Impact of varying nitrate concentrations on germination of a suite of semi-arid region riparian plant species

Background/Question/Methods

In the southwestern United States, as streams have been dewatered due to groundwater pumping or diversion for agricultural or municipal purposes, treated wastewater effluent is increasingly being discharged into stream channels to support riparian ecosystems. Changing the amount of nitrate can alter community dynamics, and effluent-dominated streams may exhibit different community dynamics than natural streams. Little information exists on the effects of elevated nitrate concentrations on seed germination of herbaceous riparian species of the southwestern United States. Among the facultative riparian species in riparian forests of southwestern USA are Bromus diandrus (BRDI), Hordeum murinum (HOMU), and Sisymbrium irio (SIIR). Mimulus guttatus (MIGU), Nasturtium officinale (NAOF), Veronica anagallis-aquatica (VEAN), and Polypogon monspeliensis (POMO) are common obligate riparian (wetland) species. In light of the increasing use of effluent discharge, we asked if varying nitrate concentrations would impact germination timing and total germination of facultative and obligate riparian species. Seeds of the aforementioned species were collected from multiple individuals and populations located in several southwestern riparian ecosystems. For each species, 6 replicate Petri dishes containing 25 seeds each were treated with distilled water or 0.025M, 0.05M, or 0.075M KNO₃ solution and placed in a growth chamber for 30 days utilizing a random-block design. 

Results/Conclusions

Overall, facultative riparian species had lower total germination and more varying germination responses to treatments than obligate riparian species. BRDI had a delayed germination response and significantly more germination in response to the lowest nitrate solutions than control or the highest nitrate solution. HOMU also had delayed germination, but had significantly higher germination in response to the lowest nitrate solution. SIIR had significantly higher germination under control conditions. All wetland species germinated rapidly with almost 100% germination in response to all treatments and control. The facultative riparian species studied likely will exhibit differing community dynamics in southwestern effluent-dominated riparian ecosystems. While BRDI will likely increase in abundance in nutrient enriched ecosystems, HOMU and SIIR may decrease in abundance. Based on our small sampling of wetland species, common wetland species may not be affected at the germination stage by the increased nitrates due to effluent discharge.