Both reductions of greenhouse gas emissions and carbon sequestration have the potential to reduce global climate warming and avoid dangerous climate change. We assessed the sequestration potential as well as possible risks and benefits of carbon amendments (16 ± 4% of soil organic C) from Miscanthus × giganteus in different carbonization stages of a temperate grassland soil together with pig slurry: (1) untreated dried biomass (feedstock), (2) hydrothermally carbonized biomass (hydrochar) and (3) pyrolyzed biomass (biochar) in comparison to a control (only pig slurry application).
The field study was complemented by a laboratory incubation study, followed by a growth experiment with Lolium perenne. In the field, greenhouse gas emissions (CO2, N2O, and CH4) were monitored weekly over 1.5 years and over three months in the lab. Initial nitrogen losses via ammonia emissions after substrate–slurry application were assessed in an additional greenhouse study.
We found that biochar reduced soil and ecosystem respiration in incubation and in the field, respectively. Additionally, biochar improved methane oxidation, though restricted by emissions outbursts due to slurry amendment. It also reduced N2O emissions significantly in the lab study but not in the field. Hydrochar and feedstock proved to be easily degradable in incubation, but had no effect on ecosystem respiration in the field. Feedstock amendment significantly increased N2O emissions in incubation and one year after application likewise in the field. In a growth experiment subsequent to the incubation, only biochar amendment increased L. perenne biomass (+29%) significantly, likely due to N retention. In the field, biochar caused a significant shift in the plant species composition from grasses to forbs, whereas hydrochar significantly reduced yields within two growth periods (2011 and 2012). Ammonia emissions were significantly higher with feedstock and biochar compared to the control or acidic hydrochar. The overall results indicate that biochar is better suited for C sequestration and GHG mitigation in grasslands than hydrochar or the uncarbonized feedstock. However, NH3 emission reductions may only occur when the biochar is neutral or slightly acidic.