Impact of nitrate enrichment on wetland and dryland seed germination and early seedling development

Questions: Atmospheric and agricultural inputs of nitrogen have increased significantly. Because riparian zones act as buffers for nutrient transfer and treated municipal effluent and untreated agricultural run-off are released into streams, and because desert riparian ecosystems contain a wide range of functional groups with respect to water relations, we asked if wetland and dryland species respond differently to increasing levels of soil nitrate. Specifically, we asked if seed germination rate, total percentage germination and seedling development varied by species and functional group in response to increasing nitrate levels. Location: Southwestern US. Methods: We subjected seeds of common wetland and dryland species to distilled water (control), 0.025 M (low), 0.05 M (intermediate) and 0.075 M (high) KNO₃ treatments for 30 days. We determined seed germination rates, total percentage germination and seedling development for each species. Generalized linear mixed models tested for differences between treatments. Results: Seeds of wetland species germinated rapidly and in high percentages, and developed into seedlings regardless of nitrate treatments. Dryland species had unique responses to nitrate treatments, with some species exhibiting high germination rates in control treatments and others showing strong germination responses at low to intermediate nitrate levels. A larger number of dryland germinants developed into seedlings under control to low nitrate treatments. Conclusions: Riparian plant community dynamics arising from germination responses could change under increased nitrogen inputs, as some species are seemingly unaffected and others significantly impacted. Based on our limited species pool and treatment conditions, the functional group 'wetland species' produces a consistent germination and early seedling response regardless of nitrate concentration, while 'dryland species' responses are variable and species-specific. Species known to be nitrophiles can be inhibited by high nitrogen concentration as germinants, highlighting the importance of investigating plant community response to changing conditions at a variety of life stages.