OOS 46-4 - Precipitation legacies amplify nitrogen losses from nitric oxide emissions in dryland ecosystems

Background/Question/Methods
Anthropogenic activity is increasing the intensity and variability of precipitation regimes globally, with important consequences for many ecosystem functions. Precipitation can directly affect water limited ecosystem processes, while also establishing legacy effects where past precipitation alters future controls over these same processes. Nitrogen (N) cycling in drylands is highly sensitive to water availability, but legacies of past precipitation may also affect ecosystem N loss pathways. Specifically, wetting dry soils stimulates ecosystem N loss from nitric oxide (NO) emissions, but it is not clear if past rainfall alters soil N accumulation and microbial communities that control the magnitude of NO emissions. To better understand how changing precipitation regimes affect N loss from drylands, we ask: how does the legacy of past growing seasons’ precipitation mediate pulsed NO emissions when dry soils are wetted? To answer this question, we excluded or added precipitation during the winter and summer growing seasons (n = 4 plots per treatment) and measured NO pulses following experimental wetting. We also measured soil inorganic N and microbial biomass to assess drivers of pulsed NO emissions.

Results/Conclusions
Our results demonstrate that precipitation legacies affected NO emissions; both increasing and decreasing winter precipitation amplified the magnitude of summer NO pulses. In the summer, cumulative NO emissions over the 24 hours post wetting were higher from the winter precipitation exclusion plots (2750 ± 972 µg N-NO m^-2) and the winter water addition plots (2449 ± 408 µg N-NO m^-2) compared to control plots (1506 ± 397 µg N-NO m^-2; p = 0.003), even though soil moisture did not differ among treatments (p = 0.97; average gravimetric water content = 0.49 ± 0.24 %). The increase in NO emissions with winter rainfall exclusion was associated with elevated soil inorganic N, while the increase in NO emissions with previous water addition was associated with an upward trend in microbial biomass. Taken together, these results suggest that precipitation legacies may amplify N loss from dryland ecosystems, potentially limiting ecosystem N retention in a future with more extreme precipitation regimes.