Biochar For Carbon Sequstration And Its Effects On Soil Greenhouse Gases Emission

The great emission of greenhouse gases and the appropriate treatment and disposal of biomass wastes are substantial issues that need to deal with. Converting biomass wastes into biochar under low temperature and limited oxygen conditions has recently been proven as a promising and effective approach for solving these issues. In this thesis, lab experiments were carried out to estimate the carbon sequestration potential by converting the main crop straw into biochar and investigate the effect of mineral addition during the biochar formation on improvement of carbon sequestration, and the effect of biochar addition on the greenhouse gases emission in soil, Furthermore, the mechanisms were discussed. The main contents from this thesis are as follow:（1） The crop residue straw including rice straw, wheat straw, corn straw and soybean straw were turned into biochar under 350 oC（BC350） and 500 oC（BC500）, the properties and carbon sequestration ability of biochar were determined. The yield of BC 500 was about 30% on average, which was lower than that of BC350. The carbon content of BC500 was 63.2% on average which were higher than that of BC350. The carbon retention of BC350 and BC500 were 60% and 47.7%, respectively. However, the carbon loss by potassium dichromate oxidation were 40% for BC350 and 15% for BC500, respectively, which indicated that biochar derived from high temperature was beneficial for carbon sequestration. Life cycle assessment was employed to quantify the potential of biochar technology in mitigation of greenhouse gases in our country. The results showed when the crop residue straw was turned into biochar, the net greenhouse effects potential was 5.62 × 108 t·CO2e which was equal to 6.13% of total carbon emission, which indicated that turning straw into biochar was a promising approach for straw management and carbon sequestration.（2） Three minerals including kaolin, calcite（Ca CO3）, and calcium dihydrogen phosphate（Ca（H2PO4）2） were added to rice straw feedstock at the ratio of 20%（w/w） for biochar formation through pyrolysis treatment, aiming to improve carbon retention and stabilization in biochar. Kaolin and Ca CO3 had little effect on the carbon retention, whereas Ca（H2PO4）2 increased the carbon retention by up to 29%, compared with untreated biochar. Though the carbon loss from the Kaolin-modified biochar with hydrogen peroxide oxidation was enhanced, Ca CO3 and Ca（H2PO4）2 modification reduced the carbon loss by 18.6% and 58.5%, respectively. Moreover, all the three minerals reduced carbon loss of biochar with potassium dichromate oxidation from 0.3% to 38.8%. The microbial mineralization as carbon dioxide（CO2） emission in all three modified biochars was reduced by 22.2%-88.7% under aerobic incubation and by 5%-61% under anaerobic incubation. Enhanced carbon retention and stability of biochar with mineral treatment might be caused by the enhanced formation of aromatic-C which was evidenced by CP-MAS 13 C nuclear magnetic resonance spectra（NMR） and fourier transform infrared spectroscopy（FTIR） analysis. Our results indicated that the three minerals, especially Ca（H2PO4）2, was effective in increasing carbon retention and strengthening biochar stabilization, which provided a novel idea that people could explore and produce the designated biochar with high carbon sequestration capacity and stability.（3） The lab experiments were conducted to investigate the effect of biochar, derived from rice straw under 500 oC, addition to red soil under simulated paddy-upland rotation condition on the greenhouse gases emission. Results showed that the emission of CH₄ and CO2 increased with the rise of biochar addition, and those emissions from upland condition were higher than that from paddy condition. CH₄ emission under upland and paddy condition from soil with nitrogen and 5% biochar addtion were 205 μg C?kg-1and 7.25 μg C?kg-1, respectively. Moreover, CO2 emisison under the same condition increased by 76.7% and 42.8%, respectively, compared to control soil. The emission of nitrous oxide（N2O） reduced with the rise of biochar addition. Under upland and paddy condition from soil with 200 mg N?kg-1 and 5% biochar addition, N2 O emission reduced by 95.0% and 92.7%, respectively, compared to control soil. In a word, during the incubation period, with 5% biochar addition, the carbon loss in terms of CH₄ and CO2 emisison was much lower than carbon sequestration by biochar and it could be ignored. Moreover, 5% biochar addition reduced N2 O emission better than the other biochar addition rate, all of which indicated the 5% biochar addition was optimization application rate for carbon sequestration in this study.（4） The short-term study was conducted to investigate the greenhouse gas emissions in five typical soils under two crop residue management practices: raw rice straw and its derived biochar application. Rice straw and its derived biochar（two biochars, produced at 350 oC and 500 oC were referred to as BC350 and BC500, respectively） were incubated with the soils at a 5%（w/w） rate and under 70% water hold capacity for 28 d. Incorporation of BC500 into soils reduced CO2 and N2 O emission in all five soils by 4%-40% and 62%-98%, respectively, compared to the untreated soils, whereas CH₄ emission was elevated by up to about 2 times. Contrary to the biochars, direct return of the straw to soil reduced CH₄ emission by 22%-69%, whereas CO2 increased by 4 to 34 times. For N2 O emission, return of rice straw to soil reduced it by over 80% in two soils, while it increased by up to 14 times in other three soils. When all three greenhouse gases were normalized on the CO2 basis, the global warming potential in all treatments followed the order of straw > BC350 > control > BC500 in all five soils. Moreover, the carbon sequestration efficiency in all treatments followed the order of BC500 > BC350> straw. The results indicated that turning rice straw into biochar followed by its incorporation into soil was an effective measure for reducing soil greenhouse gas emission and the effectiveness increased with the increasing biochar production temperature, whereas direct return of straw to soil enhanced soil greenhouse gas emission.（5） Effects of biochar derived from rice straw and dairy manure under 500 oC on nitrification, denitrification and N2 O emission in red soil and paddy soil were investigated through laboratory experiment. Results indicated that the variation in nitrification rate and N2 O emission depended on the type of soil and biochar. However, the similar change tendency between denitrification and N2 O emission with biochar addition to soil, in other word, biochar addition to soil increased the denitrification but decreased the N2 O emission. After 14 d incubation, the denitrification rate of red soil with dairy manure and rice straw bicochar addition, increased by 116% and 105%, respectively, while N2 O emission decreased by 65.0% and 99.1%, respectively. For paddy soil, however, there was no obvious variation in denitification rate and N2 O emission after the biochar addition. Hence, the mechanism of N2 O reduction with biochar addition was not by inhibition of denitrification.