Testing the Diffusion Limitation Hypothesis for Declining Methane Uptake in Forest Soils

Upland forest soils oxidize 22-38 Tg CH4 yr-1 (roughly 5% of the total atmospheric methane sink), and this capacity has been declining. Ni and Groffman (2018) documented a 53-89% reduction at two long-term ecological research networks in the northeastern United States and attributed it to increased precipitation via diffusion limitation. We tested five predictions of that hypothesis against 27 years of chamber flux data from the Baltimore Ecosystem Study (BES, 1998-2025; n = 9,359) and 14 years from the Hubbard Brook Experimental Forest (HBR, 2002-2015), using direct in-situ soil moisture measurements, a natural calcium silicate amendment, PRISM climate data, and NADP deposition records. Four predictions were not supported. Neither monthly precipitation nor direct soil moisture explained more than 1% of CH4 flux variance (R2 = 0.0008 and 0.0055, respectively). No seasonal moisture-flux structure matched diffusion predictions. Urban and rural BES forests diverged in their post-2012 trajectories despite sharing a regional precipitation regime (Year x Land Use interaction, p = 0.007), and a residual temporal trend persisted after controlling for moisture, temperature, and spatial pseudoreplication (p = 0.002). Structural breakpoints at 2002 (BES) and 2011 (HBR) aligned more closely with atmospheric deposition trends than with precipitation, and the moisture-flux coupling that existed before the 2002 break vanished entirely afterward. A fifth test (the Hubbard Brook calcium amendment) yielded a null result that does not discriminate between mechanisms but constrains the recovery potential of the methanotrophic community. The decline persists through 9 additional years of data. These results suggest that precipitation-driven diffusion limitation does not adequately account for the multi-decadal loss of CH4 uptake at these sites, and point toward chronic biological degradation, potentially through nitrogen-mediated inhibition of high-affinity methanotrophy compounded by structural changes from invasive earthworm activity. We outline specific molecular predictions testable through pmoA surveys of archived soils.