Biochar increases temperature sensitivity of soil respiration and N2O flux

Agriculture contributes to 10% of US greenhouse gas (GHG) emissions. Biochar, organic matter that has undergone pyrolysis, may both sequester soil C and reduce greenhouse gas emissions from agricultural soils. To examine the impact of biochar in Hawaii soils, two soils with contrasting fertility (Oxisol and Mollisol) under two different cropping  systems (zero-tillage napiergrass and conventional sweet corn) with and without biochar were collected after one year, incubated over an 8-point temperature gradient and analyzed for GHG emissions and shifts in microbial community abundance. Unexpectedly, biochar nearly doubled the temperature sensitivity of the soil respiration (p=0.017). For soil N2O flux, most soils were temperature insensitive, with the exception of the napiergrass with biochar in the Mollisol soil. Due to the high global warming potential of N2O, to further explore the mechanism behind this response the total microbial community abundance (16S rRNA) in conjunction with the abundance of the gene encoding nitrous oxide reductase (nosZ) was analyzed at two temperatures for the napiergrass with and without biochar. There was no difference between total microbial abundance and nosZ gene abundance with biochar amendment and both  total abundance and nosZ gene abundance decreased with increasing temperature (p=0.0088). However, the ratio of denitrifying bacteria did change, nearly doubling in the higher temperature (31°C) compared to the lower temperature (23°C) (p=0.0144). To simulate the addition of organic inputs or root exudates, the soils were then provided with a labile C source to examine how to the temperature-adapted communities responded. Unexpectedly, no biochar or temperature differences were found in nosZ gene abundances post-labile amendment. While the mechanisms are not well understood, these incubations indicate that biochar may exacerbate climate change by increasing both the temperature sensitivity of soil respiration and of soil N2O flux in these systems.