Organic-Matter Decomposition Responses to Whole-Ecosystem Warming in a Northern Peatland

Authors

Natalie A. Griffiths¹* (griffithsna@ornl.gov), Scott D. Tiegs², Randall K. Kolka³, Keith C. Oleheiser¹, Paul J. Hanson¹

Institutions

¹Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN; ²Department of Biological Sciences, Oakland University, Rochester, MI; ³Northern Research Station, U.S. Forest Service, Grand Rapids, MN

URLs

• https://mnspruce.ornl.gov
• http://tes-sfa.ornl.gov/

Abstract

Peatlands are carbon-rich ecosystems. Globally, peatlands cover only 3% of the Earth’s land surface but store 30% of terrestrial carbon. Peatlands in northern environments accumulate carbon over time because saturated, cold, and acidic conditions result in slow organic matter decomposition rates. However, northern peatlands are vulnerable to climate change, with warmer temperatures expected to accelerate decomposition rates. Within the SPRUCE project, multiple organic matter decomposition experiments are being conducting to better understand the responses of decomposition to warming. Researchers used a litterbag approach to measure decomposition rates of six litter types (e.g., spruce needles and fine roots, Labrador tea leaves and fine roots, and two Sphagnum species). A peat-ladder approach was also used to measure decomposition rates of peat soil at 10 cm depth increments (from 0–40 cm). After multiple years of incubation (6 years for the plant tissues and 3 years for the peat soil), results showed little effect of warming on decomposition rates.

While Labrador tea leaves and fine roots decomposed faster with warming, the other types of plant litter and peat soils did not respond to warming. These decomposition studies were conducted in shallow peat, limiting understanding of decomposition responses at deeper depths. Further, because decomposition rates are slow in peatlands, litterbag and peat-ladder approaches that integrate decomposition rates over multiple years make it difficult to understand seasonal and interannual variation and elucidate environmental drivers. Therefore, the third approach used cotton strips, which are a labile carbon source (95% cellulose), to examine decomposition responses to warming seasonally (winter, summer) and throughout the depth profile (surface to 1.3 m depth). Cotton strips (1.37 m long, 2.5 cm wide) were deployed into peat seasonally for 5 years. Results showed strong positive responses of organic matter decomposition to warming but only at depths >35 cm. At shallower peat depths, soil moisture was a more important factor, with faster decomposition rates when peatland water level was high. The cotton strip results suggest that soil moisture may be a more dominant factor affecting organic matter decomposition than temperature in near-surface peats, thus explaining the general lack of warming response in the litterbag and peat-ladder studies. These findings suggest that the direct (warming) and indirect (drying) effects of temperature on decomposition should be incorporated into models examining future climate change impacts on peatland carbon cycling.