Biochar improves methane oxidation activity in rice paddy soil

Paddy fields are regarded as an important anthropogenic source of atmospheric CH4 and play a significant role in global warming. Biochar refers to the highly aromatic substance remaining after thermal decomposition of biomass under complete or partial exclusion of oxygen for the purpose of creating a soil amendment. Its application is widely accepted to be a promising method to decrease CH4 emission from paddy soil. The balance between CH4 production and consumption ultimately determines whether a paddy soil is a net source or a sink of atmospheric CH4. However, there are few studies concerning the effects of biochar amendments on methanogenic and methane oxidation activities in paddy soils. Meanwhile, the feedstock used for biochar production has a substantial impact on the physiochemical characteristics of biochar. These characteristics are then related to the actual environmental function in soil, such as response to methanogenic and methane oxidation activities. In this study, a one-year field experiment was conducted to gain insight into the potential effects of bamboo biochar (BB) and rice straw biochar (SB) amendments at the rate of 22.5 t/hm2 on the methanogenic and methane oxidation activities in rice rhizosphere soil. The SB had more hydrophilic groups such as carboxyl and hydroxyl, higher pH and electrical conductivity (EC) values and lower bulk density than the BB did. Soil water content, pH, and EC values in the SB treatments were greater than those in the BB treatments. Generally, urea application did not have notable impact on soil water content, pH and EC values. Compared with the control treatment, methanogenic activitiy in the rhizosphere soil at the rice seedling stage was significantly increased with the SB amendment. There was no significant difference in the methanogenic activity between the control and the BB treatments during the whole period of rice growth. Differences in the labile components and ash contents between the two biochars may have resulted in the divergence of methanogenic microorganisms in the rice rhizosphere soil, and consequently the difference in methanogenic activity between the BB and SB treatments. The methane oxidation activity in the rhizosphere soil increased with the addition of BB and SB during the rice growth period. Nevertheless, statistical analysis indicated that significant differences were only observed in the SB treatment at the seedling and maturing stages of rice development. In comparison with the BB treatment, SB application was more effective in increase of soil pH and EC values, soil K and P contents, and soil aeration, which may have contributed to the significant improvement of methane oxidation activity in rhizosphere soil with SB application. The general trends of biochar impact on methanogenic and methane oxidation activities were not observably changed with urea application. In conclusion, SB could be used to reduce paddy soil CH4 emission through increasing soil methane oxidation activity.