N2O production by heterotrophic N transformations in a semiarid soil

Emissions of N2O from soils of the Southwestern US are thought to result from the activity of anaerobic denitrifying bacteria, but the seasonal dryness and sandy texture of these soils are more conducive to the activities of aerobic microbes. Here, we present incubations of semiarid soils with added compounds known to stimulate the N-cycling processes ammonification (proteins, oligopeptides, and amino acids (AAs)), nitrification (NH4 + and NO2 ), and denitrification (NO3 glucose). Nonflooded ( 34 kPa) incubations with added organic N determined that oligopeptides (four AA in length) resulted in the highest potential N2O flux over a 12-d incubation period (66 ng N2O g 1 soil d ), three times that of proteins (21 ng N2O g 1 soil d ) or AAs (24 ng N2O g 1 soil d ). Initial N2O production in incubations with added organic N decreased by more than 63% with addition of cycloheximide, an inhibitor of fungal activity, but additions of a bacterial inhibitor (streptomycin) increased N2O flux by 100%. Additions of NH4 + and NO2 resulted in little NO3 production during the 12-d incubation, indicating that autotrophic N transformations were limited. Flooded soil (0 kPa) incubations with added NO3 and glucose resulted in considerable N2O production by day 2 (200 ng N2O g 1 soil d ), but 0 kPa incubations without glucose produced less than 10 ng N2O g 1 soil d 1 revealing C, rather than water, limitations on denitrification in semiarid soils. Incubation of soils ( 34 kPa) with N-labeled substrates known to stimulate N mineralization and nitrification processes showed differences in N2O production after addition of glutamine (3 ng N2O g 1 soil d ), NH4 + (16 ng N2O g 1 soil d ), NO2 (26 ng N2O g 1 soil d ), and NO3 (1 ng N2O g 1 soil d ). All N treatments produced similar native N2O efflux of 12 ng N2O g 1 soil d 1 through the incubation period. The limitations of C and H2O and minimal autotrophic N activity suggest that heterotrophic N-cycling processes may be responsible for most of the in situ N transformations and N2O production in this system. Published by Elsevier B.V.