COS 75-4 - Near-term biogeochemical effects of riparian restoration: greenhouse gas flux and soil chemistry response to Russian olive (Elaeagnus angustifolia) removal along the Rio Grande

Abstract: Negative impacts of invasive vegetation on riparian hydro-biogeochemical ecosystem services are justifiable reasons for active management to restore native plant communities. Invasive plants with disproportionate functional significance are of particular interest, such as Russian olive (Elaeagnus angustifolia) which is symbiotic with N₂-fixing actinobacteria, and accelerates local N cycling and N loss. This is therefore a model invasive plant to evaluate vegetation removal success in the context of riparian restoration from an ecosystem service perspective. Here, we report on soil physio-chemical changes, nutrient availability, and greenhouse gas (GHG) fluxes, 3-12 months following the removal of ~140 hectares of Russian olive along the middle Rio Grande, compared to intact Russian olive stands and stands of native cottonwood (Populus deltoides).

GHG fluxes (CO₂, CH_4, and N₂O) were higher from removal plots, and positive from soils to the atmosphere (Kruskal-Wallis; all P < 0.05). We observed significant date by treatment interactions for CH₄, N₂O, and NH₃ fluxes (linear mixed models; interaction terms all P < 0.05). CO₂ flux was positively correlated with soil NH₄⁺ (r = 0.36), but NO₃^- did not correlate with measured GHG flux. CO₂ flux positively correlated with soil moisture (r = 0.37), and N₂O negatively correlated (r = -0.18) with soil temperature (range of 17-38°C). Soil NH₄⁺ concentration was highest in removal plots (factor of 0.5-2x higher than intact or cottonwood). Soil NO₃^- concentrations in plots with intact Russian olive were 3x higher than in removal plots, and 60x higher compared to native cottonwood soils over the course of the experiment.

Our results suggest that compared to native cottonwood-dominated stands, the presence of Russian olive in Rio Grande riparian ecosystems contributes to higher soil N levels and greater fluxes of GHG to the atmosphere. There are also legacy effects measurable within months of Russian olive removal which manifest as increased GHG flux, and potentially higher rates of N mineralization from N-rich soil in the absence of established vegetation. Longer-term monitoring is ongoing and will be used to track the N flux trajectory of the restoration effort, but our data highlight that there are significant biogeochemical effects of invasive vegetation that persist beyond its removal. We advocate that restoration projects incorporate an understanding of invasive vegetation effects on soil chemistry and biogeochemistry to forecast ecosystem service impacts of removal projects.