Nitrogen speciation and pyrolytic patterns of N-15-labelled soil and compost fractions

A mixture of urban waste, wheat straw and K (NO3)-N-15 was subjected to 80- day laboratory composting in order to analyse the qualitative and quantitat ive speciation patterns of the newly formed N compounds in the different co mpost fractions, i.e. water-soluble fraction (WS), colloidal fractions - hu mic acid-like (HA) and fulvic acid-like (FA), and alkali-insoluble organic fractions. In a further experiment, the compost was added to a mineral soil (3.1 g compost per 100 g soil), which was subjected to additional incubati on for 80 days. The above organic fractions in addition to two particulate ones were isolated from the soil. The stable isotope ratios (N-15/N-14) Of the resulting compost (incorporating 21.8% N as N-15) and soil fractions we re analysed to monitor the distribution of the N in the different soil orga no-mineral compartments. Finally, a preliminary identification of the major groups of nitrogen-bearing molecules in soil and compost fractions was car ried out by Curie-point analytical pyrolysis. It was found that, after the incubation experiments, most of the newly formed N-compounds tend to concen trate in the alkali-insoluble residue (30% N-15 enrichment) but, about 28% of the N remains in colloidal fractions (HA + FA). Upon pyrolysis, the FA y ielded typical anhydrosugar and furan compounds suggesting an origin from c arbohydrate material. The N-15 in this soil fraction amount to 0.7% total s oil nitrogen (TSN) (up to 9% of the N from the nitrate added). The HA showe d a similar composition, but comparatively released higher amounts of carbo hydrate-derived products, typical methoxyphenols and some nitrogen-containi ng compounds, suggesting a lignoprotein content (9% N derived from the nitr ate added = 1.7% of the TSN). The WS showed a very complex composition yiel ding after pyrolysis a series of methoxyphenols and carbohydrate-derived pr oducts and substantial yields of fatty acids. The slight differences betwee n the pyrolytic patterns of the different soil and compost fractions sugges t a predominantly physical incorporation of the compost to the soil, as wel l as a non-selective biodegradation of the different C and N forms. The iso topic ratios indicate that the tendency for accumulation of stable N forms followed the order soluble < colloidal < particulate soil fractions. (C) 20 01 Elsevier Science B.V. All rights reserved.