Background/Question/Methods The distinct rhizomorphic mats formed by ectomycorrhizal Piloderma fungi are significant features of the organic soil horizons of coniferous forests throughout the Pacific Northwest and have been shown to cover up to 49% of the forest floor. Piloderma mats create a unique soil environment and likely support the establishment of distinct microbial communities and activities. As part of an NSF-funded Microbial Observatory at the H.J. Andrews Experimental Forest in western Oregon, this study aimed to elucidate seasonal dynamics of Piloderma mats and their associated microbial communities. Paired mat and non-mat soil samples were taken at each of five sites in the summer, fall, early spring and late spring. The potential for chitin degradation was determined using N-acetylglucosaminidase (NAGase) activity, and DNA was extracted for molecular analysis. Fungal ITS and bacterial 16S community profiles were compared using T-RFLP analysis and their biomass was determined by quantitative PCR.
Results/Conclusions Piloderma mats had consistently greater NAGase activity, averaging ~60% higher than non-mats across all dates. The lowest activities for both sample types were seen in the summer when soils were driest. Mat NAGase activity increased 40% in the fall, while the non-mats showed no significant difference compared to the summer sampling, indicating that the increased activity in mats is not solely a response to increased moisture. The early and late spring samplings had higher activities for mats and non-mats (80% and 60% greater than the summer, respectively). There were not significant differences between the spring dates for either sample type. Fungal community profiles revealed peaks consistent with Piloderma to be dominant in mat samples, contributing up to 70% of the total fluorescence. Multivariate analysis revealed significant grouping of mat fungal communities and strong fungal indicator species for both mat and non-mat samples. Bacterial communities had significant clustering of mats and non-mats on the site level although patterns were not consistent across sites. Fewer bacterial indicator species were identified and were weaker compared to fungal indicators. Although shifts in both the fungal and bacterial communities occurred on a temporal scale, no significant trends were revealed across sites.