Microbiology in the Critical Zone: Examining Subsoil Microbial Responses to Wildfire and Snowmelt in Mediating Terrestrial Biogeochemical Cycles and Integrating Across Spatial and Temporal Scales

Abstract

Understanding the temporal and depth-related spatial controls on microbial functional redundancy and community interactions remains a central issue in microbial ecology and Critical Zone science. Microorganisms are influenced by and influence biogeochemical reactions in the soil profile, interacting with soil, water, air, and rock throughout the Critical Zone. Snowmelt driven pulses of water and nutrients structure microbial communities, influencing rates of microbial nutrient cycling and fluctuations of greenhouse gases in forest soils. Wildfires shape the biogeochemistry of a landscape, and burn history is prevalent in US Southwestern mixed-conifer forests. Fire disturbance impacts soil and microbial communities both directly and indirectly. Fire can impact soil functioning and structure through removal of organic matter, alteration of soil structure and porosity, loss of carbon, erosion, and marked alteration of microbial community through surface burning. Indirect effects include alterations to the soil physicochemical environment and vegetation cover, impacting regional watershed functions directly immediately after fire and indirectly through changes in landscape and nutrient loadings. Subsoils hold a substantial amount of carbon that exists in both organic and inorganic forms and are sensitive to changes in temperature, moisture and disturbance regimes. Subsoils are understudied reservoirs of microbial activity and understanding the integrated interactions of microorganisms throughout the soil profile with the soil physical, chemical environment is a key component of this research. The contributions of moisture, substrate, topographic, depth and temperature controls on microbial nutrient cycling throughout the soil profile in a high elevation forest were assessed in a zero-order basin (ZOB, approximately 16 ha) located in the Jemez River Basin Critical Zone Observatory (JRB-CZO) in northern New Mexico that experienced a mixed-severity burn in 2013. Integrated and co-located measurements of bulk soil, soil gas, soil porewater and stream water chemistry measurements were taken from 2013-2017 to assess the influence of burn status, ecosystem recovery, and seasonal snowmelt dynamics and subsoil biogeochemical processes. To determine the immediate post-fire disturbance impacts on microbial functionality 22 soil pits were excavated to 40 cm and measured potential activities of seven hydrolytic enzymes involved in carbon (C), nitrogen (N), phosphorus (P) acquisition. Fire resulted in decreased activity for select carbon and nitrogen degrading enzymes in surface (0-2 cm) soils and altered N and P acquisition strategies with depth suggesting potential nutrient scavenging or increased internal microbial cycling with depth as a response to fire. Digital soil mapping demonstrated consistently higher potential enzyme activities in the convergent zones of the catchment, which were primarily correlated with higher soil moisture, clay content, and vegetative cover as quantified through normalized difference vegetation index (NDVI). Controls over enzyme activity differed in surface vs. subsurface soils where physical interactions with clay and moisture became more important in deeper soils. Seasonal pulses in water and solutes create selective pressures on resident microbiota, affecting nutrient cycling and CZ processes. The goal of the second 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 forested watershed with respect to seasonality. To this end, samples were collected from two depths (0-10 cm and 30-40 cm) at 4 time points - during snowmelt, before-, during- and after the monsoon season - over multiple years across an instrumented watershed. The dissolved organic C and N pools, microbial biomass, microbial exoenzyme activities involved in C, N, and P cycling were measured to understand microbial contribution to water-driven pulses influence changes in microbial ecological traits through time and the resulting influence of watershed biogeochemical signals. Concentrations of dissolved nitrogen peaked during the spring snowmelt event. CO2 respiration peaked during the growing season but was evident under snowpack and at deeper depths. Dissolved organic carbon, nitrogen and bulk DNA concentrations increased during the summer growing season. Increase in specific enzyme activities of carbon- and nitrogen-acquiring enzymes peaked during the fall senescence period and spring snowmelt event. It was hypothesized that there would be a seasonal trade-off between growth and resource acquisition. Indeed, a key finding from this study was a positive correlation between microbial biomass carbon and dissolved organic carbon and a negative correlation between specific enzyme activity and dissolved organic carbon that varied as a function of seasonality and depth, providing further evidence for the yield-acquisition-stress trait trade-off framework. As increase in plant available labile carbon compounds increased in the growing season, so too did microbial biomass and, conversely, up regulation of enzymes involved in C and N acquisition increased as available carbon decreased. This study demonstrates how seasonality and snowmelt influence trade off of microbial ecological traits through time that impact distribution and response of mobile solute fluxes (e.g. nitrogen). These responses are important for understanding community level functional ecological dynamics in high-elevation forested catchments.The mechanisms that underly soil respiration vs. weathering were determined by analyzing soil gas CO2 and O2 ratios from soil for four years post fire recovery and with respect to seasonality. Snowmelt is an important contributor to water storage and nutrient cycling, and drives subsurface weathering dynamics. Measurements of soil CO2 and O2 were used to determine biogeochemical processes in soil in order to calculate the apparent respiratory quotient (ARQ). Deviations from ARQ=1 indicate when other processes besides aerobic respiration and diffusion control gas concentrations. When ARQ < 1, CO2 and O2 consumptive processes dominate and when ARQ >1, processes that result in CO2 release govern. A key finding from this study revealed evidence for increased respiration in subsurface soils immediately following wildfire disturbance. This was consistent with the elevated enzyme activities observed in the previous study and potential for nutrient scavenging in subsurface soils as organic matter is volatilized in surface soil and mineral soil layer becomes exposed. As the ecosystem began to recover, and plants began to establish, the fluorescence indices of organic matter (FI) and humification degree (HIX) increased, propagating downward driving the subsurface snowmelt driven chemical weathering front. Overall, the interactions of surface vs subsurface CZ processes and their responses is not well understood, and key insights were revealed through this research showing the dynamic interactions between microbes and the soil physical and chemical environment in an ecosystem undergoing post-fire recover. This research demonstrates how fire impacts nutrient cycling in the subsurface, how seasonality and snowmelt influence tradeoffs in microbial ecological traits through time, and how processes that govern soil carbon storage, release and transport are altered as a function of fire disturbance, seasonality, and vegetation recovery. Collectively, these results lead to a greater understanding of the dynamic and interacting feedbacks between surface and subsurface soil properties that influence the distribution and responses of carbon throughout the soil profile, impacting critical zone structure and function. As a variety of global change stressors continue to escalate, understanding how the subsoil responds will become increasingly important in understanding ecosystem carbon balance.