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 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.
Earth Research Institute and Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106; phomyak@uci.edu.
Peter M
0000-0003-0671-8358
PM
Homyak
Earth Research Institute and Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106;
Joseph C
JC
Blankinship
Earth Research Institute and Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106;
Kenneth
K
Marchus
Department of Environmental Sciences, University of California, Riverside, CA 92521.
Delores M
DM
Lucero
Department of Environmental Sciences, University of California, Riverside, CA 92521.
James O
JO
Sickman
Earth Research Institute and Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106;
Joshua P
JP
Schimel
D000393:Air Pollutants / Q000032:analysis / Q000378:metabolism*; D003889:Desert Climate; D017753:Ecosystem; D006813:Humidity; D009569:Nitric Oxide / Q000032:analysis / Q000378:metabolism*; D010944:Plants; D006109:Poaceae / Q000378:metabolism*; D012989:Soil Pollutants / Q000032:analysis / Q000378:metabolism*
D000393:Air Pollutants; D012989:Soil Pollutants; D009569:Nitric Oxide
N cycling; NO pulses; chemodenitrification; drylands; nitric oxide