PREDICTION OF MINERALIZABLE NITROGEN IN SOILS ON THE BASIS OF AN ANALYSIS OF EXTRACTABLE ORGANIC N

Most of the nitrogen (N) in agricultural soils is organically bound, w hile the N uptake by plants and also the N losses from the soil-plant system into the environment are as inorganic N. The electro-ultrafiltr ation (EUF) method and the extraction by a CaCl2 solution extract an o rganic N fraction (Norg) that is thought to provide information about the amount of rapidly mineralizable N in soils. This paper aims to ill ustrate various aspects regarding the biological meaning of the Norg-f ractions extracted by these two extraction methods and also the opport unities and limitations for predicting the mineralizable N based on an Norg analysis. From an evaluation of numerous data on EUF and CaCl2 e xtractable Norg fractions we concluded that these methods extract N co mpounds which can be used as indices for easily mineralizable soil N. However, both methods extract only some of the rapidly mineralizable N in soils, and some of the Norg ecxtracted appears to be from the more recalcitrant soil organic N. This was particularly true for the EUF-m ethod. It may therefore be desirable to improve both the extractabilit y and the selectivity of the extraction methods. This may be achieved by measuring extractable amino-N-compounds instead of the total extrac table Norg. Evaluating the numerous field experiments done during the last decade shows that the calibration factors obtained for extractabl e Norg were not the same for different growing seasons, geographical r egions and management practices. Theoretically, for each combination o f these factors separate calibrations would be necessary. It is this i nflexibility which appears to be the most serious drawback for the use of extractable Norg fractions in practice. A possible solution may be to combine the flexibility of a simulation model with additional info rmation obtained by the analysis of extractable soil organic N. Furthe r work in this direction may be desirable.