Testing the precipitation-driven 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. A recent study documented a 53 %–89 % reduction at two long-term ecological research networks in the northeastern US and attributed it to increased precipitation via diffusion limitation (the precipitation-driven, or climatic, diffusion limitation hypothesis). We tested five predictions of that hypothesis against 27 years of chamber flux data from the Baltimore Ecosystem Study (BES, 1998–2025; n=9359) and 13 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 (measured 2011–2020, after the 2002 BES break) 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 (β=0.249, p=0.002), it did not eliminate the residual multi-decadal decline (βyear=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 (Year × LandUse 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.