Nitrifier controls on soil NO and N2O emissions in three chaparral ecosystems under contrasting atmospheric N inputs

Abstract

High rates of atmospheric N deposition can increase ecosystem N availability and stimulate N losses from soils via nitric oxide (NO; an air pollutant at high concentrations) and nitrous oxide (N2O; a strong greenhouse gas) emissions as predicted by N saturation theory. However, it remains unclear whether theories developed in mesic ecosystems apply to drylands, where plant N uptake and N availability are often decoupled. NO and N2O are produced during the oxidation of ammonia (i.e., nitrification) by ammonia-oxidizing archaea (AOA) or ammonia-oxidizing bacteria (AOB). Because AOB may be favored in N-rich environments and may emit more NO and N2O than AOA, high atmospheric N inputs may favor both NO and N2O emissions. To assess whether atmospheric N deposition favors AOB- and AOA-derived N emissions, we selectively inhibited AOA and AOB and measured NO and N2O from soils collected from three dryland sites exposed to relatively low (3.8 kg ha−1 = Low N) or high (11.8 kg ha−1 = High N-A; 15.6 kg ha−1 = High N–B) atmospheric N inputs. We found that while the High N–B deposition site had the lowest AOA:AOB ratio (2.3 ± 0.6), consistent with expectations, this site did not emit the most NO and N2O. Rather, AOA emitted between 21 and 78% of the NO from our sites, with higher AOA-derived NO emissions from relatively coarse-textured soils in the Low N deposition site. In addition to nitrification, other processes also emitted NO and N2O, especially in the High N-A site where non-nitrifier NO and N2O emissions were ∼2–4 × higher than the other sites, and where finer textured soils may favor denitrification. Interactions between soil texture and N availability, rather than shifts in nitrifier communities, likely determine whether atmospheric N deposition is retained in these dryland sites or reemitted to the atmosphere as NO or N2O.