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 ma tter is transformed continuously to different chemical compounds. To o btain a better understanding of these changes, we used C-13 nuclear ma gnetic resonance (C-13 NMR) and pyrolysis-field ionization mass spectr ometry (Py-FIMS) to characterize plant: tissue, isolated fractions, an d whole surface soils and subsoils from a forest system and a maize (Z ea mays L.) system, Both methods indicated that chemical components of the light fraction (LF) were similar to those in the plant material h om 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 de composition 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 obs erved between the LF and sand-size fraction (SSP) of the soil under ma ize. The mass spectra showed that fewer lignin monomers and dimers, Li pids, and alky-aromatic compounds were present in the SSF compared wit h the LF. Carbon-13 NMR data indicated that the SSF contained relative ly lesser amounts of carbohydrates and aliphatic compounds and had a h igher degree of aromaticity than the LF. Differences between the organ ic matter in the soils under forest and maize reflected the effects of deforestation, cultivation, and cropping to maize on soil organic mat ter, Carbon-13 NMR results indicated that the surface soil tinder maiz e had less O-alkyl and alkyl C but more aromatic and carboxyl C than t he forest soil, in addition, Py-FIMS results indicated that lipids and sterols, which are derived from plant material, were reduced in the s oil under maize, Microbial degradation of these high-molecular-weight compounds probably resulted in their transformation into polysaccharid es in the soil humus. The presence of numerous N-compounds in the soil s under maize was attributed to N from fertilizers that had been stabi lized in heterocyclic forms.