Aridity and plant uptake interact to make dryland soils hotspots for nitric oxide (NO) emissions

Abstract

Nitric oxide (NO) is an important trace gas and regulator of atmospheric photochemistry. Theory suggests moist soils optimize NO emissions, whereas wet or dry soils constrain them. In drylands, however, NO emissions can be greatest in dry soils and when dry soils are rewet. To understand how aridity and vegetation interact to generate this pattern, we measured NO fluxes in a California grassland, where we manipulated vegetation cover and the length of the dry season and measured [δ 15 -N]NO and [δ 18 -O]NO following rewetting with 15 N-labeled substrates. Plant N uptake reduced NO emissions by limiting N availability. In the absence of plants, soil N pools increased and NO emissions more than doubled. In dry soils, NO-producing substrates concentrated in hydrologically disconnected microsites. Upon rewetting, these concentrated N pools 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 h postwetting. Plants acted as an 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. 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 drylands.nitric oxide | chemodenitrification | drylands | NO pulses | N cycling N itric oxide (NO) is an important trace gas; it regulates the oxidative capacity of the atmosphere and indirectly influences Earth's climate (1). In the troposphere, NO catalyzes the production of hydroxyl radical, a powerful oxidant and cleanser of atmospheric contaminants. High concentrations of NO also favor the production of ozone (O 3 ), an urban pollutant and contributor to radiative forcing (1). Globally, fossil fuel combustion and biomass burning are major sources of NO, but soils are also a substantial source (2). Roughly 25-60% of terrestrial NO emissions originate from drylands (arid and semiarid environments) (3), which cover one-third of the terrestrial land surface (4), suggesting arid environments are important to global NO production. Climate models predict an expansion of drylands and intensifying droughts in existing arid regions (5), and when coupled with increasing rates of nitrogen (N) deposition and changes in the magnitude and frequency of precipitation events (6), increased soil NO emissions are possible (7). Paradoxically, arid soils are often described as infertile because biological processes are limited by water and N (8), raising the questions of why drylands are NO emission "hotspots."NO is produced in soils through both abiotic and biotic processes (2). Chemodeni...

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