Extractability of 15N-labelled plant residues in soil by electro-ultrafiltration

An aerobic incubation of 15N-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-Norg) was mineralized during the soil incubation; and (3) to compare the course of extractable Norg with inorganic N accumulation. A loamy sand (Colluvisol) derived from mottled sandstone, Hessia, Germany was used. Rape labelled with 15N was incorporated in the soil and incubated for 80 d at 20°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 Norg in EUF soil extracts. The extractability of 15N-labelled Norg by EUF was, on average, 7.7 times greater than the share of total labelled Norg on the total soil Norg (labelled + unlabelled). This indicates that the EUF technique extracts with relative selectivity Norg derived from the green manure. Net N mineralization of applied N occurred up to day 40 of incubation and 38% of the applied Norg was mineralized. Labelled EUF-Norg was maximal on day 2 of incubation followed by a decrease up to day 40. After day 40, labelled EUF-Norg remained stable. This course clearly indicates that some of the EUF-Norg had been mineralized. Nevertheless, the decrease of labelled EUF-Norg 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.