Oilfield reclamation in mixed grass prairie restores nematode but not plant communities

Background/Question/Methods

Energy development in North American grasslands involves developing oil well pads and access roads. Reclamation/revegetation is constant: interim reclaims are conducted when drilling is complete and pumping commences, and final reclamation occurs upon cessation of well production. Reclamation methods include re-sculpting the landscape and applying stored topsoil; seeds of native and non-native plants are used to establish ground cover, minimize erosion and provide forage for cattle. Reclamation that focuses on establishing native plant communities is predicated on “if you build it, they will come”: higher trophic levels will recolonize these areas because of improved habitat. To test whether reclaims successfully re-establish native plants and attract diverse nematode communities, we designed a study to explore how final reclaims differ from surrounding prairie in composition and structure of vegetation and associated invertebrate communities. We selected 15 sites across the Little Missouri National Grassland in western North Dakota and established 3 100m transects at each, one on the reclaim and two on adjacent native rangeland at distances of 50m and 150m. We determined plant species cover at 15 points along each transect using 0.1m²Daubenmire frames and collected soil for determination of physical/chemical characteristics and extraction/identification of nematodes to trophic groups.

Results/Conclusions

Vegetation communities responded more strongly to reclamation than nematodes. Greater amount of bare ground (F_2,28=7.3, P=0.003) and cover of exotic plants (F_2,28=5.2, P=0.012) on reclamations compared with adjacent rangeland suggest incomplete recovery. Additionally, reclamations had lower native plant cover (F_2,28=18.5, P<0.0001) and richness (F_2,28=11.8, P=0.0002). Reclamations had higher exotic plant richness (F_2,28=8.3, P=0.002) with fewer exotic plant species collected from adjacent rangeland. Strong effects of reclamation on plant communities contrasted with only weak effects for nematode trophic group abundances (notably, only bacterivorous nematode abundances showed a response and were more abundant on reclamations; F_2,28=4.6, P=0.02). Comparisons of plant and nematode community structure using NMS ordination revealed that vegetation communities were distinct between reclaimed well pads and adjacent native rangeland, while no clear pattern was observed for nematode communities. These findings suggest that recolonization of reclamations by native plant species is limited, preventing the succession of these sites to communities more typical of mixed-grass prairie rangeland. In contrast, nematode community functional composition and structure appear largely similar across the landscape. If we presume that intact rangelands represent fully functional nematode communities, then soil ecosystem functioning is adequate in reclamations while reclaimed plant communities do not meet the same functional standards.