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The aim of the present experiment was to determine the degradation level of humic acids (HA) extraced from arable soil by three selected strains. A short-term experiment was carried out in 2014 at State Key Laboratory of Crop Stress Biology for Arid Areas, Northwest A&F University. HA was extracted from arable soil and was used as the sole carbon (C) and nitrogen (N) source in solid culture medium. Three soil HA-degrading strains Bacillus licheniformis (strain 1), Microbacterium resistens (strain 2) and Stenotrophomonas maltophilia (strain 3), were chosen to further investigate their individual and combined effects on the C degradation rate and structural properties of humic acids in the medium. The experiment included seven treatments: T1, T2, T3, T1+T2, T1+T3, T2+T3 and T1+T2+T3.

Results showed that the total colony-forming units (CFU) of S. maltophilia (strain 3) showed higher amount in the treatments it was associated with. Carbon respiration (CR) and microbial biomass carbon (MBC) in various treatments varied between 4.04 (T2) to 12.13 (T2+T3) mg dish(-1) and 1.32 (T2) to 4.96 (T3) mg dish(-1), in terms of the C degradation rates in humic acids, the sum of CR and MBC, followed the trend T2+T3 (52.05%) > T1+T3 (49.29%) > T1 (44.25%) > T3 (40.61%) > T1+T2 (38.39%) > T1+T2+T3 (37.09%) > T2 (18.91%). Infrared spectroscopy revealed that medium of all treatments resulted in higher levels of carbohydrate, aliphatic compounds, alcohols and nitrogenous substances after degradation process. T1 exhibited the lowest aromatic C content compared with the content before degradation, while that in T1+T2+T3 was the highest. Thus, B. licheniformis (strain 1) might have a relatively strong degradation capacity for aromatic C.