Spatio-temporal heterogeneity in extracellular enzyme activities tracks variation in saprotrophic fungal biomass in a temperate hardwood forest

Abstract

Saprotrophic fungi are the dominant producers of organic matter-degrading extracellular enzymes in forest soils, but the extent to which they drive spatio-temporal variation in enzyme activities is unclear. We quantified the relationships between saprotrophic fungal biomass and enzyme activities in a mature temperate hardwood forest by placing soil-filled ingrowth bags inside trenches. Trenches were located in replicate stands (n = 7) dominated by either arbuscular mycorrhizal (AM)- or ectomycorrhizal (ECM)-associated tree species, which reflect differences in soil substrate quality and nutrient availability. Additionally, we assessed the extent to which the relationships between saprotrophic fungal biomass and enzyme activities varied across seasons and were altered by experimental nitrogen (N) addition. Overall, both fungal production and enzyme activities differed between mycorrhizal types and across seasons, but were unaffected by N addition. Additionally, enzyme activities exhibited strong, positive relationships with saprotrophic fungal biomass. However, the relative production of carbon (C)-degrading enzymes was greater in stands dominated by AM-associated trees in the late growing season, and the relative production of N-degrading enzymes was greater in the late growing season across both AM- and ECM-dominated forest stands. These patterns reflected tree- and season-driven changes in microbial C and N limitation. As such, while the common assumption that microbial biomass and enzyme activities are positively correlated is robust for saprotrophic fungi, enzyme produced per unit biomass may not be consistent among stands and across seasons.