Spectroscopic and chromatographic investigation of soil organic matter composition for different agrosystems from arid saline soils from Southeastern Tunisia

In Tunisia, several studies estimated the soil organic matter (SOM) stocks. However, little is known about SOM composition and dynamics especially in arid regions. Globally, arid soils of Tunisia are saline. This study aims to investigate the composition and stability of SOM in salt-affected soils under different vegetation covers in Southeastern Tunisia. Two depths are sampled in ten profiles from different localities. SOM composition was assessed by FTIR spectroscopy and gas chromatography-mass spectrometry (GC/MS). The results showed that, despite the differences of studied soils, SOM contents are low (< 20 g/kg) and decrease with soil depth. The lowest content of SOM is attributed to native soils which suggest that SOM in arid saline soils is mainly controlled by available litter input. FTIR spectroscopy revealed that SOM is predominated by polysaccharides, aliphatic C, and aromatic C. Polysaccharides and aliphatic C increased with soil depth for agricultural and native soils. However, aromatic C increased with soil depth for agricultural soils, while it decreased for native soils. Agricultural soils are enriched in aliphatic C and polysaccharides compared with native soils that had higher content of aromatic C. Thus, the chemical composition of SOM in arid saline soils is strongly influenced by litter input quality and decomposition rate of SOM. GC/MS analyses which showed the predominance of mid-chain n-alkanes confirm those finding. Because of their relative recalcitrance and/or their protection by association with soil minerals, polysaccharides, mid-chain n-alkanes, aliphatic C, and aromatic C may have crucial effect on SOM stabilization in arid saline soils.