Analysis of carbon and nitrogen forms in soil fractions after the addition of 15N-compost by 13C and 15N nuclear magnetic resonance

A quantitative laboratory assessment of the different C and N forms in soil humus fractions was carried out by incubation of a mineral substrate after the addition of N-15-labeled compost. The experimental design included (i) preparation of the N-15-labeled organic matter (city refuse compost, 640 g kg(-1) wheat straw and (KNO3)-N-15 composted for 80 days), (ii) a further 80 day incubation of a mixture of the labeled compost with a mineral soil (32 g kg(-1)), (iii) measurement of stable isotope ratios, and (iv) isolation and structural comparison by C-13 and N-15 cross-polarization, magic-angle spinning nuclear magnetic resonance (NMR) of different organic fractions, i.e., soluble, colloidal (humic and fulvic type), and particulate (free organic matter and humin), from both the compost and the compost-treated soil. The results showed that the amide forms dominated in all of the newly formed N compounds, but an increased amount of alkali insoluble organic fractions was observed after incubation of the soil. The analysis of the insoluble, particulate fractions shows that nonextractable amides constitute the major pool of newly formed N compounds. The particulate soil fraction isolated by flotation in CHBr3-MeOH contained 16.8% of the total soil N and 26% of the N-15. The C-13 NMR spectra showed that the fulvic acid-like fraction (7.6% of the soil N, 8.8% of N-15) consisted almost completely of a C=O-containing carbohydrate material, whereas the humic acid-like fraction (20.3% of the total soil N, 8.6% of N-15) resembled an oxidized lignoproteic fraction containing the most significant aromatic domain. The water soluble fraction was, in both soil and compost, the one with the highest isotopic abundance of N-15 (96%), but the N-15 NMR spectrum revealed minor amounts of soluble mineral N in this fraction and the remainder consisting of amide compounds.