Plain Language Summary

Fossil fuel combustion and industrial agriculture have increased atmospheric nitrogen (N) pollution. Atmospheric N can travel with prevailing winds and be deposited on soil surfaces in terrestrial ecosystems. This can in turn increase N cycling and export to aquatic ecosystems, where excess N acts as a pollutant. The timing and amount of rainfall can influence how much N is transported from terrestrial to aquatic ecosystems, but it remains unclear how future N deposition and precipitation patterns will interact to influence stream water quality and drinking water security. To address this, we used a simulation model to investigate (a) how changes in the timing and/or amount of precipitation influence N export from watersheds, and (b) how the effects of precipitation change with increased atmospheric N deposition. We found that N export to streams increases with precipitation intermittency and variability, particularly when N deposition is low. Under high N deposition, export to streams is substantially elevated, regardless of precipitation timing. Our findings suggest that under future climate change, prolonged droughts that are followed by more intense storms may increase hydrologic N export and pose a major threat to water quality in dryland watersheds.

Key points

• Simulating how N deposition interacts with precipitation seasonality can enable us to better predict when dryland watersheds become N-saturated
• As rainfall regimes become more intermittent and/or variable, streamflow nitrate export is likely to increase, particularly when a watershed is N-limited
• Under future climate change in drylands, prolonged droughts that are followed by more intense storms may pose a major threat to water quality

Abstract

Atmospheric nitrogen (N) deposition and climate change are transforming the way N moves through dryland watersheds. For example, N deposition is increasing N export to streams, which may be exacerbated by changes in the magnitude, timing, and intensity of precipitation (i.e., the precipitation regime). While deposition can control the amount of N entering a watershed, the precipitation regime influences rates of internal cycling; when and where soil N, plant roots, and microbes are hydrologically coupled via diffusion; how quickly plants and microbes assimilate N; and rates of denitrification, runoff, and leaching. We used the ecohydrological model RHESSys to investigate (a) how N dynamics differ between N-limited and N-saturated conditions in a dryland watershed, and (b) how total precipitation and its intra-annual intermittency (i.e., the time between storms in a year), interannual intermittency (i.e., the duration of dry months across multiple years), and interannual variability (i.e., variance in the amount of precipitation among years) modify N dynamics and export. Streamflow nitrate (NO3−) export was more sensitive to increasing rainfall intermittency (both intra-annual and interannual) and variability in N-limited than in N-saturated model scenarios, particularly when total precipitation was lower—the opposite was true for denitrification which is more sensitive in N-saturated than N-limited scenarios. N export and denitrification increased or decreased more with increasing interannual intermittency than with other changes in precipitation amount. This suggests that under future climate change, prolonged droughts that are followed by more intense storms may pose a major threat to water quality in dryland watersheds.