Long term comparison of GHG emissions and crop yields in response to direct straw or biochar incorporation in rice-wheat rotation systems: A 10-year field observation

Returning straw directly to the field soil is an important strategy that is widely advocated for the disposal of cereal straw. Straw carbonization to form biochar is an innovative approach for treating straw, and its incorporation into the soil is beneficial for improving soil quality. To assess the long-term impacts of these two technologies on greenhouse gas (GHG) emissions and crop yields, a decade-long field study, comparing three treatments, namely direct incorporation of straw from the crop into the field (SCF), incorporation of biochar from straw into the field (BCF), both combined with chemical fertilizer, and the incorporation of chemical fertilizer alone into the field (CF), was conducted on a rice-wheat rotation system in eastern China. Results showed that SCF and BCF achieved the same rice yield as CF treatment alone, while BCF caused a slight decrease in wheat yield. However, there was no significant difference in wheat yield between SCF and BCF, with no significant 'season x treatment' interactions observed. Over the ten-year observation period, rice yields were found to be greatly influenced by air temperature and precipitation, whereas wheat yields were more dependent on precipitation. In terms of GHG emissions, SCF increased annual methane (CH4) emissions by 144 % but reduced annual nitrous oxide (N2O) emissions by 40 %, resulting in an 80 % increase in annual global warming potential (GWP) compared with CF. On the other hand, BCF reduced CH4 and N2O emissions by 58 % and 25 %, respectively, relative to SCF, in the rice season. Despite slightly higher N2O emissions in the wheat season, BCF reduced the annual GWP by 51 %, relative to SCF. Additionally, the relative CH4 emissions of SCF/CF and BCF/ CF decreased over time, with BCF showing lower CH4 emissions than CF after five years. Overall, the findings suggest that straw carbonization into biochar before soil incorporation could be a promising approach to mitigating GHG emissions and maintaining crop yields compared with incorporating straw directly into the soil in the rice-based rice-wheat rotation systems.