This phenomenon could be explained by the fact that the
methanogenic bacteria became acclimatized after the long lag
phase and developed a tolerance and ability to prevail over a high
ammonia level of ca. 8 to 14 g-N kg− 1 CM. Acetate was the most
abundant species among VFA (data not shown), which demonstrated
the effective adaptation of the microbial population to high ammonia
levels (10). Acclimatization was not observed at both 55 and 65 °C.
This may be attributed to the higher production of ammonia than
that at 37 °C (data not shown). Also, at these high temperatures, the
fraction of unionized ammonia, which is more toxic to the methanogens, is higher (12, 22).