Nitrogen loss and retention in dryland ecosystems determined by seasonality and atmospheric inputs

Background/Question/Methods

Drylands comprise 41% of Earth’s land surface with some areas exposed to some of the highest rates of atmospheric nitrogen (N) deposition in the world. High rates of N deposition can saturate ecosystems with N, leading to acidification, eutrophication, and elevated soil greenhouse gas emissions. However, the mechanisms controlling when drylands become saturated with N remain unexplored relative to mesic sites, where the concept of N saturation was developed. To better understand dryland N saturation, we are quantifying N losses via hydrologic and gaseous pathways and the environmental factors that control them. We hypothesized that i) N losses would increase in proportion to atmospheric N inputs and that ii) the transition from the dry to the wet season causes a flush of N loss because ecosystem production and nutrient uptake take time to up-regulate, creating a window when available N is vulnerable to loss. To test these hypotheses, we are studying three chaparral watersheds along a gradient of atmospheric N deposition (Low, Mid, High ranging from ≤ 5 to ≥ 30 kg N ha-1 y-1). We measured soil N emissions during two dry periods and one wet period and hydrologic losses over two water years.

Results/Conclusions

The High N deposition site, where we expected the highest N emissions, produced 1.1±2.6 ng N-N2O m-2 s-1 and 37.6±28.2 µg N-NO m-2 s-1, demonstrating most N emissions are from NO. Similar to the High N deposition site, NO emissions averaged 30.9±23.2 µg N m-2 s-1 at the Mid N site and 34.12±29.8 µg N m-2 s-1 at the Low N site, suggesting that all three sites intrinsically lose N during the dry-to-wet transition as we hypothesized. Ongoing measurements during the wet season will be used to understand whether watersheds exposed to lower rates of atmospheric N deposition proportionally export less N, especially as plants take up N and ecosystem N sinks recover from drought stress.