The Science

The dynamics and availability of soil nutrients that limit plant and microbial growth underpin ecosystem responses to changing environmental conditions. Researchers investigated climate impacts on peat nutrient availability within the framework of the large-scale Spruce and Peatland Responses Under Changing Environments (SPRUCE) warming and CO₂-enrichment experiment in a nutrient-limited bog at the southern end of the boreal peatland range. Above- and below-ground warming exponentially increased nutrient availability throughout the belowground peat profile, especially in recent years, as the carpet of Sphagnum mosses at the peat surface died in the warmest experimental treatments. However, nutrient dynamics were not yet affected by elevated CO₂. 

The Impact

Peatlands cover less than 3% of the world’s land surface but hold at least one third of global soil carbon in deep deposits of peat. Increases in peat nutrient availability in response to warming could impact plant and microbial community growth and decomposition, and therefore affect peatland carbon storage. However, the magnitude and timing of the observed increases in peat nutrient availability with warming in the SPRUCE experimental plots were not captured in the virtual space of ELM-SPRUCE—a special version of the Energy Exascale Earth System Model (E3SM) land model (ELM) developed for simulating the unique vegetation, hydrology, and soil biogeochemistry in peatland ecosystems. This mismatch pinpoints a need for improved model mechanisms controlling nutrient cycling to predict future peatland climate responses. 

Summary

Warming is expected to increase the net release of carbon from peatland soils, contributing to additional future warming. This positive feedback may be moderated by the response of peatland vegetation to rising atmospheric CO₂ or to increased soil nutrient availability. Researchers asked (1) whether a gradient of whole-ecosystem warming (from +0°C to +9°C) would increase plant-available nitrogen and phosphorus in an ombrotrophic bog in Northern Minnesota and (2) whether elevated CO₂ would modify the nutrient response. They tracked changes in plant-available nutrients across space and time and compared with other nutrient pools. Afterwards, they assessed whether nutrient warming responses were captured by a point version of the land-surface model, ELM-SPRUCE. They found that warming exponentially increased plant-available ammonium and phosphate, but that nutrient dynamics were unaffected by elevated CO₂. The warming response increased by an order of magnitude between the first and fourth year of the experimental manipulation, perhaps because of dramatic mortality of Sphagnum mosses in the surface peat of the warmest treatments. Neither the magnitude nor the temporal dynamics of the responses were captured by ELM-SPRUCE. Relative increases in plant-available ammonium and phosphate with warming were similar, but the response varied across bog microtopography (raised hummocks and depressed hollows) and with peat depth. Plant-available nutrient dynamics were only loosely correlated with inorganic and organic porewater nutrients, likely representing different processes. Future predictions of peatland nutrient availability under climate change scenarios must account for dynamic changes in nutrient acquisition by plants and microbes, as well as microtopography and peat depth.

Principal Investigator

Colleen Iversen
Oak Ridge National Laboratory
iversencm@ornl.gov

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
brian.benscoter@science.doe.gov

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

The SPRUCE experiment is supported by the Biological and Environmental Research (BER) Program within the U.S. Department of Energy’s (DOE) Office of Science. Oak Ridge National Laboratory (ORNL) is managed by UT-Battelle, LLC, for DOE under contract DE-AC05-00OR22725. The researchers thank Sarah Bellaire, Alana Burnham, Kelsey Carter, Ingrid Slette, and Nathan Stenson for their help in the field or laboratory. They also thank the editorial staff and anonymous reviewers for their helpful feedback. The contributions of Stephen D. Sebestyen and funding for the Marcell Experimental Forest and laboratory analysis by the Forest Service were provided by the Northern Research Station of the U.S. Department of Agriculture Forest Service.

Related Links

• SPRUCE Website
• Dataset Information

References