Abstract
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.
Highlights
Empirical models of soil organic carbon (SOC) turnover and sequestration are increasingly recognizing an important role for manganese (Mn2+), which is a co-factor for an enzyme, Mn-peroxidase (MnP), involved in oxidative decomposition of organic matter (Berg et al 2015; Stendahl et al 2017; Keiluweit et al 2015; Jones et al 2020)
Total N% was greater across Podzol plots but C:N ratios were equivalent between soil types for both substrates (Table 1)
For the subset of Agaricomycete fungi we found almost 500 species in the combined saprotrophic and ectomycorrhizal fungal communities, which were clearly aligned by soil type (p < 0.001) and, to a lesser degree, substrate (p = 0.028) (Fig. 2)
Summary
Empirical models of soil organic carbon (SOC) turnover and sequestration are increasingly recognizing an important role for manganese (Mn2+), which is a co-factor for an enzyme, Mn-peroxidase (MnP), involved in oxidative decomposition of organic matter (Berg et al 2015; Stendahl et al 2017; Keiluweit et al 2015; Jones et al 2020). Widespread increases in SOC could in theory greatly offset greenhouse gas CO2 emissions, and a large effort is collectively underway to examine how land use and management practices, such as afforestation, tree species selection and retention of harvest residues, might influence soil C stocks (Jandl et al 2007; Jastrow et al 2007; Mayer et al 2020). More direct interventions, such as applications of biochar or crushed basalt to soils, have drawn attention for their potential in enhancing SOC storage (Beerling et al 2018). As a potential key driver of decomposition and organic matter accumulation it is worth considering whether management systems affecting Mn availability might prove to be another avenue for influencing SOC sequestration
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