Evaluating soil microbial community structure and diversity as measures of success for road restoration

Background/Question/Methods   Reclamation treatments of forest roads range from road abandonment to road removal by full recontouring of the road prism. Measurements of how soil biological and physical properties change following restoration treatments are critical to understanding whether current road removal efforts on U.S. public lands are effective in enhancing function of ecological systems. Our research takes advantage of an active road restoration program on the Clearwater National Forest in northern Idaho. Soil samples were collected from 30 transects across a range of road restoration treatments (abandon and recontour) and 10 reference (never-roaded) transects. Treatment and reference transects were located within the same drainages, controlling for geology, slope, climate, and soil type. Soil samples were characterized for total moisture, hydraulic conductivity, particle size, soil organic matter (SOM), and total C. We used Real Time Polymerase Chain Reaction (RT-PCR) to determine the relative abundance of soil bacterial and fungal populations. EcoPlateTM C utilization assays with and without the addition of bacterial and fungal inhibitors (streptomycin and cycloheximide, respectively) provided measurements of rate and diversity of C utilization by different microbial groups. We hypothesized that road removal via recontouring would increase the rate of ecosystem recovery (represented by increased hydraulic conductivity, higher soil C, and higher microbial diversity and fungal:bacterial ratio) relative to abandoned roads.

Results/Conclusions   Consistent with these expectations, SOM in recontoured roads (11.5% g C/g dry soil) was significantly greater relative to abandoned roads (4.1% g C/g dry soil) (ANOVA, F3,38 = 7.23, p=0.0006); further, no difference was detected in SOM levels between reference transects and recontoured roads (12% and 11.5% g C/g dry soil, respectively). The substantial differences in soil C might, in turn, contribute to differences in both the rate of C utilization and microbial community structure. Preliminary results from C utilization assays support this hypothesis. The rate and diversity of C utilization was higher on the recontoured and reference areas compared to abandoned roads. Additions of cycloheximide to EcoplateTM assays for recontoured and reference soils lowered overall C utilization diversity but had no measurable effect on abandoned treatment C utilization, suggesting that recontoured roads and reference sites contain higher fungal:bacteria ratios. Dominance of fungal activity in forest soils has been shown to profoundly accelerate the recovery of compromised ecological systems.