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

Abstract

A mixture of urban waste, wheat straw and K 15NO 3 was subjected to 80-day laboratory composting in order to analyse the qualitative and quantitative speciation patterns of the newly formed N compounds in the different compost fractions, i.e. water-soluble fraction (WS), colloidal fractions — humic 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 incubation for 80 days. The above organic fractions in addition to two particulate ones were isolated from the soil. The stable isotope ratios ( 15N/ 14N) of the resulting compost (incorporating 21.8% N as 15N) and soil fractions were analysed to monitor the distribution of the N in the different soil organo-mineral compartments. Finally, a preliminary identification of the major groups of nitrogen-bearing molecules in soil and compost fractions was carried out by Curie-point analytical pyrolysis. It was found that, after the incubation experiments, most of the newly formed N-compounds tend to concentrate in the alkali-insoluble residue (30% 15N enrichment) but, about 28% of the N remains in colloidal fractions (HA+FA). Upon pyrolysis, the FA yielded typical anhydrosugar and furan compounds suggesting an origin from carbohydrate material. The 15N in this soil fraction amount to 0.7% total soil nitrogen (TSN) (up to 9% of the N from the nitrate added). The HA showed a similar composition, but comparatively released higher amounts of carbohydrate-derived products, typical methoxyphenols and some nitrogen-containing compounds, suggesting a lignoprotein content (9% N derived from the nitrate added=1.7% of the TSN). The WS showed a very complex composition yielding after pyrolysis a series of methoxyphenols and carbohydrate-derived products and substantial yields of fatty acids. The slight differences between the pyrolytic patterns of the different soil and compost fractions suggest a predominantly physical incorporation of the compost to the soil, as well as a non-selective biodegradation of the different C and N forms. The isotopic ratios indicate that the tendency for accumulation of stable N forms followed the order soluble<colloidal<particulate soil fractions.