Impact of pyrolysis and hydrothermal biochar on gas-emitting activity of soil microorganisms and bacterial and archaeal community composition

• Pyrochar did not change N2O and CO2 emissions, but hydrochar amendment induced high CO2 emissions.
• Pyrochar had no effect on overall bacterial and archaeal abundance.
• Hydrochar reduced the overall archaeal abundance.
• Pyrochar had nor or little effect on bacterial and archaeal community composition.
• Hydrochar considerably changed the bacterial and archaeal community composition.

Most biochar studies are focusing on the usage of char produced by pyrolysis (pyrochar). However, only dry biomass can be subjected to pyrolysis. It is beneficial to produce biochar by hydrothermal carbonization (hydrochar) from wet biomass to avoid energy use for drying. The objective of this study was to compare the effects of pyrochar and hydrochar on greenhouse gas-emitting activity, abundance and composition of the soil bacterial and archaeal community. Three different moisture contents (40%, 60% and 80% of water holding capacity) and two N fertilization steps (with and without N addition) were investigated. The microcosm study was conducted in 120 mL glass bottles with septum caps for periodic headspace gas analysis. N2O and CO2 emissions from pyrochar were in the same range as the char-free control. Hydrochar, however, caused high N2O emissions in the fertilized high moisture treatment and significantly higher CO2 emissions in all treatments compared to the control. Pyrochar increased CH4 emission in the unfertilized treatments, whereas hydrochar had no effect except a small reduction in the fertilized and highest moisture treatment. Enzyme activity in all pyrochar microcosms was in the same range as the char-free control, but lower in unfertilized hydrochar microcosms. Pyrochar soil amendment did not change bacterial and archaeal abundance. Hydrochar decreased archaeal abundance in the majority of the treatments. T-RFLP analysis revealed that pyrochar, hydrochar and control each developed a distinct bacterial community. Pyrochar had no effect on archaeal communities, whereas hydrochar induced the formation of significantly different communities compared to the control. Furthermore, hydrochar reduced the abundance of Acidobacteria and Firmicutes, while it remarkably increased the abundance of Bacteroidetes and Proteobacteria. The results suggest that the addition of hydrochar induces considerably stronger effects on soil microbial communities than the addition pyrochar.