Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2005
Publication Date: 6/1/2006
Citation: Mclain, J.E., Martens, D.A. 2006. N2o production by heterotrophic n transformations in a semi-arid soil. Applied Soil Ecology. 32:253-263.
Interpretive Summary: Nitrous oxide is a potent greenhouse gas produced by soil microbes. Because of its implications for global warming, it is crucial to quantify the contribution of different microbial pathways to total nitrous oxide production. Only then can accurate predictions of nitrous oxide efflux from soils into the atmosphere can be made. Traditional methods for measuring microbial sources of this gas made use of experimental flooding of the soil and addition of carbon sources. By use of laboratory soil incubations with addition of various substrates, we show that flooding of the soil and addition of carbon sources leads to overestimation of nitrous oxide production in semi-arid soils. Instead, the dominant nitrous oxide production pathway in semi-arid soils is heterotrophic nitrification. The results of the study may aid scientists in developing more accurate models of soil nitrous oxide production and may ultimately aid atmospheric scientists in more accurately predicting future nitrous oxide concentrations in the atmosphere.
Technical Abstract: Traditional measures of denitrification activity have never shown a consistent relationship with in situ N2O production in non-agricultural soils. Here we present soil incubations with added compounds known to stimulate N cycling processes, including ammonification (peptides, proteins, and amino acids), nitrification (NH4+ and NO2-) and denitrification (NO3- ± glucose) in semi-arid soils. Non-flooded (-34 kPa) incubations with added organic N determined that peptides resulted in the highest N2O flux over a 12-d incubation period (66 ng N2O g-1 soil d-1), compared to proteins (21 ng N2O g-1 soil d-1) or amino acids (24 ng N2O g-1 soil d-1). Nitrous oxide fluxes from organic N incubations decreased by more than 75% with addition of cycloheximide, an inhibitor of fungal activity. In contrast, additions of streptomycin, a bacterial inhibitor, resulted in increased N2O flux. Flooded soil (0 kPa) incubations with added NO3- and glucose resulted in short-term N2O production of more than 200 ng N2O g-1 soil d-1, but 0 kPa incubations without glucose produced less than 10 ng N2O g-1 soil d-1, suggesting a moisture and C limitation on denitrification in semi-arid soils. Incubations of soils (-34 kPa) with 15N-labeled substrates representing successive steps of N mineralization/nitrification showed 15N2O production after addition of glutamine (3.2 ng 15N2O g-1 soil d-1), NH4+ (15.8 ng 15N2O g-1 soil d-1), NO2- (26.3 ng 15N2O g-1 soil d-1) and NO3- (1.5 ng 15N2O g-1 soil d-1), but similar native N2O efflux was measured regardless of substrate added, suggesting that heterotrophic N cycling processes are responsible for most N transformations and N2O production in this system.