Transformation of plant residues into soil organic matter: chemical characterization of plant tissue, isolated soil fractions, and whole soils

During the stabilization of plant residues into soil humus, organic matter is transformed continuously to different chemical compounds. To obtain a better understanding of these changes, we used C nuclear magnetic resonance (C NMR) and pyrolysis-field ionization mass spectrometry (Py-FIMS) to characterize plant tissue, isolated fractions, and whole surface soils and subsoils from a forest system and a maize (Zea mays L.) system. Both methods indicated that chemical components of the light fraction (LF) were similar to those in the plant material from which the LF was derived, but a lesser amount of carbohydrates and a greater amount of sterols in the LF signalled the early stages of decomposition of organic matter in soil. Accumulation of alkyl C in the maize LF was attributed to microbial structures or metabolites. Larger differences in the abundance and range of organic components were observed between the LF and sand-size fraction (SSF) of the soil under maize. The mass spectra showed that fewer lignin monomers and dimers, lipids, and alky-aromatic compounds were present in the SSF compared with the LF. Carbon-13 NMR data indicated that the SSF contained relatively lesser amounts of carbohydrates and aliphatic compounds and had a higher degree of aromaticity than the LF.Differences between the organic matter in the soils under forest and maize reflected the effects of deforestation, cultivation, and cropping to maize on soil organic matter. Carbon-13 NMR results indicated that the surface soil under maize had less O-alkyl and alkyl C but more aromatic and carboxyl C than the forest soil. In addition, Py-FIMS results indicated that lipids and sterols, which are derived from plant material, were reduced in the soil under maize. Microbial degradation of these high-molecular-weight compounds probably resulted in their transformation into polysaccharides in the soil humus. The presence of numerous N-compounds in the soils under maize was attributed to N from fertilizers that had been stabilized in heterocyclic forms.