title: Nitrate and nitrite microgradients in barley rhizosphere as detected by a highly sensitive denitrification bioassay.
abstract: A highly sensitive denitrification bioassay was developed for detection of NO3- and NO2- in rhizosphere soil samples. Denitrifying Pseudomonas aeruginosa ON12 was grown anaerobically in citrate (30 mM) minimal medium with KClO3 (10 mM) and NaNO2 (3 mM), which gave cells capable of NO2- reduction to N2O but incapable of NO3- reduction to NO2-. Growth on citrate minimal medium further resulted in the absence of N2O reduction. When added to small soil samples in O2-free vials, such cells could be used to convert the indigenous NO2- pool to N2O, which was subsequently quantified by gas chromatography. Cells grown in KClO3-free citrate medium with 10 mM NaNO3 as the electron acceptor were capable of reducing both NO3- and NO2-, and these cells could subsequently be added to the sample to convert the indigenous NO3- pool to N2O. Concentrations of both NO3- and NO2- were thus determined as N2O, with a detection limit of approximately 10 pmol of N. The bioassay could be used to determine NO3- and NO2- pools in 10-mg soil samples taken along a microgradient in the rhizosphere of field-grown barley plants. At both low (10%, wt/wt) and high (18%, wt/wt) water content, relatively high levels of NO2- were found in the rhizosphere compared with bulk soil. Under dry conditions, NO3- was also more abundant in the rhizosphere than in the bulk soil, whereas such a difference was not observed at the high water content. The roles of plant metabolism and bacterial nitrification and denitrification processes for NO3- and NO2- availability in the rhizosphere are discussed.
authors.affiliation: Department of Ecology and Molecular Biology, Royal Veterinary and Agricultural University, Frederiksberg, Denmark.
authors.forename: S J
authors.initials: SJ
authors.lastname: Binnerup
authors.forename: J
authors.initials: J
authors.lastname: Sørensen
mesh_terms: D001681:Biological Assay / Q000379:methods; D005069:Evaluation Studies as Topic; D001467:Hordeum; D009566:Nitrates / Q000378:metabolism*; D009573:Nitrites / Q000378:metabolism*; D010944:Plants / Q000378:metabolism*; D011550:Pseudomonas aeruginosa / Q000378:metabolism; D012987:Soil / Q000032:analysis
chemical_list: D009566:Nitrates; D009573:Nitrites; D012987:Soil