Effect of Agro-industrial Solid Waste and Bacillus and Pseudomonas Bacteria on Ecophysiological Indices in a Loam Soil

Solid waste (SW) of olive oil industries contain valuable organic and inorganic materials can be useful soil quality and its microbial activity. However, these wastes contain fatty acids and recalcitrant materials which require special microorganisms for complete decomposition in soil environment. The aim of this study was to investigate soil biological and ecophysiological indicators (as soil quality indices) after addition of oil refinery plant solid waste and inoculation of Bacillus and Pseudomonas bacteria to soil.

The experiment was conducted in a completely randomized design with factorial arrangement and three replications. Factors included SW (0, 2 and 4%), inoculated bacteria (no bacteria, native Bacillus sp. native Pseudomonas sp., Bacillus persicus and Pseudomonas fluorescens), and sampling time (0, 1, 2, 3, 4, 5, and 6 months). Soil without SW and uninoculated with bacteria was regarded as control. Soil-SW mixtures incubated at lab temperature (~25 ºC) and 0.7 FC moisture content conditions for 6 months. Sampling was carried out before incubation start and after each month of incubation and organic carbon (OC), basal respiration (BR), substrate induced respiration (SIR), and microbial bimass carbon (MBC) were measured in the samples and ecophysiological parameters including metabolic quotient (qCO2), microbial carbon ratio (Cmic), and available carbon index (CAI), were calculated. Data analysis and mean comparisons were done by Tukey method (p < 0.05) using SAS software package.

The results of variance analysis showed that the effect of all factors and their interactions were significant (p < 0.05) on the studied parameters exept qCO2. The highest values of BR and SIR were obtained in the 4% SW treatment inoculated with Pseudomonads. Adding SW to the soil and inoculating it with Pseudomonads had an important role in the increase of these two parameters. However, despite the positive role of SW, bacterial inoculation did not increase MBC. Where the highest amount of MBC was observed in the 4% SW treatment uninoculated with bacteria. The lowest amount of all three parameters was observed in the 0% SW treatment inoculated with Bacillus persicus. SW addition to soil increased qCO2, and its higher values were observed in the mixtures inoculated with native Pseudomonas sp. SW application and bacteria inoculation to soil decreased Cmic. The highest and lowest Cmic values were observed in soil without SW inoculated with Pseudomonas fluorescens and soil contained 4% SW inoculated with Bacillus persicus, respectively. The mean Cmic values in 0, 2, and 4% SW-mixtures were 10.32, 5.06, and 4.76 mg Cmic gCorg-1, respectively. The trend of Cmic changes over time showed that this parameter had ascending trend in uninoculated soil and soil inoculated with Bacillus (native and persicus) bacteria. SW addition to soil and soil inoculation with Pseudomonads increased CAI. The highest and lowest CAI values were observed in 4% SW-mixtures inoculated with Pseudomonas fluorescens and soil inoculated with native Bacillus sp., respectively. The trend of CAI changes was greatly fluctuated over time and was only positive in Bacillus inoculated soils.

Overall, the addition of oil refinery plants solid waste to the soil and its inoculation with pseudomonads, although reduced microbial carbon ratio, increased microbial basal respiration and available carbon index. Therefore, it seems that if this waste is added to the soil, its inoculation with pseudomonas bacteria can accelerate the decomposition of the waste in the short term and release its nutrients.