Importance of biochar as a key amendment to convert rice paddy into carbon negative.

Biochar being made up of recalcitrant carbon (C) compounds is considered a negative emission technology (NET) due to its indirect removal of atmospheric carbon dioxide (CO). However, there is no clear report about how biochar remains a NET when organic amendment application in rice paddy results in a huge emission of greenhouse gases (GHG) particularly, methane (CH). To evaluate the net impact of biochar application on the net global warming potential (GWP) in rice paddy, no organic amendment (control), fresh manure, compost, and biochar treatments were selected during the whole investigation period. Compared to compost, biochar application decreased annual CH and NO emissions by 55 and 31 %, respectively. In comparison to the control, biochar application increased CH emission by 163 % but decreased NO emission by 19 %. Soil organic carbon (SOC) stock would annually deplete by 2.2 Mg C ha under control; however, biochar application could increase the SOC stock by 18.1 Mg C ha which was 63 and 33 % higher than fresh and compost treatments, respectively. As a result, the control had a net GWP of 10 Mg CO-eq ha however, this impact was increased with fresh manure and compost application by around 319 and 159 %, respectively. Interestingly, biochar application converted rice paddy into a C sink having a net GWP of -0.104 to -0.191 Mg CO-eq ha. Since there was a comparable difference in grain yield among organic amendments, greenhouse gas intensity (GHGI) which is the net GWP per grain yield was significantly high in compost application of approximately 3.1 Mg CO-eq Mg grain being 127 % higher than control. However, the biochar application had a -0.02 Mg CO-eq Mg grain which was 1.4 Mg CO-eq Mg grain lower than the control. Conclusively, biochar application could be a considerable option in maintaining soil quality and productivity without contributing any GHG emissions and their associated impacts.