CHEMICAL CHARACTERIZATION AND DECOMPOSITION OF ORGANIC-MATTER FROM 2 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 usin g solid-state C-13 nuclear magnetic resonance spectroscopy and fractio nation of organic N by steam distillation. In the mor humus soil, whic h had received 180 kg (NH4)2SO4 ha-1 annually and was pH 4.3, there wa s 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 fra ctions were similar for both soils. The intensities of NMR signals bet ween 0 and 40 ppm (alkyl-C) and between 160 and 200 ppm (carbonyl-C) i ncreased with decreasing particle size. The intensities of the NMR sig nals between 60 and 90 ppm (O-alkyl-C) and between 90 and 110 ppm (ace tal- and ketal-C) decreased with increasing particle size. Comparison of the NMR spectra of the > 2000 mum fractions from both soils with th ose 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 r esonances between 40 and 60 ppm were attributed to alkyl-amino-C becau se their intensities agreed with the amino-N determinations obtained d uring organic N fractionation. During incubation in soil microcosms, t he larger sized fractions decomposed more rapidly than the smaller fra ctions. However, all the correspondingly sized particle fractions from the two soils decomposed at the same rate except the > 2000 mum fract ions. The largest size fraction from the mor humus soil decomposed fas ter than that from the mull humus soil. This difference in decompositi on rate could not be attributed to differences in the chemical composi tion of the > 2000 mum fraction.