Reclamation seed mix research for ecosystem resilience on compacted soils within the shale gas footprint
Reclamation of disturbed, compacted soils is a major challenge for ecological restoration of shale gas development. Loss of topsoil structure and subsurface compaction can greatly impair vegetation establishment and succession for the long-term with cascading impacts on the ecosystem, including encroachment of invasive plants. Well staging pads can be greatly reduced in size for the lifetime of gas production (estimated at 20-50 years for the Marcellus Shale in the Mid-Atlantic, U.S.). In 2014 we established research plots with the objectives to broaden the understanding of (1) community assembly, and (2) ecosystem services, of four native perennial warm season grasses during the establishment phase of reclamation on highly compacted soils. We simulated typical subsoil compaction as a control to compare with compaction alleviation with a 30cm subsoiler. We used a fixed density replacement series design to assess the establishment success of mixtures of Sorghastrum nutans, Panicum virgatum, Andropogon gerardii, and Dichanthelium clandestinum. We hypothesize that increasing richness in a seeding mixture will increase multifunctionality including the ecosystem services: weed suppression, compaction alleviation, erosion control, and wildlife habitat provision. Soil compaction, we argue, will be associated with lower biomass of the perennial grasses, and therefore a decline of some ecosystem services.
Perennial warm season grasses exhibit high dormancy and slow growth in the first season and therefore our preliminary results of ecosystem services include only weed suppression by the cover crop. Biomass and percent cover of the cover crop, Avena sativa, did not differ by soil treatment. Greater A. sativa biomass improved weed suppression with subsoiling, but had no impact on weed biomass in compacted soils. Weed cover (F=6.3, p=0.01) and biomass (F=6.4, p=0.01) were lower in the compacted treatment, with correspondingly more bare ground (F=12.0, p=0.0007). Fewer weeds in the compacted treatment could be a result of impeded growth of weed propagules from the compacted subsoil. We expect this trend will reverse in the next two seasons with increased weed suppression by perennial grasses in the non-compacted treatment. An expansion of ecosystem services provided by the diverse mixtures is expected, yet to a lesser extent in compacted soils. For example, mixtures with bunch grasses S. nutans and A. gerardii may to provide less weed suppression but greater habitat value, and improve in weed suppression with the addition of P. virgatum. These deep-rooted grasses are anticipated to alleviate some compaction and provide the backbones of resilience in reclaimed shale gas pads.