Revisiting the primary production control on wetland methane emission

The relationship between plant primary production and wetland methane (CH₄) emission is well established. This relationship is expected because plant production fuels methanogenesis and plants act as conduits for gas exchange between the soil and the atmosphere. Recent global increases in bottom-up measurements of wetland CH₄ provide a new opportunity to revisit the hypothesis that plant production and CH₄ flux have a positive linear relationship in wetlands. 

Using paired CH₄ and gross primary productivity (GPP) measurements from 56 wetland sites, we found that CH₄ and GPP are weakly related, with the maximum R² from linear regressions being 0.14 (p= 0.0081). Instead, we found some support for a unimodal relationship (R²= 0.24, p= 0.0016) between GPP and CH₄ flux. While flooded sites exhibited strong GPP-CH₄ relationships, sites where the mean annual water table depth was below the soil surface showed weak or no GPP-CH₄ relationship. This suggests that variable degrees of CH₄ oxidation, among other factors, could be weakening the apparent relationship between GPP and CH₄. 

In this presentation, we will discuss processes that could disrupt the expected positive linear relationship between plant production and CH₄ using multi-scale examples from syntheses, lab and field experiments. We will also explore why the GPP-CH₄ relationship may not scale across space and time and how this affects the utility of GPP as a CH₄ predictor.