Altered precipitation regimes affect NO production via nitrification in dryland soils

Changing precipitation regimes may alter soil nitrogen (N) cycling by breaking down the transfer of microbial metabolic products. As a result, prolonged dry periods can build up soil ammonium (NH₄⁺), a fraction of which can be nitrified to produce nitric oxide (NO) when soils wet up. The fraction of NO emitted can vary as a function of soil N concentrations and the community composition of nitrifying microorganisms. Specifically, increased soil N concentrations may stimulate NO production via nitrification, and ammonia oxidizing bacteria (AOB) have been shown to produce more NO during nitrification relative to ammonia oxidizing archaea (AOA). Here, we ask: how do soil N concentrations and nitrifier community composition respond to changing precipitation regimes, and what consequences does this have for soil NO emissions? To answer this question, we measured soil NO emissions, soil mineral N concentrations, and the contribution to AOB and AOA to NO production after manipulating rainfall at our field site in Southern California. Since 2018, we have used shelters to exclude rain from experimental plots (6 x 8 meter) during the summer or winter and used drip irrigation to increase the summer or winter precipitation at other plots (n = 4 plots per treatment). In the winter of 2020, soil NO emissions were only detected in the winter rainfall exclusion treatment, where emissions reached 20.2 ± 3.97 ng N-NO m² hr^-1 within 24 hours of wetting soils in the field. Soils in the winter rainfall exclusion plots also had the highest soil NH₄⁺ concentrations (p < 0.05), suggesting that the accumulation of NH₄⁺ during sustained dry periods increased NO production following rewetting of soils. Additionally, using a nitrifier inhibition assay in the lab, we show that AOB were responsible for increased NO production in the winter exclusion treatment. These results demonstrate that NH₄⁺ accumulation over extended winter dry periods can stimulate soil NO production by AOB, representing an important N loss pathway from dryland ecosystems.