3.2 Rhizosphere secretions
The rhizosphere secretions detected in this study could be divided roughly into 15 types: hydrocarbons, acids/esters, alcohols, amines, ketones, nitriles, ammonium salts, aromatic hydrocarbons, hydrazines, amides, sulfonyls, oxides, ethers, aldehydes, and carbohydrates; however, the last seven types were detected only in the low salinity rhizosphere soils (L-R). Acids/esters were found in all soil samples and accounted for a relatively high proportion (43-83%), except in the L-NR (30%) and H-R soils (16%). The other rhizosphere secretions widely present in all soil samples were hydrocarbons, but in a modest proportion, ranging from 1.07% to 9.6% (Figure 2A).
Alcohols were not detected in H-NR, and their relative proportion in the bulk soil was between 4.9% and 7.6%, higher than in the rhizosphere soil (0.24% to 3.2%). Amines were not detected in L-R, the two sample types with the highest relative proportions were L-NR (21%) and H-NR (13%), whereas CK soil had the lowest proportion (0.39%). The relative proportion of ketones in CK was 22%, which was significantly higher than in M-NR, L-NR and H-R (2.9%, 0.24% and 3.4%, respectively). Nitriles were detected only in highly saline soil, with relative proportions of 0.83% in H-NR and 0.19% in H-R (Figure 2A).
The six abundantly detected rhizosphere secretions were acids/esters, ammonium salts, amines, ketones, hydrocarbons, and alcohols (Figure 2A-D). For acids/esters in the bulk soil, the relative proportion increased with an increase in soil salinity, whereas it was mostly opposite in the rhizosphere soil. The relative proportion of ketones in CK soil (22%) was significantly higher than in the other soil types, while amines was significantly higher in L-NR than in the other soil sample types. The relative proportion of alcohols in CK (7.6%) was significantly higher than in the other soil sample types, and it was higher in the bulk soil than the rhizosphere soil (at medium and low salinity).