Flow regulation has reduced the exchange of water, energy, and materials between rivers and floodplains. As such, flow regulation can alter dynamics of riparian soil nitrogen (N) cycling. Flow regulation also has advanced the spread of nonnative plants, which can alter ecosystem-level nutrient dynamics by differing from native species in key physical or physiological traits. We asked whether flood regime (short [SIFI] vs. long [LIFI] inter-flood interval) and/or plant species (Populus deltoides ssp. wislizeni [Rio grande cottonwood], Tamarix chinensis [salt cedar], and Elaeagnus angustifolia [Russian olive]) influence soil N pools and fluxes along the middle Rio Grande, New Mexico (USA). We present data from twelve riparian forests.  Eight sites were dominated by P. deltoides with an understory containing E. angustifolia.  Four sites were dominated by T. chinensis. Half of the sites were designated SIFI sites, based on historical overbank flood frequency (every 2-3 years), and the remaining sites were designated LIFI sites (last flood event occurred ≥ 15 years ago). We collected data on soil inorganic N concentrations, potential rates of soil net N mineralization (PNM), rates of organic and inorganic N consumption by plant roots, leaf fall, and leaf litter N content.

Results/Conclusions

PNM rates were consistently higher at LIFI vs. SIFI sites, but flood regime had no effect on pools of soil N across sites. T. chinenis and P. deltoides consumed organic N in addition to mineral forms, but preferred forms of N (NH₄ > organic N > NO₃) and rates of N uptake were similar across species. The atmosphere was the primary source of N for E. angustifolia (mean foliar δ¹⁵N: 0.09). P. deltoides had lower mean leaf litter N content (0.47 %) relative to T. chinensis (0.66 %) and E. angustifolia (2.15 %).  However, high levels of leaf fall within P. deltoides stands resulted in high N inputs to soils, thus supporting greater pools of soil inorganic N (QTY) as compared to stands of T. chinensis. PNM rates were similar across P. deltoides and T. chinensis sites. E. angustifolia leaf fall within P. deltoides-dominated sites was strongly correlated with pools of soil inorganic N (R² = 0.59, P = 0.001), but had no effect on PNM rates. Invasion by T. chinensis and E. angustifolia alters pools of soil N, but in opposing directions due to differences in leaf litter production and N content.