Nitrate and nitrite microgradients in barley rhizosphere as detected by a highly sensitive denitrification bioassay
1992
Binnerup, S.J. | Sorensen, J.
A highly sensitive denitrification bioassay was developed for detection of NO3(-1) and NO2(-1) 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(-1) reduction to N2O but incapable of NO3(-1) reduction to NO2(-1). 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(-1) 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(-1) and NO2(-1), and these cells could subsequently be added to the sample to convert the indigenous NO3(-1) pool to N2O. Concentrations of both NO3(-1) and NO2(-1) were thus determined as N2O, with a detection limit of approximately 10 pmol of N. The bioassay could be used to determine NO3(-1) and NO2(-1) 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(-1) were found in the rhizosphere compared with bulk soil. Under dry conditions, NO3(-1) 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(-1) and NO2(-1) availability in the rhizosphere are discussed.
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