Snowmelt and seasonality influence trade-offs of microbial functional traits in forest soils

Background/Question/Methods

Understanding the temporal and depth related spatial controls on microbial functional redundancy and community interactions remains a central issue in microbial ecology. Microorganisms are influenced by and influence biogeochemical reactions throughout the soil profile, interacting with multiple Critical Zone compartments. Snowmelt driven pulses of water and nutrients structure microbial communities, influencing rates of microbial nutrient cycling and fluctuations of greenhouse gases in forest soils. The goal of this study was to understand the relative contributions of moisture, substrate, topographic, depth and temperature controls on microbial nutrient cycling throughout the soil profile in a high elevation forest. These seasonal and highly dynamic ecosystems are important carbon sinks in the Western U.S. where the productivity and disturbance regimes of these forests are largely driven by bimodal precipitation. These pulses in water and nutrients create selective pressures on resident microbiota, representing a trade-off in ecological traits through time. To this end, we examined (1) how microbial community and functional diversity change along soil depths (0-10 cm and 30-40 cm) and through time during episodic moisture pulse events and (2) the underlying mechanisms, revealed by random forest machine learning models, that dictate spatial and depth-related functional and community level controls through time.

Results/Conclusions

We hypothesized a seasonal trade-off in microbial functional traits between growth, resource acquisition, and stress response that dictate biogeochemical processes. Soil sampling occurred during snowmelt, before-, during- and after the monsoon across an instrumented watershed. In order to understand these tradeoffs, we measured dissolved organic carbon and nitrogen pools, biomass, microbial exoenzyme activities, community (16S and ITS rRNA), and the composition of DOM in soil water and stream water extracts using ultraviolet absorbance of fluorescence excitation-emission matrices. We found variations in the above measured variables during the growing season where concentrations of TDN peaked during the spring snowmelt event. CO2 respiration peaked during the growing season but was evident under snowpack and at deeper depths. DOC, DON and bulk DNA concentrations increased during the summer growing season. Carbon and nitrogen acquiring enzymes peaked during the fall senescence period and spring snowmelt event. The composition of soil DOM in the soil water shifted to more humic acid-like and biological origin during the spring snowmelt indicating rapid connectivity. This study demonstrates how seasonality and snowmelt influence trade offs in microbial ecological traits through time that impact distribution and response of mobile solute fluxes throughout the soil profile.