Stability of peatland carbon to rising temperatures

Abstract

Peatlands contain one-third of soil carbon (C), mostly buried in deep, saturated anoxic zones (catotelm). The response of catotelm C to climate forcing is uncertain, because prior experiments have focused on surface warming. We show that deep peat heating of a 2 m-thick peat column results in an exponential increase in CH₄ emissions. However, this response is due solely to surface processes and not degradation of catotelm peat. Incubations show that only the top 20-30 cm of peat from experimental plots have higher CH₄ production rates at elevated temperatures. Radiocarbon analyses demonstrate that CH₄ and CO₂ are produced primarily from decomposition of surface-derived modern photosynthate, not catotelm C. There are no differences in microbial abundances, dissolved organic matter concentrations or degradative enzyme activities among treatments. These results suggest that although surface peat will respond to increasing temperature, the large reservoir of catotelm C is stable under current anoxic conditions.

Figure 1. Schematic of the SPRUCE site located in northern Minnesota.

Three boardwalks transect the site, with experimental treatments plots branching radially off of those boardwalks. Numbers indicate the target temperatures, relative to ambient conditions, established within each enclosure. ‘Amb' plots indicate that no temperature treatment has been added. Inset shows an aerial overview of the site with the experimental enclosures installed in the context of the surrounding bog.

Figure 2. Peat temperatures throughout DPH treatment period.

The seasonal progress of (a) absolute peat temperatures at 2 m below the hollow surface throughout the DPH treatment period and (b) the temperature depth profiles associated with the 16 June 2015 coring event. This coring event took place 10 months after the deep peat temperature differentials were stable. In a, blue denotes control (+0 °C) plots, green denotes +2.25 °C plots, gold denotes +4.5 °C plots, orange denotes +6.75 °C plots, and red denotes +9 °C plots. In b, circles denote control (+0 °C) plots, squares denote +2.25 °C plots, diamonds denote +4.5 °C plots, triangles denote +6.75 °C plots and inverted triangles denote +9 °C plots. In the absence of air warming during this phase of the experiment, anticipated energy loss reduced the separation among treatment temperatures at the surface.

Figure 3. Seasonal CH₄ fluxes from S1 bog.

Seasonal CH₄ flux vs in situ temperatures from 1.2 m diameter collars during (a) fall 2014, (b) winter 2014-2015 and (c) summer 2015. Black and grey dots distinguish between daily averages for two different sampling times during each season. Significant correlations between flux and temperature as exponential regressions are indicated on the graphs by black lines for September (a) and June (c), and grey lines for October (a) and July (c).

Figure 4. CH₄ production in anaerobic incubations.

Temperature response of CH₄ production from (a) surface and (b) deep peat samples that were anaerobically incubated within 1 °C of in situ temperatures after ∼4 (closed symbols, September 2014) and 13 (open symbols, June 2015) months of deep peat warming. Circles represent values from 25 cm, triangles represent results from 75 cm, squares represent results from 100 cm, diamonds represent results from 150 cm and inverted triangles represent results from 200 cm. Temperatures reflected in situ temperatures at time of collection. The temperature response of deep peat (b) for each season was analysed separately due to a distinct bimodal distribution. Dark line indicates significant regression results. NS, not significant.

Figure 5. CO₂:CH₄ ratios in anaerobic incubations.

Peat samples were collected from five depths and anaerobically incubated within 1 °C of in situ temperatures after ∼4 (closed symbols; September 2014) and 13 (open symbols; June 2015) months of deep peat warming. The circles indicate results from peat collected from 25 cm, triangles indicate results from 75 cm, squares indicate results from 100 cm, diamonds indicate results from 150 cm and inverted triangles represent results from 200 cm. The line indicates the regression result for the 25 cm incubations. NS, not significant. The log scale is worth noting.

Figure 6. Isotopic composition of respiration products and substrates.

Depth profiles of (a) ¹⁴C for solid peat, DOC, CH₄ and dissolved CO₂ (DIC), and (b) differences in stable carbon isotopic (δ¹³C) composition between DIC and CH₄ (α_C=[(δ¹³CO₂+1000)/(δ¹³CH₄ +1000)]) with depth during DPH (June 2015). In a, closed symbols represent values from control plots prior to DPH when no treatment (that is, +0 °C) was applied, open symbols represent values from +9 °C treatment plots during DPH (June 2015). In a, black squares denote radiocarbon peat values, closed blue circles denote radiocarbon DOC values from control (0 °C) plots, blue triangles denote radiocarbon DIC values from control plots, magenta diamonds denote radiocarbon DOC values from +9 °C plots, magenta inverted triangles denote radiocarbon DIC values from +9 °C plots, blue stars denote radiocarbon CH₄ values from control plots, and magenta x's denote radiocarbon CH₄ values from +9 °C plots. In b, blue circles denote α_C from control plots, turquoise squares denote α_C from +2.25 °C plots, gold diamonds denote α_C from +4.5 °C treatment plots, orange triangles denote α_C from +6.75 °C plots and magenta inverted triangles denote α_C from +9 °C plots. Note the age difference between solid peat and all DOC and DIC values.

Figure 7. Microbial community structure in treatment plots.

Characterization of in situ microbial community structure by non-metric multidimensional scaling (NMDS) in (a) control plots, (b) +2.25 °C plots, (c) +4.5 °C plots, (d) +6.75 °C plots and (e) +9 °C plots. Symbols represent averages of two samples collected from each plot/depth within each temperature treatment plot from pre-DPH (2014, closed symbols) and 13 months post-initiation of the DPH (2015, open symbols) experiment. Error bars represent 1 s.d. of replicate results. Circles represent results from the control (0°C) plots, squares represent results from the +2.25°C plots, diamonds represent results from the +4.5°C plots, triangles represent results from the +6.75°C plots, and inverted triangles represent results from the +9°C plots. Final sequence data were normalized by cumulative sum scaling (CSS) and beta diversity indices were estimated based on Bray–Curtis and weighted as well as unweighted Unifrac distances. Significant differences in beta diversity were analysed by a PERMANOVA test on weighted Unifrac distance metrics with 1,000 permutations followed by Bonferroni correction of P-values.