Original Application of Solid-State Nuclear Magnetic Resonance Spectroscopy to Characterise Soil Organic Matter Transformations Under Crude Oil Contamination in the Niger Delta

The transformation of soil organic matter under chronic crude oil contamination in the Niger Delta has profound implications for soil fertility recovery, carbon cycling, and bioremediation effectiveness, yet the molecular-level characterisation of these transformations has been hampered by the complexity of the soil matrix and the limitations of traditional chemical extraction methods. Solid-state nuclear magnetic resonance spectroscopy offers unique capabilities for directly characterising organic carbon functional group chemistry in intact soil matrices without fractionation artefacts, but its application to Niger Delta oil-contaminated soils represents an underexplored methodological frontier. This study applied multiple solid-state nuclear magnetic resonance techniques, including cross-polarisation magic angle spinning 13C NMR, direct polarisation 13C NMR, 1H-13C heteronuclear correlation, and diffusion-ordered spectroscopy, to characterise organic matter transformations in a chronosequence of Niger Delta soils spanning 2 to 35 years of crude oil contamination history. Samples were collected from Rivers and Bayelsa States under controlled sampling protocols ensuring stratigraphic preservation. Spectral deconvolution and multivariate analysis of nuclear magnetic resonance spectra were used to extract quantitative organic carbon functional group data. An original spectral deconvolution algorithm was developed and validated for the complex overlapping spectral signatures observed in petroleum-contaminated tropical soil matrices. Contaminated soils showed systematic decreases in O-alkyl carbon forms representing carbohydrate structures and increases in alkyl and aromatic carbon forms across the contamination chronosequence, indicating progressive humification of petroleum carbon and degradation of labile native organic matter. These molecular transformations were correlated with soil biological activity and nutrient availability data. This study provides an original molecular characterisation of Niger Delta soil organic matter transformation chemistry with direct implications for bioremediation monitoring.

Keywords: solid-state NMR, soil organic matter, crude oil contamination, Niger Delta, spectral deconvolution