Wildfires are a natural landscape disturbance in many climates and forest types, but the cumulative impact of human-caused climate change, historical fire management and suppression, and changing species diversity in forests has led to an increase in the size and (or) severity of wildfires in certain regions across the globe. There are a significant number of research studies on the effects of wildfire on human health, forest ecology, hydrology, and the physical, chemical, and biological properties of soils. However, research on the impact of wildfire on watersheds including toxicity in aquatic organisms, water chemistry, and fluvial sediment quality is less extensive, focusing primarily on water quality indicators such as nutrients and sediment flux. Recent research has shown that wildfires contribute to the environment significant amounts of polycyclic aromatic hydrocarbons (PAHs), which are compounds produced during the incomplete combustion of organic material, and are known to be toxic and mutagenic compounds. The primary objective of this paper is to review the recent literature that pertains to the contamination of surface waters and sediments and source apportionment of wildfire-derived PAHs to determine where research gaps remain. Additional objectives are to assess the use of molecular ratios to apportion PAH sources and, finally, to create a roadmap for future studies in designing and conducting research that seeks to determine sources of wildfire-derived PAHs in water and sediment.

Soil microbes are key to nutrient cycling and soil formation, yet the impact of soil properties on microbe biomass remains unclear. Using 240 soil cores of 0–15 cm depth, taken at random points across six cattle-grazed pastures on an undulating landscape, we evaluated the biomass of microbes in soil as affected by naturally occurring variation in soil organic carbon (SOC), clay content, and local topography. The study pastures varied in historic land-use for crops or forage seeding. SOC was found to be greater in topographically low areas. In contrast, clay content was not related to topography, and clay deposition possibly varies with glaciation legacy. Microbial biomass carbon (MBC) was correlated positively with SOC, increasing from 700 mg kg⁻¹ MBC at 25 g kg⁻¹ SOC to 2240 mg kg⁻¹ MBC at 90 g kg⁻¹ SOC. Most likely, SOC promotes MBC through the release of water-soluble organic carbon. However, the response of MBC to clay content was negative, decreasing from 1340 mg kg⁻¹ MBC at 5% clay to 880 mg kg⁻¹ MBC at 30% clay. Small voids in association with clay particles likely restrict the access of microbes to SOC. The relationship between SOC and MBC illustrates the important role of SOC for soil function, in terms of nutrient availability and development of soil structure via the contribution of microbes. Lastly, there was considerable spatial variability in MBC across the 65 ha site, highlighting the importance of land-use histories and gradients in environmental variables, to determine the biomass of microbes in soil.