N) faster than the dominant big sagebrush ecosystems and disproportionately contribute to emissions at larger scales. We investigated the role of ecosystems that could be potential hot spots (i.e. wetlands, sub-irrigated hay meadows) for landscape-scale soil greenhouse gas (GHG) and C storage dynamics in a big sagebrush steppe landscape in southwestern Wyoming. To do this, we measured weekly trace gas fluxes (carbon dioxide, methane, and ammonia), the soil C and N pools, and net N mineralization across four ecosystem types (wetlands, hay meadows, upland, and sloping big sagebrush), replicated at three locations over the summer of 2021. Using the NatureServe ecosystem classifications, we calculated the proportion of the landscape represented by each ecosystem, excluding forested and montane areas, which we did not measure. We then scaled up the soil GHG fluxes and soil C values to represent the relative contribution of each ecosystem type to landscape C and N dynamics.

Results/Conclusions

We found significant differences in carbon dioxide fluxes, methane fluxes and inorganic N among ecosystem types, with the wetlands and hay meadows storing more and cycling C and N faster than the big sagebrush ecosystems. We did not find significant differences in ammonia fluxes among ecosystem types. On average, the carbon dioxide emissions per m² from the hay meadows and wetlands were 613% and 264% greater than those from the big sagebrush ecosystems respectively. On average, methane emissions per m² from the wetlands were 165,890% greater than that consumed in the big sagebrush ecosystems. Though hay meadows and wetlands cover approximately 13% and 1% of the landscape respectively, on average, hay meadows accounted for 47% of carbon dioxide fluxes and wetlands accounted for 96% of methane fluxes. Despite big sagebrush ecosystems covering approximately 86% of the landscape, GHG dynamics are dominated by fluxes occurring in hot spots (i.e. wetlands and hay meadows). Given that these ecosystem types are primarily introduced to the landscape, this work contributes to our knowledge of how land use can affect C and N dynamics at large scale and can inform our understanding of how to best represent these hot spots in ecosystem and Earth-system models.