Fate of Decomposed Fungal Cell Wall Material in Organic Horizons of Old-Growth Douglas-fir Forest Soils

Abstract

Organic horizons of old-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forests are colonized by ectomycorrhizal fungi, which can form large patches or mats. Respiration, N mineralization, chitinase activity, and amino sugar C and N turnover are higher in soils colonized by ectomycorrhizal mats. To test if microbial activity in mat soil is driven by decomposition of fungal biomass, we incubated mat and non-mat soil with ¹³C¹⁵N-labeled, chitin-enriched fungal cell wall material. Cell wall material was readily decomposed during 22-d incubations in both soils, and cell-wall-derived ¹³C and ¹⁵N were traced into mineralized pools, extractable organic pools, microbial biomass, and microbial phospholipid fatty acids (PLFAs). Basal microbial activity was higher in mat soils. Although total respiration was higher in mat soils with cell walls added, more cell wall C was mineralized in non-mat than mat soils; this was inferred to reflect greater preferential respiration of cell wall C over soil organic matter (SOM). Mineralization of N from cell wall material was low, with no difference between soils, and positive priming of N mineralization from SOM occurred after 3 wk. Both the proportion of cell wall N assimilated and N growth yield efficiency (GYE) were higher than for cell wall C. The relative abundance of fungi was higher in cell-wall-amended soils, and cell-wall-derived ¹³C was found in fungal and Gram-negative bacterial PLFAs. Cell wall C assimilation and C GYE were higher in mat soils, which could be linked to greater mineralization of SOM or to differential activity of select microbial taxonomic groups. Chitin-rich fungal cell wall material was utilized as both a microbial N and C source; N was predominantly assimilated and C was predominantly metabolized. It is possible that fungal necromass may contribute to N stabilization in these soils.