The organic N-pool in soils provides an important part of the N metabolized by rice. As this pool is, however, very large compared to the yearly uptake, -which may involve a few percent only -direct evidence of its importance is difficult to establish. We tried therefore to distinguish different fractions with supposedly different availability. We did not succeed in developing a satisfactory fractionation scheme, but improved existing fractionation schemes by applying a sequential extraction using an alkaline extraction followed by an acid one; the residue of the latter was still an important portion of the total.
These three fractions were measured both in pot cultures and in an experimental field with different N applications schemes, before and after rice growth. The NaOH-extractable fraction nearly always increased during rice growth, except in the last growth period in the rice field. The acid-extractable fraction decreased in all experiments, sometimes a little, sometimes considerably. The residual fraction usually showed a decrease except in the second, dry period in the field, when there was a stark increase. Probably processes occur shifting the ratios between the fractions in a way different from uptake by rice.
The NaOH-extractable N showed a small increase when ammonia was used in the pot cultures, but decreased when nitrate was used. Although about 80%7 of this nitrate is denitrified, rice growth was stimulated by nitrate application, probably because of its oxidative action on organic matter. The same effect was found with H 2 0 2 ; this compound caused the alkaline and acid extractable fractions to decrease as well. We think that nitrate exerts its favourable role on N uptake for rice growth mainly by its oxidative capacity. Ammonia does the same, after it is -automatically -oxidized to nitrate in the oxygenated water of the rice field.