Detection of denitrification, by a ¹⁵N racer technique, of nitrogen released from azolla and blue-green algae in a flooded soil

Abstract

Denitrification of nitrogen from applied NH4+ and NO3-, and nitrogen released from Azolla caroliniana, Anabaena variabilis and Nostoc muscorum, was studied in a flooded soil. Denitrification did not occur in Azolla-, Anabaena-, Nostoc- or ammonium sulfate-treated soil, incubated at 18° to 25°C, until after 30 days since time was required to develop a surface-oxidized layer where nitrification could first take place. About 32, 45 and 49% of the total nitrogen in Azolla, Anabaena and Nostoc was released as mineral-N in 60 days, with 96, 93 and 93% respectively of this being lost as N2. Thus potentially serious losses of nitrogen from Azolla and blue-green algae may be avoided if their incorporated residues in flooded soils are left no longer than 3 weeks before planting a rice crop. Denitrification started within 3 days of incubation in the sodium nitrate-treated soil. About 10 and 75% of the ISN applied as ammonium sulfate and sodium nitrate, respectively, was lost as N, in 60 days. In addition, a substantial amount of 15N+4- N was formed from the applied 15NO-3-N (about 9% of the total amount added) in 60 days, indicating that a dissimilatory pathway also existed in this soil.