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The stable fraction of soil organic matter (SOM) has received considerable attention due to concerns about the sustainability of agricultural systems. The main structural characteristics of the SOM of an Entisol in the semi-arid region of Paraiba state, Brazil, after 6 years of cultivation of annual crops with different organic fertiliser treatments, were determined. Six different treatments were evaluated, as follows: (1) gliricidia prunings (Gliricidia sepium Jacq. Walp) incorporated into the soil before planting (GI); (2) cattle manure incorporated before planting (MI); (3) gliricidia applied after planting (GS); (4) manure and gliricidia incorporated before planting (MI + GI); (5) manure incorporated before planting and gliricidia applied after planting (MI + GS); and (6) a control treatment. Spectroscopic techniques were used to analyse carbon in whole soil samples (0-20 cm), as well as in fulvic acid (FA) and humic acid (HA) extracts. Surface-applied gliricidia resulted in the highest SOM content. Laser-induced fluorescence spectroscopy showed that MI + GS contributed to increases in the degree of humification, whereas for the HA fraction, the highest degree of humification occurred with the GS treatment. The ultraviolet-visible absorbance spectra of FA fractions indicated the presence of fresh soluble SOM in all treatments except GS, and mid-infrared region spectra revealed no differences in the chemical structure of the HA fractions. Application of gliricidia prunings (GI and GS), which are nitrogen rich and highly labile, resulted in low HA transformations compared with the control based on nuclear magnetic resonance analysis. Therefore, gliricidia, whether incorporated before planting or applied after planting, decreases the labile fractions of SOM, probably because of its higher decomposition rates.