## 4 Discussion
### The Complexity of UDMH Biodegradation in Soil  
From Figure 6, one can see that the extra bacteria obviously improved the biodegradation ability of autochthonous microorganisms. Usually, a high concentration of UDMH, bonded with the soil solids in the form including the UDMH adsorbed by the mineral phase and chemically adsorbed by the soil organic matter, could exist for approximately 30 years [27; 48]. Therefore, biodegradation of UDMH in the soil was a complicated process still influenced by organic matter, autochthonous microorganisms, and metal ions [49]. The prerequisite release of a contaminant from the adsorbed phase to the aqueous phase for its degradation by microorganisms determined the susceptibility to microbial degradation, \begin{table}
\begin{tabular}{c c c c c} \hline
**No.** & **Compound Name** & **CAS No.** & **Formula** &
\begin{tabular}{c} **Characteristic Ions, \(m/z\)** \\ **(Relative Abundance, \%)** \\ \end{tabular} \\ \hline
1 & Dimethylamine & 124-40-3 & C\({}_{2}\)NH\({}_{7}\) & 44 (100); 45 (56); 28 (26); 46 (3) \\ \hline
2 & Formaldehyde dimethylhydrazone & 2035-89-4 & C\({}_{3}\)N\({}_{2}\)H\({}_{8}\) & 72 (100); 71 (71); 42 (61); 57 (15) \\ \hline
3 & Tetramethylhydrazine & 6415-12-9 & C\({}_{4}\)N\({}_{2}\)H\({}_{12}\) & 88 (100); 73 (89); 44 (73); 42 (63) \\ \hline
4 & Acetaldehyde dimethylhydrazine & 7422-90-4 & C\({}_{4}\)N\({}_{2}\)H\({}_{10}\) & 86 (100); 44 (51); 42 (48); 85 (34) \\ \hline
5 & \(N\)-Nitrosodimethylamine & 62-75-9 & C\({}_{2}\)N\({}_{2}\)H\({}_{6}\)O & 74 (100); 42 (37); 43 (15); 41 (4) \\ \hline
6 & 1,1,4,4-Tetramethyltetrazene & 6130-87-6 & C\({}_{4}\)N\({}_{4}\)H\({}_{12}\) & 116 (100); 43 (53); 72 (42); 42 (32) \\ \hline
7 & 1-methyl-1H-1,2,4-triazole & 6086-21-1 & C\({}_{3}\)N\({}_{3}\)H\({}_{5}\) & 83 (100); 56 (29); 84 (5); 40 (4) \\ \hline \end{tabular} Compounds with the NIST MS library match \(>\)85% both in soil and solution.  
\end{table} 
Table 2: The list of transformation compounds of unsymmetrical dimethylhydrazine (UDMH) detected in aqueous mineral salt medium (MSM) and soil. thereby influencing the effectiveness of the biodegradation process [50]. The fate of UDMH was largely influenced by the competing processes of degradation and sorption [22]. The extra bacterial culture should be added as soon as possible to biodegrade UDMH against the chemical absorption. (a) Organic matter: In Figure 6, the final degrading percentage of \(n\)OM sample was only 66.5%, and the final degrading percentage of the SS sample was 81%. It could be inferred that the difference between the two samples was caused by chemical absorption of soil organic matter. The soil samples collected from 5-10 cm below the ground surface contain abundant dead plant matter, which forms the humic substances. The organic matter content in various soils is very different, so the natural degradation ability of UDMH in different areas is variable. (b) Indigenous microorganisms: The samples in \(n\)SS had a higher final degrading percentage than those in SS, and the \(n\)SS sample could even have a final degrading percentage of 92%, which was almost the same as the P4 + SS sample. Those proved that autochthonous microorganisms could have some biodegradation effect on UDMH in the soil. Microbiological studies also showed there were certain bacteria that could biodegradation hydrazine fuels in natural environment [49]. The relationship between exogenous bacteria (P4, M12) and indigenous microorganisms was competitive. In the first two days, the degrading percentage of UDMH for the P4 + SS sample was higher than the P4 + \(n\)SS sample (Figure 3). The biodegradation capacity of P4 was influenced in \(n\)SS soil by the autochthonous microorganisms with the competition of the nutrient. When P4 gained the competitive advantage in \(n\)SS soil after two days, the degrading percentage of the P4 + \(n\)SS + broth sample began to exceed that of the P4 + SS sample.