Manganese (Mn) has been identified as a regulatory bottleneck in the accumulation of humus because of its role as an enzymatic co-factor in the breakdown of recalcitrant C by Mn-peroxidase (MnP). We tested this abiotic limit on decay via contrasting soils along a podzolization gradient of coastal British Columbia, where an inverse exponential relationship between soil organic carbon (SOC) and exchangeable Mn had been observed. Moderately weathered soils (Brunisols) had an average 3.6-fold increase in MnP activity within the upper soil profile in comparison to highly weathered Podzols. An ordination of the Agaricomycete fungal community, which are responsible for MnP production in soils, confirmed significant differences in assemblages across soil types for saprotrophic fungi, particularly species within the Agaricales, Trechisporales and Auriculariales. Ectomycorrhizal fungi of Pseudotsuga menziesii were equally aligned with soil type and select taxa more abundant on Brunisols may have supplemented MnP activity. A laboratory incubation with an Mn amendment produced significant interactions in MnP activity by soil type. Surprisingly, MnP activity of both Brunisol substrates declined substantially with an amendment (-56% and -40% for forest floor and mineral soil, respectively), in contrast to Podzols (-30% and +26%, respectively). This inhibitory response was linked to considerable uptake of the amendment, and underscores how Mn²⁺ operates directly on fungi as a regulator of mnp transcription for MnP production. Our study highlights a new perspective concerning the abiotic drivers underpinning the large, expansive soil C stocks across perhumid temperate rainforests of the Pacific Northwest.