THE HETEROGENEOUS NATURE OF MICROBIAL PRODUCTS AS SHOWN BY SOLID-STATE C-13 CP MAS NMR-SPECTROSCOPY/

Homoionic Na-, Ca-, and Al-clays were prepared from the <2 mu m fracti ons of Georgia kaolinite and Wyoming bentonite and mixed with sand to give artificial soils with 5, and 25% clay. The artificial soils were inoculated with microbes from a natural soil before incubation. Unlabe lled and uniformly C-13-labelled (99.9% atom) glucose were incorporate d into the artificial soils to study the effects of clay types, exchan geable cations and clay contents on the mineralization of glucose-carb on and glucose-derived organic materials. Chemical transformation of g lucose-carbon upon incorporation into microbial products and metabolit es, was followed using solid-state C-13 CP/MAS NMR spectroscopy. There was a significant influence of exchangeable cations on the mineraliza tion of glucose-carbon over a period of 33 days. At 25% clay content, mineralization of glucose-carbon was highest in Ca-soils and lowest in Al-soils. The influence of exchangeable cations on mineralization of glucose-carbon was more pronounced in soils with bentonite clay than t hose with kaolinite clay. Statistical analysis of data showed no overa ll effect of clay type on mineralization of glucose-carbon. However, t he interactions of clay type with clay content and clay type with clay content and exchangeable cations were highly significant. At 25% clay content, the mineralization of glucose-carbon was significantly lower in Na- and Al-soils with Wyoming bentonite compared with Na- and Al-s oils with Georgia kaolinite. For Ca-soils this difference was not sign ificant. Due to the increased osmotic tension induced by the added glu cose, mineralization of glucose-carbon was slower in soils with 5% cla y than soils with 25% clay. Despite the differences in the chemical an d physical characteristics of soils with Ca-, Na- and Al-clays, the ch emical composition of organic materials synthesised in these soils wer e similar in nature. Assuming CP/MAS is quantitative, incorporation of uniformly C-13-labelled glucose (99.9% atom) in these soils resulted in distribution of carbon in alkyl (24-25%), O-alkyl (56-63%), carbony l (11-15%) and small amounts of aromatic and olefinic carbon (2-4%). H owever, as decomposition proceeded, the chemistry of synthesised mater ial showed some changes with time. In the Ca- and Na-soils, the propor tions of alkyl and carbonyl carbon decreased and that of O-alkyl carbo n increased with time of incubation. However, the opposite trend was f ound for the Al-soil. Proton-spin relaxation editing (PSRE) subspectra dearly showed heterogeneity within the microbial products. Subspectra of the slowly-relaxing (long T-1(H)) domains were dominated by alkyl carbon in long- and short-chain structures. The signals due to N-alkyl (55 ppm) and carbonyl carbon were also strong in these subspectra. Th ese subspectra were very similar to those obtained for microbial and f ungal materials and were probably microbial tissues attached to clay s urfaces by polysaccharide extracellular mucilage. Subspectra of fast-r elaxing (short T-1(H)) domains comprised mostly O-alkyl and carbonyl c arbon and were probably microbial metabolites released as neutral and acidic sugars into the extracellular environment, and strongly sorbed by clay surfaces.