Long- and short-term temperature responses of microbially-mediated boreal soil organic matter transformations

Abstract:

Microorganisms use exoenzymes to decay soil organic matter into assimilable substrates, some of which are transformed into CO₂. Microbial CO₂ efflux contributes up to 60% of soil respiration, a feature that can change with temperature due to altered exoenzyme activities (short-term) and microbial communities producing different exoenzymes (longer-term). Often, however, microbial temperature responses are masked by factors that also change with temperature in soil, making accurate projections of microbial CO₂ efflux with warming challenging.

Using soils along a natural climate gradient similar in most respects except for temperature regime (Newfoundland Labrador Boreal Ecosystem Latitudinal Transect), we investigated short-vs. long-term temperature responses of microbially-mediated organic matter transformations. While incubating soils at 5, 15, and 25°C for 84 days, we measured exoenzyme activities, CO₂ efflux rates and biomass, and extracted DNA at multiple times. We hypothesized that short-term, temperature-induced increases in exoenzyme activities and CO₂ losses would be smaller in soils from warmer regions, because microbes presumably adapted to warmer regions should use assimilable substrates more efficiently and thus produce exoenzymes at a lower rate.

While incubation temperature generally induced greater exoenzyme activities (p<0.001), exoenzymes’ temperature responses depended on enzymes and regions (p<0.001). Rate of CO₂ efflux was affected by incubation temperature (P<0.001), but not by region. Microbial biomass and DNA sequencing will reveal how microbial community abundance and composition change with short-vs. longer-term temperature change. Though short-term microbial responses to temperature suggest higher CO₂ efflux and thus lower efficiency of resource use with warming, longer-term adaptations of microbial communities to warmer climates remain unknown; this work helps fill that knowledge gap.