Controls on spatial variation in porewater methane concentrations across United States tidal wetlands

Tidal wetlands can be a substantial sink of greenhouse gases, which can be offset by variable methane (CH 4 ) emissions under certain environmental conditions and anthropogenic interventions. Land managers and policymakers need maps of tidal wetland CH 4 properties to make restoration decisions and inventory greenhouse gases. However, there is a mismatch in spatial scale between point-based sampling of porewater CH 4 concentration and its predictors, and the coarser resolution mapping products used to upscale these data. We sampled porewater CH 4 concentrations, salinity, sulfate (SO 4 2− ), ammonium (NH 4 + ), and total Fe using a spatially stratified sampling at 27 tidal wetlands in the United States. We measured porewater CH 4 concentrations across four orders of magnitude (0.05 to 852.9 μM). The relative contribution of spatial scale to variance in CH 4 was highest between- and within-sites. Porewater CH 4 concentration was best explained by SO 4 2− concentration with segmented linear regression ( p < 0.01, R 2 = 0.54) indicating lesser sensitivity of CH 4 to SO 4 2− below 0.62 mM SO 4 2− . Salinity was a significant proxy for CH 4 concentration, because it was highly correlated with SO 4 2− ( p < 0.01, R 2 = 0.909). However, salinity was less predictive of CH 4 with segmented linear regression ( p < 0.01, R 2 = 0.319) relative to SO 4 2− . Neither NH 4 + , total Fe, nor relative tidal elevation correlated significantly with porewater CH 4 ; however, NH 4 + was positively and significantly correlated with SO 4 2− after detrending CH 4 for its relationship with SO 4 2− ( p < 0.01, R 2 = 0.194). Future sampling should focus on within- and between-site environmental gradients to accurately map CH 4 variation. Mapping salinity at sub-watershed scales has some potential for mapping SO 4 2− , and by proxy, constraining spatial variation in porewater CH 4 concentrations. Additional work is needed to explain site-level deviations from the salinity-sulfate relationship and elucidate other predictors of methanogenesis. This work demonstrates a unique approach to remote team science and the potential to strengthen collaborative research networks. • Methane variance was highest along environmental gradients and between sites. • Sulfate was negatively and non-linearly correlated with methane concentration. • Salinity was a significant, but weaker proxy for methane concentration.