Greenhouse gas emissions vary in response to different biochar amendments: an assessment based on two consecutive rice growth cycles

The efficiency of biochar to mitigate greenhouse gas (GHG) emission from rice paddy soils is not consistent. Furthermore, which factor dominates this mitigation efficiency is not clear. In the present 2-year greenhouse experiment, the effects of biochars derived from two feedstocks (wheat straw and saw dust) and two pyrolysis temperatures (500 °C and 700 °C), and applied at two different rates (0.5 wt% and 3 wt%) on methane (CH₄) and nitrous oxide (N₂O) emissions, and the total global warming potential (GWP_t), and GHG intensity (GHGI) were measured. The results showed that biochar applications did not alter GHG emission flux patterns in either rice cycle. In 2015, the N₂O emissions were 24.6–71.2% lower under six biochar treatments than under the urea control treatment. Moreover, total CH₄ emissions were mitigated by 13.3–92.6% and 27.7–53.5% under six and five biochar treatments in 2015 and 2016, respectively. Overall, lower GWP_t and GHGI were observed under most of the biochar treatments compared with the urea control treatment in both rice cycles. The multivariate analysis of variance (MANOVA) results of the data from both years suggested that the biochar effects on reducing GHG emissions changed with either individual factors or their interactive effects. The responses of the GWP_t and GHGI varied mainly with biochar application rate and pyrolysis temperature (P < 0.005); compared with that derived from a relatively low pyrolysis temperature and applied at a relatively low rate, biochar derived from a relatively high pyrolysis temperature and applied at a relatively high rate exerted relatively higher GWP_t and GHGI mitigation efficiencies. The influence of the feedstock source was not as prominent as the application rate and pyrolysis temperature, which will expand the scope of biochar applications.