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

Abstract

Upland forest soils oxidize 22-38 Tg CH4 yr-1, roughly 5% of the total atmospheric methane sink. A recent study 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). Four predictions were not supported. At the individual-measurement scale, neither monthly precipitation nor direct soil moisture explained more than 1% of CH4 flux variance (R2 = 0.0008 and 0.0055). While precipitation emerged as a significant interannual predictor when data were aggregated to annual-site means ({beta} = 0.249, p = 0.002), it did not eliminate the residual multi-decadal decline ({beta}year = 0.211, p = 0.007). No seasonal moisture-flux structure matched diffusion predictions. Urban and rural BES forests diverged despite sharing a regional precipitation regime (YearxLandUse interaction, p = 0.007), and a residual temporal trend persisted after controlling for moisture, temperature, and spatial pseudoreplication (p = 0.002). A structural breakpoint at 2002 (BES) and a putative shift at 2011 (HBR) aligned with atmospheric deposition trends rather than precipitation. A fifth test, the Hubbard Brook calcium amendment, yielded a null result that does not discriminate between mechanisms but constrains methanotrophic recovery potential. 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.