EXTRACTABILITY OF N-15-LABELED PLANT RESIDUES IN SOIL BY ELECTROULTRAFILTRATION

An aerobic incubation of N-15-labelled green manure in soil was performed: (1) to determine the extractability of organic and inorganic N fractions from decomposing plant residues in the soil; (2) to provide evidence that electro-ultrafiltration-extractable organic N (EUF-N-org) was mineralized during the soil incubation; and (3) to compare the course of extractable N-org with inorganic N accumulation. A loamy sand (Colluvisol) derived from mottled sandstone, Hessia, Germany was used. Rape labelled with N-15 was incorporated in the soil and incubated for 80 d at 20 degrees C. The same incubation procedure was done without rape for control. Samples were taken 13 times during the incubation to analyse for NH4+, NO3- and N-org in EUF soil extracts. The extractability of N-15-labelled N-org by EUF was, on average, 7.7 times greater than the share of total labelled N-org on the total soil N-org (labelled + unlabelled). This indicates that the EUF technique extracts with relative, selectivity N-org derived from the green manure. Net N mineralization of applied N occurred up to day 40 of incubation and 38% of the applied N-org was mineralized. Labelled EUF-N-org was maximal on day 2 of incubation followed by a decrease up to day 40. After day 40, labelled EUF-N-org remained stable. This course clearly indicates that some of the EUF-N-org had been mineralized. Nevertheless, the decrease of labelled EUF-N-org did not correspond to the accumulation of labelled inorganic N. The major decrease took place from day 2 to day 9, but about 80% of the increase of labelled inorganic N was observed after day 9.