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Abundance of ammonia-oxidizing Archaea and Bacteria and their contribution to nitrous oxide emissions in woody perennial cropping systems Hans, Geetkamal Kaur


Nitrification is one of the processes responsible for the emissions of nitrous oxide (N₂O), an ozone-depleting greenhouse gas, from agricultural soil. In the first step of nitrification, catalyzed by the enzyme ammonia monooxygenase, ammonia is converted into hydroxylamine which further chemically decomposes into N₂O. The contribution of nitrifying Archaeal species to N₂O emissions in woody perennial cropping systems is not well understood. The first study reported here examined the effect of nitrogen application, mulch and soil depth on abundances of nitrifying Archaea and Bacteria, and total Archaea and Bacteria in an apple orchard and a grape vineyard. Soil samples were collected five times (2013-2015) and at three depths. Quantitative polymerase chain reaction was used to determine the abundances of total Archaeal (16SrRNAArch) and Bacterial (16SrRNABac) and nitrifying Archaeal (amoAArch) and Bacterial (amoABac) genes. Total Bacteria were more abundant than total Archaea and both decreased as depth increased. Nitrifying Archaeal abundance increased with depth. Abundance of nitrifying Bacteria was lower than that of nitrifying Archaea when compost was applied and there was no effect of mulch. The second study examined the effect of selective nitrification inhibitors, urea concentration, and soil depth on N₂O emissions in closed in vitro microcosms. Soil samples were collected from a grape vineyard in 2015 and 2016 at two depths (0-15, 15-30 cm). Increasing urea concentrations increased N₂O emissions. When urea was added, soils collected from 0-15 cm produced about 4 times more N₂O than soils collected from 15-30 cm soil depth. Addition of a Bacterial or an Archaeal inhibitor reduced N₂O fluxes from soils at 0-15 cm. These data show that nitrifying Archaea are present in these woody perennial soils, and in vitro studies suggest nitrifying Bacteria may be more important contributors to N₂O emissions than nitrifying Archaea at higher ammonia concentrations.

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