Manganese limitations and the enhanced soil carbon sequestration of temperate rainforests

Manganese (Mn) has been identified as a regulatory bottleneck in carbon (C) turnover because of its role as an enzymatic co-factor in the oxidative decomposition of C by Mn-peroxidase (MnP). We tested this limit on decay using forest soils from coastal British Columbia with contrasting Mn concentrations. 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. Ordination of the Agaricomycete fungal community, which are responsible for MnP production, confirmed significant differences in assemblages between soil types for saprotrophic fungi, particularly species within 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 added Mn in Brunisols, presumably by saprotrophic fungi, and underscores how Mn likely operates directly on fungi as a regulator of gene transcription for MnP production. Our study highlights a new perspective concerning the abiotic drivers underpinning the expansive soil C stocks across perhumid temperate rainforests of the Pacific Northwest.