Nitrogen (N) limits ecosystem productivity across drylands, yet many drylands can lose N as if they were N saturated—when N availability outpaces ecosystem N sinks. Some of these N losses are controlled by microbial and abiotic processes that produce nitric oxide (NO) and nitrous oxide (N2O) after soils wet up, representing an N loss pathway to the atmosphere with implications for both regional air quality and Earth’s climate. While many biotic and abiotic processes are known to produce N emissions following wetting of dry soils, controls over the magnitude and duration of the emissions remain elusive, and it is not clear how these processes may respond to increasing rates of atmospheric N deposition. Thus, we ask: what processes control gaseous N loss from dryland soils in response to wetting and how do they respond to increasing soil N availability? Using an automated chamber system, we measured soil N trace gas emissions every 30 minutes over a 24-hour period after adding water and N to two arid sites in Southern California. We also used isotopically labeled nitrate and ammonium tracers to characterize N trace gas production pathways.
Results/Conclusions
We observed large N emission pulses after wetting dryland soils. Nitric oxide emissions accounted for the greatest cumulative gaseous N loss from both sites, with fluxes reaching 800 ng NO-N m2 s-1 and remaining elevated over the entire measurement period. Ammonium (NH4+) addition was positively correlated with NO emissions at both sites (p < 0.05), but nitrate (NO3-) addition was not (p > 0.1). This suggests that nitrification drives NO emissions, and that higher N availability can stimulate N loss from dryland soils. We also observed a pulse in soil N2O emissions from both sites; emissions reached 1000 ng NO-N m2 s-1 within 15 minutes of wetting but lasted less than four hours. While NO3- addition was not correlated with cumulative N2O emissions (p > 0.1), 15N-NO3- tracer was converted to N2O within 15 minutes of adding water, suggesting that rapid denitrification may be the predominant N2O production pathway. Together, these results show that N emission pulses after wetting dry soils can contribute to N loss from dryland ecosystems, and that NO production via nitrification may become an increasingly important N loss pathway as rates of atmospheric N deposition increase.