Uncoupling of Ammonia Oxidation from Nitrite Oxidation, and Its Impact upon Nitrous Oxide Production in Non-Cropped Oregon Soils.

Abstract

The factors controlling the relative contributions of ammonia (NH₄⁺/NH₃) oxidizing archaea (AOA) and bacteria (AOB) to nitrification and nitrous oxide (N₂O) production in soil remain unclear. A study was conducted to examine the contributions of AOA and AOB to nitrification, nitrite (NO₂^-) accumulation, and NO₂^- effected N₂O production in three non-cropped soils. Nitrification rates in the three soils ranged 7-fold from 0.15 to 1.08 µmol N g^-1 d^-1, with AOA contributing 64-71% of the total activity. AOA and AOB driven NO₂^- accumulation represented 8-100% of NO₂^- + NO₃^- accumulation, persisted over 48h and was accompanied by acetylene sensitive, and NH₄⁺ dependent N₂O production. NH₄⁺ and NO₂^- dependent N₂O production was stimulated, both when AOA and AOB or just AOA alone were active. Evidence was obtained to suggest that the soil NO₂^- oxidation potential was inadequate relative to NH₄⁺ oxidation potential leading to NO₂^- accumulation. By adding the NO₂^--oxidizing bacteria (NOB) Nitrobacter vulgaris, NO₂^- accumulation and N₂O production were prevented, while the overall rate of nitrification was unaffected. N₂O-N yields amounted to 0.05±0.01% of NO₂^- + NO₃^--N accumulated in the presence of NH₄⁺ alone, and 0.28±0.11% in the presence of both NH₄⁺ + NO₂^-. Regression analysis of NO₂^- concentration and N₂O formation rates revealed a positive, non-linear relationship for both total and AOA nitrification activity. V_max values ranged from 0.62-0.05 nmol N₂O g^-1 d^-1, and K_m values ranged from 0.30-0.02 µmol NO₂^- g^-1 soil. The results of our work have provided new insights into nitrifier denitrification as a source of N₂O. The relative roles of AOA and AOB in N₂O production is complex, but we have shown that NO₂^- accumulation and the potential activity of soil NOB play a critical role in nitrifier denitrification.