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

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...

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Published in:Proceedings of the National Academy of Sciences - PNAS 2016-05, Vol.113 (19), p.E2608-E2616
Main Authors: Homyak, Peter M., Blankinship, Joseph C., Marchus, Kenneth, Lucero, Delores M., Sickman, James O., Schimel, Joshua P.
Format: Article
Language:English
Subjects:
Air Pollutants - analysis
Air Pollutants - metabolism
Arid zones
Biological Sciences
Desert Climate
Ecosystem
Emissions
Humidity
Hydrology
Nitric oxide
Nitric Oxide - analysis
Nitric Oxide - metabolism
Photochemistry
Plant ecology
Plants
PNAS Plus
Poaceae - metabolism
Soil Pollutants - analysis
Soil Pollutants - metabolism
Vegetation
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Summary: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 [δ15-N]NO and [δ18-O]NO following rewetting with 15N-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.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1520496113