Fires are expected to become more common in the western United States as a result of climate change. The long term impacts of increased fire frequency on ecosystem carbon storage are not well understood, and are likely to vary among different ecosystems. In July 2014, the Watermelon Hill fire burned 4,000 hectares of arid grassland in eastern Washington (Lincoln and Spokane Counties). The objective of this study was to determine the immediate and long-term impacts of the fire on soil organic matter and nematodes. I sampled six sites affected by the fire, along with paired unburned reference sites, from September 2014 through April 2016. I extracted nematodes from samples using Baermann funnels, estimated total organic matter using AFDM, and measured soil respiration in the field using a Licor Infrared Gas Analyzer. I used samples from September 2014 in a laboratory microcosm experiment to determine the capacity of nematode populations to recover under suitable abiotic conditions. I also analyzed soil samples from September 2014 for 14C to determine whether the fire had preferentially combusted younger or older organic matter.
In September 2014, soil organic matter and moisture were 29% and 73% lower, respectively, in soils from burnt sites compared to unburned sites. Nematode abundances were 67% lower in burnt sites in September 2014, and the difference was similar for all functional groups. These differences persisted through April 2015, nine months after the fire, but were no longer significant in April 2016. Soil respiration was 39% lower in burnt sites in 14 months after the fire, but were also no longer significant in April 2016. In the microcosm experiment, bacterivorous and fungivorous nematodes in burnt soils recovered to the same abundances as unburned soils after 60 days, but only when grass litter was added as a substrate. These results suggest that wildfires may have large effects on the soil food web, and that their rate of recovery may depend on the presence of plant litter that is frequently lost in such fires. Overall, the field sampling and microcosm experiment suggest that a full growing season, and the subsequent deposition of new surface litter, was necessary for the soil food web, organic matter and moisture retention to recover from this wildfire.