EXTRACTABILITY OF LABELED MICROBIAL BIOMASS-N BY ELECTROULTRAFILTRATION AND CACL2 EXTRACTION

A laboratory soil incubation and a pot experiment with ryegrass were c arried out in order to examine the extractability of microbial biomass N by using either 10-mM CaCl2 extraction or the electro-ultrafiltrati on (EUF) method. The aim of the experiment was to test the hypothesis whether the organic N (Norg) extracted by EUF or CaCl2 from dried soil samples represents a part of the microbial biomass. For the Laborator y incubation a N-15-labelled Escherichia coli suspension was mixed wit h the soil. For the pot experiment a suspension of N-15-labelled bacte ria was applied which had previously been isolated from the soil used. Soil samples of both treatments, with and without applied bacterial s uspension, were extracted by EUF and CaCl2. The extractability of appl ied microbial biomass was estimated from the difference in extractable Norg between the two treatments. In addition, the N isotopic composit ion in the upper plant matter, in the soil, and in organic and inorgan ic N fractions of EUF and CaCl2 extracts was analysed, Both experiment s showed that the applied microbial biomass was highly accessible to m ineralization and thus represented potentially mineralizable N, Howeve r, this mineralizable N was not extractable by CaCl2 or by the EUF met hod, It was, therefore, concluded that the organic N released on soil drying and which was thus extractable was derived from the non-biomass soil organic matter. The result suggests that both extraction methods may provide a suitable index for mineralizable N only in cases where the decomposable organic substrates are derived mainly from sources ot her than the living sail biota.