Fluctuations in available mineral nitrogen in a flooded rice soil on the sub-coastal plains of the Adelaide river, N.T

Abstract

Soil from the 0-3 inch and 3-12 inch layers of nitrogen fertilized and unfertilized cropped and fallow plots was sampled at two=weekly intervals throughout the growing period of flooded rice. The soil was extracted with sodium acetate-acetic acid (pH 4.8) and ammonium, nitrate, and nitrite nitrogen determined. Nitrite nitrogen levels fell from 0.4 p.p.m. before flooding to less than 0.1 p.p.m. after flooding. Ammonium nitrogen reached peak mean levels of 57 and 55 lb an acre in the surface 12 inches of the soil in fertilized and unfertilized plots respectively, eight weeks after flooding. The effect of added nitrogen was lost within four weeks of flooding. Nitrate nitrogen in the surface 12 inches of soil reached peak levels of 40 and 10 lb an acre two weeks before flooding and 28 and 25 lb an acre eight weeks after flooding in fertilized and unfertilized plots respectively. Changes in the levels of available mineral nitrogen are discussed in relation to soil pH, Eh, and temperature