Carbon stability and soil N2O emissions: Pyrolyzed or unpyrolyzed manure?

Abstract

Pyrolysis is a useful alternative to current waste management practices. Manure biochars can be used as C-rich soil amendments, reducing the risk of greenhouse gas (GHG) emissions derived from unpyrolyzed manure decomposition. In this study, two manure biochars and their feedstock were used as soil treatments to evaluate the impact of pyrolysis on CO2 and N2O emissions, C stability, and global warming potential (GWP) for one year under field conditions. The experiment included five treatments: unamended soil (control), pig manure (PM), pig manure biochar (PMB), dairy manure (DM), and dairy manure biochar (DMB). The amendments were applied at a 1% w/w, adding approximately 0.5 kg C m−2. All amendments increased soil CO2 emissions; however, C mineralization was lower in pyrolyzed manures than in their corresponding feedstocks. CO2–C emissions were 8.35%, and 63% of the C from biochar and manure treatments, respectively. Biochars reduced soil N2O emissions, producing a negative N2O emission factor (N2O Ef), whereas manure treatments induced N2O fluxes for 105 days, with an N2O Ef of 0.27% after one year. Soil C stock variations were 16.4 and 6.58 t CO2-eq avoided ha−1 in treatments of biochars and manures, respectively. The mitigation potential of unpyrolyzed manure was affected by the high CO2 and N2O fluxes, whereas the biochar treatments reduced the GWP due to the changes in soil C stock with a low impact on soil emissions. Manure was more stable after pyrolysis, indicating that manure biochars are more favorable as soil amendments for environmental purposes. This could reduce the C footprint of pig and dairy farms, promoting the enhancement of soil C stocks.