The effects of a grassland wildfire on soil nematodes, organic matter and respiration
Fires are a common disturbance in the western United States, and one which is expected to become more common with climate change. It is known that fires can have a wide variety of effects on soil nutrients, organic matter and food webs, but their effects vary with the ecosystem and the intensity of the fire. In July, 2014, a wildfire intensely burned 4,000 hectares of arid grassland in eastern Washington State (Lincoln and Spokane Counties). I sampled soil from six paired burned and unburned sites in September, 2014, and again in April, 2015, in order to determine the immediate impact of the fire on soil organic matter and nematodes, and what effects persisted after nine months. Soils from September were used in a laboratory microcosm experiment to determine the capacity of soil nematode populations to recover under suitable abiotic conditions, and to determine the importance of an intact litter layer in their recovery. I extracted nematodes from soil samples using Baermann funnels, and sorted nematodes to functional groups (e.g. bacterivore, fungivore, plant parasitic, tylenchus type, omnivore, predator). I estimated the soil organic matter content of soils using AFDM, and soil respiration rates of wetted soils using absorption by soda lime.
Soil organic matter and soil moisture were 29% and 73% lower, respectively, in soils from burnt sites in September compared to soils from unburned sites. Soil nematode abundances were 67% lower in soils from burnt sites, and the difference was similar for all functional groups of nematodes. In the microcosm experiment, nematodes remained at lower abundances in soils from burned sites even after 40 days. After 60 days, bacterivorous and fungivorous nematodes had recovered in the burned soils to the same abundances as the unburned soils, but only when grass litter was added as a substrate. These results suggest that intense wildfires may have large and persistent effects on the soil food web, and that recovery of the soil food web may depend on the presence of plant litter that is frequently lost in such fires.