Elevation of NO-N from biochar amendment facilitates mitigating paddy CH emission stably over seven years.

Biochar application into paddy is an improved strategy for addressing methane (CH) stimulation of straw biomass incorporation. Whereas, the differentiative patterns and mechanisms on CH emission of straw biomass and biochar after long years still need to be disentangled. Considering economic feasibility, a seven-year of field experiment was conducted to explore the long-term CH mitigation effect of annual low-rate biochar incorporation (RSC, 2.8 t ha), with annual rice straw incorporation (RS, 8 t ha) and control (CK, with no biochar or rice straw amendment incorporation) as a comparation. Results showed that RSC mitigated CH emission while RS stimulated CH significantly (p < 0.05) and stably over 7 experimental years compared with CK. RSC mitigated 14.8-46.7% of CH emission compared with CK. In comparison to RSC, RS increased 111-950.5% of CH emission during 7 field experimental years. On the 7th field experimental year, pH was significantly increased both in RS and RSC treatment (p < 0.05). RSC significantly (p < 0.05) increased soil nitrate (NO-N) compared with RS while RS significantly (p < 0.05) increased dissolved carbon (DOC) compared to RSC. Soil NO-N inhibition on methanogens and promotion on methanotrophs activities were verified by laboratory experiment, while soil pH and DOC mainly promoted methanogens abundance. Significantly (p < 0.05) increased DOC and soil pH enhanced methanogens growth and stimulated CH emission in RS treatment. Higher soil NO-N content in RSC than CK and RS contributed to CH mitigation. Soil NO-N and DOC were identified as the key factors differentiating CH emission patterns of RS and RSC in 2019. Collectively, soil NO-N impacts on CH flux provide new ideas for prolonged effect of biochar amendment on CH mitigation after years.