Aridity and Plant Uptake Interact to Make Dryland Soils Hotspots for Nitric Oxide (NO) Emissions.

Nitric oxide (NO) is an important trace gas; it regulates the oxidative capacity of the atmosphere and indirectly influences Earth’s climate. High concentrations of NO also favor the production of ozone, an urban pollutant and contributor to radiative forcing. Globally, fossil fuel combustion and biomass burning are major sources of NO, but soils, particularly those from dry lands, are also a substantial source. This is puzzling because for decades we have known that moist soils should favor NO emissions while wet or dry soils should constrain them. In dry lands, however, NO emissions can be greatest in dry soils and when dry soils are rewet and lowest when soils are moist, raising the questions of what factors control NO emissions and why drylands are NO emission “hotspots.”

In this talk, I will explore the possibility that vegetation and aridity can influence the well-known relationship between NO-producing processes and soil moisture, generating NO emission patterns opposite to those expected. We hypothesized that drylands act as NO emission hotspots because i) interactions between aridity and vegetation maintain a “leaky” N cycle, and that ii) periods of high NO flux are governed by a shutdown in plant N uptake and by arid conditions that stimulate NO-producing abiotic mechanisms.

To test these hypotheses, we measured NO fluxes in a California annual grassland where vegetation cover was manipulated by thinning and the length of the dry season by irrigation and rainfall exclusion. We also used a novel isotopic approach to measure δ15N- and δ18O-NO following rewetting with 15N-labeled substrates to distinguish between biotic and abiotic NO producing mechanisms and to understand what forms of N generated NO upon rewetting.

When soils were moist, plant N uptake reduced NO emissions by limiting N availability. During the dry season, when plants died, soil N pools increased and NO emissions more than doubled. In these dry soils, NO-producing substrates concentrated in hydrologically disconnected microsites. Upon rewetting, these concentrated N pools, particularly nitrite, underwent rapid abiotic reaction producing large NO pulses. Biological processes did not substantially contribute to the initial NO pulse, but governed NO emissions within 24-hours post-wetting. In the competition between biology and chemistry, dry soils may favor abiotic over biotic processes, producing NO via abiotic NO2 - decomposition.

We conclude that plants acted as a N sink, limiting NO emissions under optimal soil moisture. When soils were dry, however, the shutdown in plant N uptake, along with the activation of chemical mechanisms and the resuscitation of soil microbial processes upon rewetting governed N loss. Our measurements suggest that NO emissions significantly contribute to the openness of arid ecosystems and to the enrichment of soil δ15N during “hot moments” controlled by rapid shifts from dry to wet soil conditions. Aridity and vegetation interact to maintain a leaky N cycle during periods when plant N uptake is low, and hydrologically disconnected soils favor both microbial and abiotic NO-producing mechanisms. Under increasing rates of atmospheric N deposition and intensifying droughts, NO gas evasion may become an increasingly important pathway for ecosystem N loss in dry lands.