The Effect of Urbanization on Forest Mycorrhizae, Worms, and Soil Respiration

In previous work, I have found that urban forests tend to have few to no saplings compared to more rural forests. In an attempt to determine possible reasons for this finding, two students (William McCuen and Delaney Riggins) helped me examine worms and soil characteristics at my permanent research sites in Forest Park and at the control sites in the Mount Hood National Forest. We collected earthworms at 29 sites using a mustard extraction method, conducting three worm pulls at each site. The worms were returned to the lab where they were counted and weighed. We measured the depth of the O horizon at each earthworm collection location. Invasive earthworms have been shown to increase turnover rate of organic material in the soil, resulting in lower seedling survival. We also collected soil from each of my sites and measured 24-hour CO2 production in twelve containers to determine soil respiration rates. In addition, we harvested seedlings to quantify the amount of ECM (ectomycorrhizae) and AMF (arbuscular mycorrhizae) in the roots, another important factor for seedling survival. We found the depth of the O horizon and the amount of CO2 were both significantly greater at rural control sites than in the urban forest. The number of worms collected per site ranged from 0 to 52, with a mean of 10.8. Neither the number of worms nor the biomass of worms was significantly different among locations. There was, however, a tendency of lower worm biomass at the control sites, and the depth of the O horizon was negatively correlated with the biomass of worms, although R2 was low. Worms are best collected in early spring, and because we collected worms in June and July, we feel the drier soil and warmer temperatures may have played a role in the absence of worms at some sites. We did find thinner O horizons in areas with a higher biomass of worms, although the relationship was weak. This preliminary study shows a potential negative impact of invasive worms on urban forest soils