| Rice is one of the most important food crops and its production is of special importance in food security. However, soil degradation has exerted a threat to agriculture production as a result of the integrated effects of natural factors and human activities. Traditional straw returning to the field can increase soil fertility and improve soil quality, but also results in a substantial increase in CH4 emission. However, up to date, there are only some short-term experiments with biochar application at high-rate by one-off returning focusing on the mechanism of biochar addition on soil amendment in middle- and low-producing areas and soil greenhouse gases mitigation. There are no researches, based on the low-rate of straw carbonization and returning, about the effects of rice straw-derived biochar addition on soil amendment and the control of soil greenhouse gases emissions. This study mainly compared the long-term effects of bamboo and rice straw biochar amendment at high and low rate on soil property, rice growth and CH4 emission in paddy fields. Further, physicochemical analysis, Q-PCR and 454 sequencing techniques were applied to reveal the mechanism of biochar’s role in reducing greenhouse gas emission. The primary results of this study are summarized as follows:(1) Through a five-year old locating test, we compared the effects of bamboo and rice straw biochars at high rate by one-off returning on soil property and rice yield. Results showed that under the one-off returning to the soil with biochars at high rate (22.5 t ha-1) (HSC) mode, the effects of HSC amendment on soil improvement and rice yield increase over weighed those of bamboo biochar (BC). Compared with corresponded controls, the rice yield in treatments with rice straw biochar amendment could be increased by 13.5% and 6.1% respectively in the first two years under the one-off returning to the soil with biochars at high rate mode. This might be due to the higher level of pH, CEC and nutrient substances (K and P) in SC. However, the positive effects of this high rate biochar application mode turned to be weakened as time went on. This suggested that the one-off returning to the soil with biochars at high rate mode showed an apparently positive effect on soil amendment, but was not sustainable in the long term.(2) The effects of direct rice straw return (RS treatment) on soil property and rice yield were compared with rice straw-derived biochar application at low dose (LSC treatment) through a 2 year field experiment. Results showed that in comparison with corresponding controls, LSC treatment significantly increased rice yield by 7.6% in the first year and 10.7% in the second year. RS treatment did not show more significant effect on soil property improvement and rice yield increase than LSC treatment. Spearman analysis suggested that LSC treatment increased rice yield by increasing soil organic carbon content and available K and P content while decreasing available Al content. Although each year, the content of biochar addition for LSC treatment was only one tenth of the HSC treatment, the positive effects of LSC treatment on rice yield increase might be sustainable in the long term.(3) Through field experiments, we studied the effects of LSC treatment on CH4 emissions from paddy fields and its microbial mechanism. Results of the field experiment suggested that compared with RS treatment, LSC addition could significantly reduce CH4 emissions by 80.9% in the first year and 87.9% in the second year. Q-PCR analysis showed that compared with RS treatment, LSC addition could significantly decrease methanogenic 16S rRNA genes abundances and promote methanotrophic pmoA genes abundances. Results of 454 pyrosequencing showed that compared with RS treatment, LSC treatment could significantly decrease the diversity of soil methanogens and methanotrophs, and significantly cut down the percentage of Methanosarcina and Methanosaeta, which could convert acetate substrates into methane. Meanwhile, LSC treatment could significantly decrease the percentage of Methanobacteriumand and Methanospirillum (H2 plus CO2 are suitable substrates for these two methanogenic microbes). Therefore, the effect of LSC on decreasing CH4 emissions from paddy fields mainly benefited from the inhibition to methanogens and stimulation to methanotrophs. |
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