EXTRACTABILITY OF NEWLY IMMOBILIZED NITROGEN BY ELECTROULTRAFILTRATION (EUF) AND CACL2 EXTRACTION

Applied inorganic N is rapidly immobiliezed in soils, provided that or ganic carbon is available to soil microbes. This newly immobilized N i s highly accessible to remineralization and may therefore be an import ant source of N for crop growth. The objective of our study was to det ermine the extractability of newly immobilized inorganic N as organic N fraction in EUF and 10 mM CaCl2 soil extracts. A soil incubation exp eriment was carried out over 80 days and the immobilization of applied (15NH4NO3)-N-15 was induced by addition of cellulose. For both extrac tion procedures a 'different method' was used to estimate the apparent immobilization of inorganic N, the remineralization of the newly immo bilized N and its extractability as an organic N fraction (Norg). In a ddtion, the isotopic composition of EUF extractable organic and inorga nic N was determined. Apparent net immobilization took place in cellul ose amended soil up to day 30 of incubation. This newly immobilized N was highly accessible to remineralization. 47 % of the newly immobiliz ed N (labelled and unlabelled) was apparently remineralized from day 3 0 to 80. Labelled N was completedly immobilized on day 16 and 29 % was release again up to day 80. Both extraction methods failed to extract this potential of remineralizable N as an extractable Norg fraction. The results of the present study showed that EUF and CaCl2 extractable Norg was unsuitable as index for easily mineralizable N in cases wher e remineralization of newly immobilized N plays a major role.