Soil nitrogen cycling in dry lands: Precipitation legacy effects on microbial N loss pathways

The transition from dry-to-wet soils is often characterized by an acceleration of the nitrogen (N) cycle, representing a period of interest to biogeochemists studying future N cycling responses to global changes. In particular, wetting dry soils can produce large emission pulses of nitric oxide (NO; an air pollutant at high concentrations) and nitrous oxide (N₂O; a powerful greenhouse gas), but the mechanisms governing the N losses remain unresolved. The asynchronous timing of when N becomes bioavailable and when ecosystem N sinks activate (e.g., plant N uptake) post wetting has often been used to explain why N is lost when dry soils wet up. However, other factors directly affecting nitrifying communities may also contribute to the emissions. For instance, ammonia oxidizing bacteria (AOB) may be favored over ammonia oxidizing archaea (AOA) in NH₄⁺-rich environments that are typically observed in dry soils. Because AOB may process N less efficiently than AOA, shifts in nitrifier activity may help promote gaseous N losses.

 

To better understand mechanisms for gaseous N loss, we studied drylands in southern California that can experience >6 months without rain, as well as other drylands where we added or excluded precipitation during the dry summer or wet winter seasons. We also selectively inhibited AOA and AOB communities to measure their contributions to soil N emissions. Excluding precipitation during the winter prior to collecting soils did not affect NO emissions, but either adding or excluding precipitation during the summer did; NO emissions after adding extra rainfall (95 ± 6 µg NO g soil^-1; p = 0.01) or excluding rainfall (105 ± 22 µg NO g soil^-1; p = 0.006) were significantly higher than the control (41 ± 6 µg NO g soil^-1), with over 50% of the emissions controlled by AOB. While most of the effects of manipulating precipitation were observed on NO emissions, N₂O increased only when we excluded precipitation in the winter wet season, averaging 0.58 ± 0.40 µg N-N₂O g soil^-1. Using isotopologues of N₂O coupled with chloroform fumigations to slow microbial activity, we found that N retention and loss trade off as dry conditions intensify. Altogether, our measurements suggest that that shifts in precipitation patterns can favor AOB-derived NO emissions when dry soils are wetted at the end of the dry season, suggesting that shifts in nitrifier activity from the legacies of past precipitation can also help explain why N is lost when dry soils are wetted.