Bioremediation has been applied to accelerate the biodegradation of contaminants and rehabilitate the contaminated environment to a condition similar to that found before contamination, either in terms of biodiversity or ecosystem functions [1] - [6] . Some approaches can be adopted to ensure proper biological activity during the bioremediation process, including biostimulation (by eliminating environmental limitations, including low availability of mineral nutrients, water, and electron acceptors, extremes of pH or temperature, etc); bioaugmentation (inoculation with selected microorganisms efficient in biodegradation of target contaminants), and application of surfactants or biosurfactants-producing microorganisms to increase the bioavailability of contaminants [2] - [12] .

The use of bioaugmentation generates much contradiction between the researches. Some studies have demonstrated that biodegradation is accelerated by inoculation [5] [8] [13] [14] , while others show neutral or even negative effects [15] [16] [17] . However, it must be emphasized, that the efficacy of the abovementioned bioremediation strategies depends on particularities of the target environment and, in several cases, they can act as complementary techniques [3] [18] . The microbial species and isolates may have positive or negative effect on biodegradation, depending on the combination of microorganisms used [14] . The success of bioaugmentation depends on the use of suitable microorganisms and their survival and activity in the target habitat.

In this study, the enrichment of hydrocarbonoclastic microbial populations was conducted under uncontrolled environmental variables such as temperature and humidity, in order to favor the selection of populations with higher adaptability to environmental variables that affect the survival and biodegradation activity. Moreover, we performed enrichment of a mixed microbial population without isolation in culture media. It was considered, for the adoption of this strategy, that uncultivable microorganisms can also play a role in hydrocarbon biodegradation.

In recent studies, the use of organic waste in biostimulation and bioaugmentation of soils contaminated with hydrocarbons hasproduced satisfactory results [18] [19] [20] . Therefore, in the present work, we proposed the use of municipal solid waste compost (MSWC) for the development of microbial inoculants capable of degrading gasoline and diesel in the soil. This residue was selected owing to its desirable characteristics to microbial inoculants, such as high microbial diversity and high capacity to adsorb hydrocarbons, which can prevent volatilization of n-alkanes of low molecular weight; it is a biologically stable material consisting of humic substances and a solid matrix, with a low C:N ratio [21] [22] . The above mentioned aspects make MSWC a good conditioner of soils [23] , improving its physical, chemical, and biological properties. Thus, the material can function as a suitable substrate for the development of inoculants for use in contaminated soils.

The substrate used for the development of inoculants was MSWC obtained from the composting facility of the municipality of Coimbra, MG, Brazil. The compost was sieved through a 5-mm sieve and analyzed for its physical and chemical properties (Table 1).

The concentration of gasoline and diesel suitable for enrichment of hydrocarbonoclastic microbial populations was determined by applying these fuels to MSWC in doses of 7500, 15,000, and 37,500 mg∙Kg⁻¹ dry weight for gasoline and 8500, 17,000, and 42,650 mg∙Kg⁻¹ dry weight for diesel. The material was mixed with a spatula and analyzed for CO₂ evolution in a respirometer equipped with an infrared detector with intermittent air flow (Sable System, NE, USA). The MSWC received new applications of fuels every 5 days, at the same initial concentrations. The influence of moisture on the biodegradation of hydrocarbons was evaluated under two humidity conditions: no moisture adjustment and moisture fixed at 60% of the water-holding capacity (WHC). Subsequent moisture adjustments of the least treatment were made when the moisture content reached 40% WHC. The C: N: P ratio was adjusted with ammonium sulfate to 100:10:2. The microcosms were incubated at 30˚C for 14 days without stirring. Compost not receiving gasoline or diesel was used as control. The experimental design was a completely randomized 4 × 2 factorial for each source of hydrocarbon (four doses of gasoline or diesel and two moisture level), with three replicates for each treatment.