Denitrification, riparian heterogeneity and nitrogen fluxes in a forested watershed

Background/Question/Methods

Understanding the nitrogen (N) cycle at landscape, regional and global scales is a great current challenge in environmental science.  Excess “reactive” N has caused degradation of air and water quality and coastal ecosystems in many areas.  The development of solutions to N pollution problems has been hindered by large amounts of “missing N” that dominate N balances at all scales.  Uncertainty about N balances has led to increased interest in N gas production as a fate of N.  However, gas fluxes are difficult to quantify because of problematic measurement techniques (especially for N₂), high spatial and temporal variability, and a lack of methods for scaling point measurements to larger areas.  A particular challenge is that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of N gas flux activity. Here, we have applied a new soil core gas flow method to quantify N₂ and N₂O fluxes and relationships between flux and soil O₂ levels, along with in situ soil O₂ sensors in a forested watershed at the Baltimore urban LTER site.  Long-term ecohydrology research in this watershed has focused on upland – riparian interactions, and on variation between hummocks and hollows within the riparian zone.

Results/Conclusions

Flux of N₂ was consistently highest in the wet riparian “hollows” relative to other landscape positions in the watershed; dry riparian “hummocks,” upland ephemeral drainage channels or uplands.  These results support the idea that wet riparian zones are “hotspots” of denitrification in watersheds.  Denitrification rates were significantly higher at 5% and 10% O₂ than under strictly anaerobic conditions.  These results suggest that denitrification in the hollows is strongly coupled to nitrate supply by mineralization and nitrification (an aerobic process).  Potential net N mineralization was negative in the hollows and positive in other sites, supporting this idea.  N₂O fluxes were much lower than N₂ fluxes (3.5% of total N gas flux at upland sites, but less than 0.1% at all other sites) and were highest from upland sites and at higher O₂ levels, suggesting that nitrification was the dominant source of N₂O in this landscape.  These results suggest that focusing on topographic heterogeneity is a robust approach to measuring and scaling N gas fluxes.  Strong relationships between flux and soil O₂  suggest that continuous measurements of soil O₂ will be a powerful tool for temporal scaling of measured rates.