Inorganic nitrogen cycling in ephemeral urban waterways of the semi-arid Southwest

Background/Question/Methods

Elevated loading of non-point source inorganic nitrogen (N) in urban runoff is a major water quality concern in water and N limited regions such as the semi-arid Southwestern US. Although ephemeral streams in drylands have long been recognized as biogeochemical hotspots, it is unclear how inorganic N cycling varies across ephemeral urban streams of distinct substrates in response to episodic wetting. Therefore, we performed stream channel wetting experiments using an isotopic label (¹⁵N as K¹⁵NO₃) to identify N-processing pathway differences in 3 ephemeral urban streams of distinct substrates, % soil C and % soil N in Tucson, AZ: 1) sand, 2) sandy loam and 3) loam. We applied the ¹⁵N label at a rate of 1.3 kg ha^-1, and wetted the experimental plots to 25% volumetric water content. We monitored CO₂, and N₂O gas fluxes for 6 hours and changes in soil inorganic and microbial N before and after the experiment.

Results/Conclusions

Fluxes of CO₂ were significantly (α = 0.05) lower in the sand (1.05 ± 0.21 SD g CO₂-C m^-2 hr^-1) than in the sandy loam and loam streams (1.77 ± 0.75 and 1.86 ± 0.87 g CO₂-C m^-2 hr^-1, respectively), while sand and sandy loam stream N₂O fluxes (6.91 ± 5.06 and 8.42 ± 7.17 mg N₂O-N m^-2 hr^-1, respectively) were significantly higher than loam N₂O fluxes (3.03 ± 2.49 mg N₂O-N m^-2 hr^-1). Approximately 21.4 %, 13.9% and 2.3% of cumulative N₂O flux in the sand, sandy loam and loam streams, respectively, was ¹⁵N₂O, suggesting a greater fraction of NO₃-N loss to denitrification in the coarser streams. We observed significant ¹⁵N enrichment of soil NH₄ from pre-experiment and control plot values of -4.7 ± 11.8‰ and -7.2 ± 11.8‰, respectively, to labeled plot values of 110.7 ± 63.0‰, indicating potential dissimilatory nitrate reduction to ammonium (DNRA) activity in these ephemeral streams. We show that N-cycling in coarser substrates is limited by water, N transport and labile carbon; and by N transport and microbial N uptake in fine substrates. We suggest that through modification and management, urban ephemeral streams can mitigate non-point N loading to surface waters in semi-arid climates.