Manganese limitations and the enhanced soil carbon sequestration of temperate rainforests

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.