Dry season length and vegetation controls on soil nitric oxide emissions from a semiarid annual grassland
Nitric oxide (NO) is an important regulator of atmospheric photochemistry. In soils, NO emissions are driven by the biological processes of nitrification and denitrification, which in turn are controlled by soil physical properties (temperature and moisture). Plant N uptake as well as the length of the dry season can also exert control on ecosystem processes that produce NO—plants can limit substrate supply to nitrifiers during the wet season and accumulation of solutes in dry soils can stimulate NO producing reactions, especially upon wet-up. To understand how dry season length interacts with plant N uptake to influence soil NO emissions, we measured NO fluxes at a California annual grassland where we manipulated vegetation cover (control, 30, 60, and >90 % thinning) and length of the dry season (control, extended, short, and none).
Thinning increased NO emissions more than threefold (~1.5 ng NO-N m-2 s-1) and extractable soil N concentrations (NH4+, NO3-, and NO2-) by 2 to 20x above controls (~30 µg N g-1). The magnitude of NO pulses in response to wetting was higher after extended drought (8 ng NO-N m-2 s-1) than in in the control (7 ng NO-N m-2 s-1) and continuously moist treatments (2 ng NO-N m-2 s-1), likely due to increases in extractable soil N concentrations as the dry season progressed. In arid and semiarid ecosystems, the plant growing season represents an important N sink, which limits NO emissions during periods when soil physical properties can favor NO-producing reactions. During the dry season, however, plant senescence and accumulation of extractable soil N pools in soils favor N loss via emission of NO.