## 4 Discussion
### Toxic Mechanism, Actuator, and Relevance to the Environment  
Apparently, the mechanism of toxicity is the digestion of BC particles, whereby gastric juices liberate toxins, which are present in the BC. This mechanism is supported by the images of dead animals that show digestive tracts with abundant (in some cases) BC traces. As no feed was applied, the only particles contained in the elutriate and found in the digestive tract can arise from the BC. Another indicator, which subscribes the digestion hypothesis, is that ciliates did not absorb BC particles. In fact, ciliates did not experience any harm from the BCs. Except for ciliates, the results show that rotifers and cladocerans, which habitat in intercellular soil water, are capable of digesting micro-particles of BC and thus release bounded and mobilize immobilized hazardous substances contained therein. Rotifers have an activity of phosphatases, \(\delta\)-N-acetylhexosaminidaseases, and lipases [59]. In the case of _D. magna_, many digestive enzymes have been found: galactosidases, esterases, trypsin, and cellulases [60]. Based on the ingestion mechanism triggered by rotifers and cladocerans, potentially toxins could enter the food chain, accumulate biologically and, at higher concentrations, potentially provoke carcinogenic, mutagenic, or reprotoxic effects to higher organisms. Naturally, PCDD/Fs and PAHs are generated during pyrogenic oxidation of hydrocarbon compounds and are ubiquitous and persistent pollutants in the environment [61; 62]. Volcanic eruptions, as well as forest and vegetation fires, will release PCDD/Fs and PAHs on a natural basis [63], which will most likely degrade in the soil. But the man-made application of BC and associated toxins, in quantitative terms, is much higher and more frequent. Hence, naturally driven degradation process may take a long time until complete dissolution and will restrict, significantly, specific soil biota, as demonstrated within this study. High concentrations of Zn and Cu (Table 3) could be an indication for increased zinc accumulation by the feedstock, as other sources of contamination can probably be excluded. As far as the authors know, there are rather no documentations known that indicate an increased zinc uptake by the used wood species. As mesquite is known for its tendency to accumulate heavy metals above average, in contrast to other trees, it was expected to find higher contents in BC 2 and BC 3, nevertheless this assumption was not confirmed by the present study. Among the PAH-16 (Table 4) used as benchmarks by many environmental authorities in many countries, benzo (a) pyrene is considered to be the most crucial. Especially fodder producers require a maximum content of benzo (a) pyrene below 0.1 mg kg\({}^{-1}\), no matter if the total PAH-16 content is below the maximum threshold of (\(<\)4 \(\pm\) 2 mg kg\({}^{-1}\)). Concentrations of benzo (a) pyrene in all samples were below 0.1 mg kg\({}^{-1}\). Even though BC 1, 2, and 3 (4.8 mg kg\({}^{-1}\), 5.3 mg kg\({}^{-1}\), 0.7 mg kg\({}^{-1}\)respectively) qualify for EBC premium quality, BC 4 (8.3 mg kg\({}^{-1}\)) was only permissible for basic quality (\(<\)12 \(\pm\) 4 mg kg\({}^{-1}\)). The highest content in all samples was shown by naphthalene in a range from 2.5-3.4 mg kg\({}^{-1}\). This is probably explicable due to improved naphthalene emergence at high temperatures in pyrosynthesis above 700 \({}^{\circ}\)C. BC 4 additionally had an outlier in phenanthren content with 1.6 mg kg\({}^{-1}\). Basically, PAHs in BC are very hydrophobic and hardly bio-accessible [7], but could potentially be liberated by the enzymatic and mechanistic processes. PCDD/Fs and PCBs belong to the POP substances. They are (P) persistent (not biodegradable), (O) organic, and (P) pollutant. Furthermore, they belong to the CMR substances. Their human toxicological effects may; therefore, be (C) carcinogenic, (M) mutagenic, and (R) reprotoxic.