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, while the N uptake by plants and also the N losses from the soil-plant system into the environment are as inorganic N. The electro-ultrafiltration (EUF) method and the extraction by a CaCl2 solution extract an organic N fraction (Norg) that is thought to provide information about the amount of rapidly mineralizable N in soils. This paper aims to illustrate various aspects regarding the biological meaning of the Norg-fractions extracted by these two extraction methods and also the opportunities and limitations for predicting the mineralizable N based on an Norg analysis. From an evaluation of numerous data on EUF and CaCl2 extractable Norg fractions we concluded that these methods extract N compounds 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-method. It may therefore be desirable to improve both the extractability and the selectivity of the extraction methods. This may be achieved by measuring extractable amino-N-compounds instead of the total extractable Norg. Evaluating the numerous field experiments done during the last decade shows that the calibration factors obtained for extractable Norg were not the same for different growing seasons, geographical regions and management practices. Theoretically, for each combination of these factors separate calibrations would be necessary. It is this inflexibility 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 information obtained by the analysis of extractable soil organic N. Further work in this direction may be desirable.