Burn severity and tree species impacts on soil chemical and biological properties from fire-prone sites in the Davis Mountains, TX

Soil is the basis of ecosystem functioning and health because of the numerous services it provides. It is a media for plant growth and habitat for animals and microorganisms. It stores, releases and cycles nutrients and atmospheric gases, regulates the storage, flow and purity of water, and also buffers wastes and toxins. Ecosystem disturbances such as wildfires can impact soil functioning by altering decomposition of soil organic matter (SOM) and nutrient transformation processes that are crucial for restoration following a disturbance. In arid and semi-arid ecosystems, high temperatures and low moisture availability limits plant growth and nutrient cycling. In these regions, fire is a critical component of the ecosystem that can enhance nutrient availability via the combustion of organic matter and vegetation. Fire not only influences aboveground composition and function but it also affects belowground processes which are important factors of ecosystem restoration, productivity and sustainability following a fire. In this study I investigated the impact of a wildfire that occurred in April 2012 in the Davis Mountains, Texas on soil chemical and biological properties important for nutrient cycling and ecosystem restoration. Soil samples (0-5 cm) were collected in June and October 2012 under three tree species (Quercus grisea (QUGR), Juniperus deppeana (JUDE) and Pinus cembroides (PICE)) and across three fire severity levels (unburned, low burn severity and high burn severity). Samples were analyzed for soil moisture, soil pH, inorganic nitrogen, SOM, soil microbial biomass (SMB), and soil enzymes (β-glucosidase, β-glucosaminidase and L-asparaginase). I hypothesized that fire will affect soil properties under QUGR (broad-leaved tree) differently compared to JUDE and PICE (coniferous trees) due to initial differences in those soil properties given that litter quality and inputs as well as microbial associations vary by tree species and influence physical, chemical and biological soil conditions. Furthermore, I hypothesized that fire will (1) reduce SOM, SMB and soil enzyme activity, and (2) increase soil moisture by removing vegetation cover, and soil pH and inorganic nitrogen (N) due to nutrient release upon SOM combustion and ash deposition. Results showed that tree species did not predominantly influence soil properties, with the exception of soil pH which was higher in QUGR and JUDE samples than in PICE samples. In June samples, burn severity affected soil pH, inorganic N, SOM content and soil enzyme activity with marked effects in high severity burns. Soil pH and NH4+-N concentrations increased while NO3--N, SOM content and enzyme activity decreased. In October samples, burn severity affected soil moisture content, soil pH, inorganic N, soil microbial biomass, and β-glucosidase and L-asparaginase activity with marked effects in high severity burns. Soil moisture content, soil microbial biomass and enzyme activity decreased while soil pH and inorganic N concentrations increased. Time since fire also affected soil properties and led to decreasing NH4+-N concentrations, SOM content and β-glucosidase and L-asparaginase activity while soil moisture content, NO3--N concentrations and soil microbial biomass N increased.