Stability of peatland carbon to rising temperatures

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.


Supplementary Figure 3: CH4 and CO2 production in legacy effect incubations
Test for legacy effects of experimental warming on (a) CH4 and (b) CO2 production from peat samples taken at multiple depths and anaerobically incubated at a common temperature (20°C) after approximately 13 months of DPH. 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. Data are plotted against the temperature treatment from which the peat was collected. Note the lack of significant temperature response across all depths.

Supplementary Figure 4: Total dissolved organic carbon (DOC) concentrations
DOC concentrations in the peat porewater (a) prior to and (b) during deep peat heating. The relative deviation of DOC (c) was calculated to account for pre-treatment differences in DOC concentrations across plots. The deviation was calculated by dividing the DOC concentration at a given temperature treatment and a given depth by the mean DOC concentration at that same depth in the two control (0° C) plots prior to DPH (c). Points represent the averages of weekly (pre-DPH) or biweekly (during DPH) sampling and standard deviations of samples from all time points are indicated by the error bars. The blue circles represent results from the control (+0°C) plot, turquoise squares represent results from the +2.25°C treatment, gold diamonds represent results from the +4.5°C treatment, orange triangles represent results from the +6.75°C treatment, and magenta inverted triangles represent results from the +9°C treatment.

Supplementary Figure 5: Microbial community structure
Depth dependence of soil microbial community structure in SPRUCE site enclosures prior to (closed symbols) and after (open symbols) exposure to deep peat heating (DPH). Circles represent 0-10 cm, squares represent 10-20 cm, diamonds represent 20-30 cm, triangles represent 30-40 cm, inverted triangles represent 40-50 cm, hexagons represent 50-75 cm, x's represent 75-100 cm, + represent 100-125 cm, stars represent 125-150 cm, crossed circles represent 150-175 cm, and crossed squares represent 175-200 cm. Community structure exhibits strong vertical stratification in the peat column as visualized in a nonparametric multidimensional scaling plot (NMDS). Pairwise community distances were determined using the weighted Unifrac algorithm. A total of 5.35 million of rRNA gene sequences were normalized by cumulative sum scaling (CSS) methods and grouped by depth. As shown in Figure 4, no significant effect of temperature treatment or time is observed on community diversity or composition.

Supplementary Figure 6: Phylum level microbial relative abundances
Depth dependence of soil microbial groups detected at the phylum level (> 1 % divergence in gene sequences) in (a) control, (b) +2.25°C, (c) +4.5°C, (d) +6.75°C, and (e) +9°C plots prior to (2014) and after (2015) exposure to deep peat heating (DPH). Bars are stacked by date such that pre-DPH (2014) and during DPH (2015) results are proximate. Purple cross hatching represents results for Acidobacteria, solid white bars represent results for Proteobacteria, blue cross hatching represents results for Verrucomicrobia, solid light gray represents results for Planctimycetes, green cross hatching represents results for Actinobacteria, dark gray solid represents results for Bacteroidetes, and solid black represents results for all other phyla. The majority of microbial populations (~70%) are taxonomically affiliated to Proteobacteria and Acidobacteria phyla. A total of 5.35 million of rRNA gene sequences were assigned to the greengenes database by RDP Classifier at 50% confidence thresholds. Phyla which represented < 1% of relative abundance were not displayed and are summarized as Other.

Supplementary Figure 7: Alphaproteobacteria and Deltaproteobacteria abundances
Depth dependence of soil microbial groups detected at the class level in (a) control, (b) +2.25°C, (c) +4.5°C, (d) +6.75°C, and (e) +9°C plots prior to (closed symbols) and after (open symbols) exposure to deep peat heating (DPH). Putative aerobic heterotrophs affiliated with the Alphaproteobacteria (circles) decreased in average relative abundance with depth, while putative anaerobes in the Deltaproteobacteria (triangles) increase with depth. Solid lines connect symbols for Alphaproteobacteria and dashed lines connect symbols for Deltaproteobacteria.

Supplementary Figure 8: Comparison of Acidobacteriia and TM1 relative abundances
Depth dependence of soil microbial groups affiliated with Class Acidobacteria in (a) control, (b) +2.25°C, (c) +4.5°C, (d) +6.75°C, and (e) +9°C plots prior to (closed symbols) and after (open symbols) exposure to deep peat heating (DPH). Circles denote Acidobacteriia and triangles denote TM1. Putative aerobic heterotrophs affiliated with the Acidobacteriia decreased in relative abundance with depth, while putative anaerobes in the TM1 class increase with depth. Solid lines connect symbols for Acidobacteriia and dashed lines connect symbols for TM1.

Supplementary Figure 9: Abundances of microbial groups in treatment plots
The abundance of (a) fungal, (b) bacterial, (c) archaeal and (d) mcrA gene copies were determined by quantitative PCR using primers targeted to amplify their respective SSU rRNA genes, and targeting the mcrA gene for methanogen populations. Microbial abundance is expressed for core samples from control (+0⁰C) and +9⁰C plots as gene copies per gram dry peat. Magenta circles represent results from control temperature treatment (0°C) and blue inverted triangles represents results from +9°C treatment plots. After thirteen months of deep peat heating (DPH) treatment, the in situ abundance of microbial groups (bacteria, archaea, fungi, and methanogen populations) shows no clear response to temperature, while strong vertical stratification is observed with peat depth.

Supplementary Figure 10: Relative abundances of methanogens out of total Archaea
Depth dependence of known methanogenic Archaeal groups in treatment plots prior to and after exposure to deep peat heating (DPH) in (a) control (+0°C), (b) +2.25°C, (c) +4.5°C, (d) +6.75°C, and (e) +9°C plots. Apparent zero abundances (e.g. pre-DPH 0-10cm in the +2.25°C plot) reflect missing data. Abundance of known methanogens gradually decreases with peat depth, while no significant effect of temperature treatment or time on relative abundance of methanogens was observed. Pre-DPH is represented by closed bars and during DPH by open bars. Whiskers represent one standard deviation of replicate sample values.