Stability Of Rice Straw-derived Biochar And Its Mechanism In Paddy Soil

Carbon sequestration is the basic for sustainable development in the context of global warming. Biomass derived charcoal (Biochar) can be preserved in the environment for hundreds and even thousands of years for its resistance to chemical and biological mineralization. Therefore, it has been widely accepted that biomass pyrolysis combined with biochar application could be an efficient approach for carbon capture and sequestration. China is the main rice-producing country in the world. Direct incorporation of rice straw into paddy soil has long been promoted. This management practice helps to conserve soil nutrients; however, it results in large quantities of methane released to the atmosphere. Therefore, the formation of rice straw-derived biochar appears to be a promising method to increase soil fertility associated with carbon storage. In this thesis, rice straw was pyrolysied under different conditions to determine the effects of pyrolysis temperature and holding time on the chemical composition and structure of biochar. Furthermore, laboratory experiments were carried out to study the biotic and abiotic oxidation of biochar in the flooded paddy soil by stable carbon isotope labeling. A suit of analytical techniques were used to investigate the change in characteristics of biochar due to bioctic and abiotic oxidation. In additon, the variances in the properties of the biochar applied in paddy field were also investigated. The obtained results could provide strong evidence on the technology of straw biochar application to realize carbon sequestration in agricultural ecosystem. The main results are summarized as follows:(1) Rice straw-derived biochars produced at different temperatures (300,400,500,600&700℃) and residence time (1,2,3&5h) were characterized. Results showed that pyrolysis temperature had a greater influence than residence time on the chemical composition and structure of rice straw-derived biochar (RSC) produced. Increasing charring temperature will decrease the volatile matter, H/C and O/C, while increase the carbon (C), fixed C and ash content of RSC. The degree of aromaticity and graphitization of RSC also enhanced with rising temperature. But RSC still arranged in turbostratic disorder. RSC produced at500℃had83.4%of C (ash-free) and98.5%of aromatic C content. The value of pH, cation exchange capacity and available phosphorus and potassium was higher for RSC produced at400-500℃. It indicated that RSC generated at500℃might be a better option for carbon sequestration and soil amendment.(2) Biotic and abiotic oxidation of RSC in waterlogged paddy soil was investigated under laboratory condition using13C labelled RSC. It showed that both biotic and abiotic oxidation play an important role in RSC mineratization. Biotic oxidation was more dominant than biotic oxidation for low-temperature RSC (350℃), while the contribution of biotic and abiotic oxidation to high-temperature RSC (500℃) was equal. RSC could be transformed to dissolved organic carbon (DOC) through abiotic process, and more than0.1%of DOC was derived from low-temperature RSC (350℃), while less than0.01%from high-temperature RSC (500℃). Low-temperature RSC (350℃) can be utilized by microorganism muvh easier than high-temperature RSC (500℃). These findings indicated that biotic and abiotic oxidation were much higher in low-temperature RSC than that in high-temperature RSC due to the higher content of labile component.(3) The changes of RSC properties through biotic and abiotic processes in paddy soil were characterized. Elemental composition did not display significant difference before and after incubation for both kinds of RSC. The aliphatic C-H and parts of aromatic components in low-temperature RSC (350℃) can be mineralized through biotic and abiotic process, and the thermal stability low-temperature RSC (350℃) was increased slightly. Both biotic and abiotic oxidation had no remarkable influence on the thermal stability and functional groups of high-temperature RSC (500℃); however, surface oxidation was increased through biotic process. The characteristics of bamboo biochar (BC) and RSC after3.5years in paddy field showed that the C content was increased firstly and then decreased afterwards. The major alteration of BC was the increasing intensity of carbonyl, while that of RSC was the decrease of aliphatic C-H and C-O-O and increase of aromaticity. Surface oxidation can be found in BC and RSC, and was more significant for RSC. Analytical results of the-3700year old straw biochar (ASC) in the ancient paddy soil suggested that it exhibited a distinctive porous vascular structure and was rich in Si. No crystalline structure of C was found in the ASC. Similarities in morphology and molecular structure between ASC and fresh RSC indicated that ASC was derived from rice straw. The ASC had a large amount of C (64.9%) and aromaticity content (82.8%). The physical incorporation or chemical reaction between the organic and mineral phase involved in stabilization mechanism of the ASC. The higher oxygen content and obvious carbonyl of the ASC implied that oxidation occurred in the scale of thousands years. All together, the above mentioned results suggested that RSC could be decomposed in short-term, associated with the formation of oxygen groups, and finally can be preserved in paddy soil for long-time scale due to the high aromaticity.