Ammonium bicarbonate,still being one of the main nitrogen fertilizers at present time in China,is easily volatilizable in soils. Various field experiments as well as practices have repeatedly shown that,if prilled and applied beneath the soil surface about 6 cm,its effect on the growth of crops will be markedly increased. In order to get some idea about the mechanism involved in its beneficial effect,the present study was undertaken.Experiments were carried out in pots under greenhouse conditions as well as in the paddy field using N¹⁵-tracer technique.The following is a summary of the results.