Spectroscopic and Chromatographic Characterization of the Composition of Organic Matter in Arid Salt-Affected Soils Under Different Vegetation Cover, Southeastern Tunisia (Gabes and Medenine)

In arid Tunisia, particularly in salt-affected soils, studies concerning the soil organic matter (SOM) dynamics and stability are scarce. SOM stability is closely linked to its chemical and molecular composition. This work aims to figure out the SOM composition in arid salt-affected soils, particularly in Lithosols and Solonchaks, under different vegetation cover (sorghum, millet, olive-grove, native vegetation). Ten sites were sampled from 0 to 10 and from 10 to30 cm depth. SOM composition was assessed at functional group and molecular level by Fourier Transform infrared (FTIR) spectroscopy and Gas Chromatography Mass spectrometry (GC/MS). The results showed that, across all studied soils, the characterization of SOM by FTIR spectroscopy revealed a homogeneous composition. Polysaccharides (1043-1032 cm(-1)), aliphatic (1435-1432 cm(-1)) and aromatic (1635-1621 cm(-1)) groups predominate the SOM's composition at different depths. Those organic compounds, known as relatively recalcitrant, enhance the SOM's stability. The GC/MS data suggest that SOM of the studied soils contains a higher abundance (up to 30.05%) of mid chain n-alkanes (soil aliphatic fraction). Those molecules which are stable recalcitrant confirm the FTIR finding. Therefore, regardless the soil type and the vegetation cover, the SOM in arid salt-affected soils seems to be recalcitrant and stable. It consolidates the soil's potential to store the green house gases.