Drought-induced reductions in net methane emissions from an ombrotrophic peatland are enhanced across a range of experimental warming treatments
Authors: Hanson Paul , Phillips Jana , Iversen Colleen , Ricciuto Daniel , Yuan Fenghui , Zhang Jingwei , Xu Xiaofeng 
Published: March 2022

&lt;p&gt;Peatlands represent a dominant source of natural CH&lt;sub&gt;4&lt;/sub&gt; emissions from the land surface to the atmosphere and the quantitative nature of CH&lt;sub&gt;4&lt;/sub&gt; emissions for future climatic conditions is a key unknown. The SPRUCE experimental warming by elevated CO&lt;sub&gt;2&lt;/sub&gt; study located in northern Minnesota has been addressing this question for &lt;em&gt;in situ&lt;/em&gt; forested peatland plots since 2016 for five different warming treatments (+0, +2.25, +4.5, +6.75 and +9 &amp;#176;C). Under predominantly wet conditions from 2016 through 2019 (2020 observations were not obtained due to COVID travel restrictions) with minimal reductions in peatland water table levels, CH&lt;sub&gt;4&lt;/sub&gt; emissions showed an exponential increase across warming treatments with no apparent impact from mid-summer drying conditions nor evidence of a clear elevated CO&lt;sub&gt;2&lt;/sub&gt; response. The CH&lt;sub&gt;4&lt;/sub&gt; emissions were less than 1 &amp;#181;mol m&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt; for ambient or low temperature treatments but ranged from 2 to 5 &amp;#181;mol m&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt; under the +6.75 and +9 &amp;#176;C warming treatments.&amp;#160; Moisture and water table levels had minimal impacts on net CH&lt;sub&gt;4&lt;/sub&gt; flux during this wet period.&lt;/p&gt;&lt;p&gt;The 2021 summer season, however, provided extremely low precipitation and high evapotranspiration that led to reduced average water table depths across the warming treatments to -0.34, -0.45, -0.54, -0.71 and &amp;#160;&amp;#160;&amp;#160;-0.83 m, respectively. These drought-induced drops in the water table led to aeration of the surface peat layers (acrotelm) and effectively shut off CH&lt;sub&gt;4&lt;/sub&gt; production in the top layers of the bog. Some evidence for limited net CH&lt;sub&gt;4&lt;/sub&gt; uptake to the bog during the driest conditions (-0.001 to -0.01&amp;#181;mol m&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;) suggested that CH&lt;sub&gt;4&lt;/sub&gt; oxidation was playing a role in the reductions of net CH&lt;sub&gt;4&lt;/sub&gt; emissions. An empirical fitted relationship for net CH&lt;sub&gt;4&lt;/sub&gt; flux as a function of peat temperatures at -0.2 m and water table depth was developed across all treatments and years. That fitted curve showed that net CH&lt;sub&gt;4&lt;/sub&gt; emissions were precluded when water table levels dropped below -0.3 m.&amp;#160; This depth corresponds to the peat acrotelm layer containing most of the live root production and activity. The ELM_SPRUCE model was used to fuse the CH&lt;sub&gt;4&lt;/sub&gt; data to investigate the causes of reduction in CH&lt;sub&gt;4&lt;/sub&gt; emission. The model was able to reconstruct the dynamics of substrates and CH&lt;sub&gt;4&lt;/sub&gt; processes under ambient and warming treatments; hydrological feedback was confirmed as warming drives water table drop, which is exacerbated by drought in the summer of 2021.This data-model integration approach suggests the roles of mechanistic models in understanding CH&lt;sub&gt;4&lt;/sub&gt; cycling in response to warming and drought interactions in future climates.&lt;/p&gt;