Integrating environmental metabolomics and proteomics to describe changes in carbon decomposition resulting in increased greenhouse gas flux under whole ecosystem warming at the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experimental site

Following the deep peat heating experiment at the SPRUCE site (Wilson and Hopple et al., 2016, Nature Communications, 7: 13723) air warming was added to treatment plots to achieve whole ecosystem warming (WEW). We have continued measuring CO₂ and CH₄ concentrations and stable C isotopes and added high resolution analyses of the environmental metabolome (FTICRMS of large molecule metabolites, GCMS of small molecule metabolites, NMR to quantify energetic substrates (e.g. acetate), and an omics analysis of lipids). We hypothesized that WEW would stimulate primary production leading to increased fresh labile organic C inputs to the surface soil. We previously showed (1) heterotrophic respiration within the peat to be largely driven by dissolved organic matter (Tfaily et al., 2014, 2018; Wilson and Hopple et al., 2016) and (2) that shallow depths are most susceptible to enhanced warming (Wilson and Hopple et al., 2016). Thus, increases in labile organic matter inputs to surface soil are expected to result in significant changes to CO₂ and CH₄ production/emission dynamics at the site. Consistent with this expectation, CH₄ and CO₂ concentrations in porewater increased with temperature, with CH₄ showing a larger effect resulting in declining CO₂:CH₄ ratios. This result is consistent with prior findings that surface peat becomes more methanogenic with temperature perturbation. Organic matter characterizations revealed significant correlations between metabolite concentrations and temperature for small sugars, amino acids, and some lipids in surface peat (≤50 cm). Small sugars and diacylglycerols increase, while amino acid concentrations decrease with temperature. We integrated environmental metabolomics with proteomics to construct a metabolic network. Changes in the metabolic network with temperature treatment were consistent with stimulation of energy-yielding pathways such as glycolysis and downregulation of amino acid synthesis. These results are consistent with increases in microbial biomass in response to increased availability of labile substrates, higher cell maintenance costs, and a reduction in the pool of available free amino acids in the porewater while increasing the production of CO₂ and CH₄ as respiration end products.