Chemical characterization and decomposition of organic matter from two contrasting grassland soil profiles

Particle size fractions of soils from the surface 6 cm of two adjacent grassland plots which, as a result of different fertilizer treatments since 1897, have either a mor or a mull humus form were analysed using solid‐state ¹³C nuclear magnetic resonance spectroscopy and fractionation of organic N by steam distillation. In the mor humus soil, which had received 180 kg (NH⁴)²SO⁴ ha⁻¹ annually and was pH 4.3, there was more C and N in the larger particle size fractions than in the mull humus soil (pH 5.8). The NMR spectra of correspondingly sized soil fractions were similar for both soils. The intensities of NMR signals between 0 and 40 ppm (alkyl‐C) and between 160 and 200 ppm (carbonyl‐C) increased with decreasing particle size. The intensities of the NMR signals between 60 and 90 ppm (0‐alkyl‐C) and between 90 and 110 ppm (acetal‐ and ketal‐C) decreased with increasing particle size. Comparison of the NMR spectra of the >2000 um fractions from both soils with those of dried grass litter from the same plots indicated the exclusive plant origin of the C in the largest size fraction of the soils. NMR resonances between 40 and 60 ppm were attributed to alkyl‐amino‐C because their intensities agreed with the amino‐N determinations obtained during organic N fractionation. During incubation in soil microcosms, the larger sized fractions decomposed more rapidly than the smaller fractions. However, all the correspondingly sized particle fractions from the two soils decomposed at the same rate except the >2000 pm fractions. The largest size fraction from the mor humus soil decomposed faster than that from the mull humus soil. This difference in decomposition rate could not be attributed to differences in the chemical composition of the >2000 pm fraction.