Diverging soil peroxidase activity under ectomycorrhizal versus arbuscular mycorrhizal conifers with increasing C:N and exchangeable manganese

Abstract

Ectomycorrhizal (EM) fungi purportedly contribute to the enzymatic decay of soil organic matter (SOM), in contrast to arbuscular mycorrhizal (AM) stands where SOM turnover may be more fully governed by free-living saprotrophic fungi. We tested this distinction in a 30-year-old mixedwood conifer trial by comparing total peroxidase activity (including manganese-peroxidase [MnP]), fungal communities and mass of the humus layer between Pseudotsuga menziesii (EM host), Thuja plicata (AM host), and a 50:50 mixture across a natural productivity gradient. Total peroxidase and MnP activity diverged between hosts as humus C:N ratio increased, culminating in 3- to 4-fold greater enzyme activity under P. menziesii on low fertility soils. This soil effect also correlated positively with exchangeable Mn, highlighting a possible further restriction on SOM turnover under EM stands. Peroxidase activity was well aligned with a subset of abundant EM fungal species, notably Piloderma olivaceum and Piloderma sphaerosporum, suggesting a much greater enzymatic contribution by these symbiotic fungi in comparison to saprotrophic fungi under T. plicata. After three decades, mass of the humus layer averaged 2.15 kg OM m⁻² and did not yet differ among stand types. However, a 3-fold range in humus mass in correlation with declining soil fertility under P. menziesii suggests that select EM fungal taxa engaged in organic N liberation can limit SOM accumulation. This research highlights the adaptive ligninolytic enzymatic capacity of EM fungal communities and underscores how dual soil properties (low N or high Mn availability) may enhance peroxidase production and SOM turnover in EM forests.