This is the first study to investigate the role of biochar in mitigating CH 4 emission from paddy soil under elevated temperature and CO 2 condition. It is imperative to better understand the emission of CH 4 influenced by different temperature and CO 2 concentration, as well as to address and elucidate the mechanistic effects of biochar amendment on CH 4 emission from paddy soil under the predicted climate change.

In this study, variations of temperature and CO 2 concentration had different influences on CH 4 emissions during the rice growing season (Fig. 2). Our results revealed that CH 4 emissions were reduced under elevated temperature or elevated CO 2 alone, where cumulative CH 4 emissions were reduced by 70.9% and 54.4%, respectively, compared with those treatments under ambient temperature and CO 2 . However, elevating temperature and CO 2 simultaneously did not exert any significant effects on the cumulative CH 4 emissions, rather increased them by 8.3%. The different responses of CH 4 emission to environmental factors might be due to the variations in rice plant growth and subsequent effects on CH 4 production. Elevated temperature significantly reduced the total and above-ground biomass of rice plants (Fig. 1). Baker et al.30 also reported the reduced rice yield under elevated temperatures. Since 80–90% of CH 4 released into the atmosphere was rice plant-mediated through the well-developed aerenchyma31,32, inhibited rice growth caused by elevated temperature can partially reduce the emissions of CH 4 through rice plants17. The optimal temperature of most mesophilic methanogens is about 35 °C, with methanogenic activity showing a decreasing trend when above this temperature33,34,35. This is one of the causes that in our study, elevated temperature significantly decreased soil methanogenic activity at rice heading stage (Fig. 3a), and then weakened CH 4 emissions. On the other hand, CO 2 enrichment strikingly promoted the biomass of rice plants (Fig. 1), which could bring more available oxygen to the rhizosphere soil. These impacts led to the decreased CH 4 emissions by depressing methanogenesis and accelerating the soil methanotrophic growth (Figs 3a and 4b). It was well supported by the results of Schrope et al.17 and Inubushi et al.36, who reported that CO 2 enrichment reduced CH 4 production and promoted CH 4 oxidation through the benefit of increased oxygen delivery through rice plants. In addition, rice biomass was similar in the control (CK) and in soil with simultaneously elevated temperature and CO 2 (tcCK) (Fig. 1). One possibility would be that CO 2 enrichment weakened the negative effects of elevated temperature on rice plants growth, providing a certain amount of substrates for CH 4 production. However, the observed results could not come to such an effect. Further research is required before conclusions can arrive. Increases in atmospheric temperature and CO 2 concentration turned out to be driving forces for CH 4 emission from paddy soil (Fig. 2). Thus, CH 4 control from paddy soil needs to be paid more attention in the future due to the predicted rise of both temperature und CO 2 .

Our results confirmed that CH 4 emissions in paddy soil under ambient condition were reduced significantly by the application of biochar. Biochar amendment did significantly reduce the cumulative CH 4 emissions by 97.2% compared to the control (Fig. 2a). The addition of biochar under ambient conditions attenuated the methanogenic activity remarkably at both the tillering and the heading stages, and improved methanotrophic pmoA gene abundance and potential activity at the heading stage (Figs 3a and 4b). These findings are similar to the results of many previous studies, which reported that biochar amendment could make the rhizosphere soil favorable for methanotrophs but unfavorable for methanogens25,26,27. Therefore, it was the stimulated methanotrophic activity and inhibited methanogenic activity caused by biochar application that led to the declined CH 4 emissions in ambient system. These results verify that the application of rice straw biochar not only stimulate rice plant productivity37,38, but also suppress CH 4 emissions from paddy soil.

Interestingly, biochar amendment also significantly decreased CH 4 emission from paddy soil under simultaneously elevated temperature and CO 2 condition. Compared to the corresponding control (tcCK), the cumulative CH 4 emissions in tcBC were reduced by 39.5% (Fig. 2d). Based on the role of biochar in decreasing CH 4 emission under ambient environmental conditions, we hypothesized that biochar addition would have notable influence on CH 4 production and oxidation under the combined condition. As was expected, methanogenic activity decreased and CH 4 oxidation potential increased when biochar was applied in simultaneously elevated temperature and CO 2 system at the heading stage (Fig. 3). Spearman correlations and the correlation circle were calculated to compare the CH 4 emission rates with biochemical and microbial gene data. We observed a significantly positive correlation between CH 4 emission rate and methanogenic activity (rho = 0.500, p < 0.01) and CH 4 oxidation activity (rho = 0.533, p < 0.01) (Fig. 5; Supplementary Table S1). This confirms the important role that methanogenic activity and CH 4 oxidation activity have in controlling CH 4 emission from paddy soil.

Figure 5: The correlation circle of CH 4 emission and biochemical and microbial characteristics during the rice growing season. Dim 1 and Dim 2 represent the ratio of respective index in the whole system. Full size image

It is well-known that variations of biochemical and microbial parameters can cause different CH 4 fluxes by influencing CH 4 production and oxidation processes. Soil CH 4 production is affected by the availability of labile carbon substrates39. Soil dissolved organic carbon (DOC) contributes to a great deal of carbon sources for methanogenic growth40. A decrease of soil DOC content from tcBC compared to tcCK at the heading time could explain the reduced CH 4 emissions observed due to the decreased methanogenic activity (Supplementary Fig. S2a). As is proved, absorption of soil organic carbon onto biochar particles may have reduced substrate availability for CH 4 production40. Liu et al.25 also illustrated that biochar amendment significantly reduced the soil methanogenic activity and CH 4 emissions from paddy soil, mainly benefiting from the lack of substrate availability for methanogens. Meanwhile, we observed a greater concentration of microbial biomass carbon (MBC) in tcBC at the heading time, which suggests a faster succession of microorganisms by consuming easily available soil organic carbon (Supplementary Fig. S2b). This may then have retarded CH 4 production and methanogenic activity by forming a more recalcitrant organic carbon pool41. Moreover, biochar addition under simultaneously elevated temperature and CO 2 condition significantly promoted rice plants growth by improving the total and above-ground biomass (Fig. 1). The stimulated rice plants could bring more oxygen to the aerenchyma tissues of rhizosphere28,42, thus inhibiting methanogenic activity and increasing methanotrophic activities (Fig. 3). Some studies also demonstrated that the high porosity and large surface area of biochar may enhance the adsorption of CH 4 25,28, providing substrates for methanotrophs and thus reducing CH 4 emissions.

Furthermore, a significantly positive correlation of CH 4 oxidation activity and soil water content (rho = 0.542, p < 0.01), and pH value (rho = 0.439, p < 0.05) indicated the important role of soil moisture and pH in affecting soil CH 4 oxidation (Fig. 5; Supplementary Table S1). Soil water content increased considerably in tcBC in comparison to that in tcCK at the heading time (Supplementary Fig. S3). This broadened the optimum range of water content for methanotrophy43,44, and then stimulated CH 4 oxidation activity (Fig. 3b). Studies have noted that the highly porous structure of biochar could increase water holding capacity, and thus increase CH 4 oxidation by restricting soil moisture fluctuations40,45,46. Schnell et al.47 also reported that CH 4 uptake rates would increase with increasing water content. Moreover, methanotrophs are usually sensitive to the fluctuation of soil pH values39. Our results showed that biochar amendment could significantly increase the pH values ranging from 5.55 ± 0.10–5.71 ± 0.01 under simultaneously elevated temperature and CO 2 condition at the heading stage (Supplementary Fig. S4). The increased pH was favorable for methanotrophs (the optimal pH value is 6.0–7.0)48, promoting methanotrophic potential (Fig. 3b).

The CH 4 emission rate displayed significant negative correlations with the abundance of methanotrophic pmoA gene (rho = −0.558, p < 0.05) (Supplementary Table S1). In simultaneously elevated temperature and CO 2 system, biochar addition improved the copy numbers of methanotrophic pmoA gene significantly, stimulating methanotrophic growth at the heading stage (Fig. 4b). As a result of the apparently promoted methanotrophic pmoA gene abundance, CH 4 oxidation activity was enhanced greatly (Fig. 3b). The observed increase in the methanotrophic pmoA gene abundance might benefit from the abundant CH 4 as the only C substrate for methanotrophs26, as well as from the oxygen condition40 and living habitat49 supplied by biochar addition. Our results were similar to Feng et al.26, who also showed that biochar addition could significantly promote methanotrophic growth with the increased abundances of pmoA gene in paddy soil, explaining the reduced CH 4 emissions. This study demonstrated that biochar incorporation resulted in the significant increases in rice biomass, pH, moisture and methanotrophic pmoA gene abundance, and decrease in labile organic carbon. These variations would inhibit CH 4 production and promote CH 4 oxidation, and thereby lower CH 4 emission under the combined condition.

The community structure and composition of methanogens and methanotrophs could exert great effects on CH 4 production and oxidation33,39. Potential activity and the copy numbers of key genes observed in this study could not represent the structure and metabolism difference of methanogens and methanotrophs communities, during the heading time, although an evidently higher methanogenic activity and less stimulatory methanotrophic growth in tcBC statistically explained parts of the difference of CH 4 emission under ambient and the combined conditions (Figs 3a and 4b). Thus, further research to assess the environmental functions and structures of methanogens and methanotrophs responsible in different systems is highly desirable. These findings suggested that although the effect of biochar addition on CH 4 mitigation was weakened, it could also assist in making paddy soil a great CH 4 sink under the combined condition (Fig. 2). It will provide valuable rice ecosystem services, which become increasingly important for CH 4 control from paddy soil in the projected warming climate.