The Retention Mechanism Of Soluble Biochar By Soil Reactive Minerals

The thermal transformation of agricultural biomass into biochar in an oxygen-depleted atmosphere has been extensively studied, because of concerns over enhancement of soil fertility and mitigation of climate change by long-term carbon sequestration. Biochar surface properties are usually controlled by the pyrolysis conditions applied and feed stocks used and thus play an important role in its interactions and reactions with soil reactive nanominerals. Soil mobile colloids can significantly reduce the activity and bioavailability of soil organic matter by forming a nanotube, which can effectively store organic aromatic carbon and is one of the main mechanisms of stability and accumulation of soil organic matter. The nano scale organo-mineral interaction between soluble biochar and soil reactive nannominerals is very important for the retention of soluble biochar and improvement of soil carbon pool. The objectives of this study were:1) To investigate the development of functional groups in biomass-derived biochar by applying two dimensional correlation spectroscopy(2D FTIR and 2D 13C NMR)and establish the relationship between functional groups and biochar properties (i.e., pH and EC); 2) to investigate the relationship between soil reactive nanominerals in soil mobile colloids and the soluble biochar by studying soils from two long-term location experiments, using the combination of high resolution transmission electron microscopy (HRTEM) and near edge x-ray absorption fine structure spectroscopy (NEXAFS), aiming to explore the mechanism of soluble biochar sequestration by reactive nanominerals.The main conclusions are listed as follows.1、Four types of agricultural biomass (i.e., rice straw, rice bran, wood dust and cow manure) and eight charring temperatures(100～800℃)were selected to produce biochars. The results showed that the properties of biochar greatly depend on the production conditions (i.e., charring temperature) and the biomass type used to produce biochar. The regression analysis showed that functional groups and biochar properties (pH and EC) are highly correlated (p< 0.05). Ether bond (C-O-C) and carbonyl (C=O) were not existed after carbonization from the result of 2D FTIR correlation spectroscopy. With increasing pyrolysis temperature, methyl (-CH3)and methylene (-CH2)gradually disappeared while more aromatization was shown by new aromatic compound. Two-dimensional (2D) 13C nuclear magnetic resonance (NMR) correlation spectroscopy showed that the agricultural biomass carbonized to biochars was a dehydroxylation/dehydrogenation and aromatization process, mainly involving the cleavage of O-alkylated carbons and anomeric O-C-0 carbons in addition to the production of fused-ring aromatic structures and aromatic C-0 groups. With increasing charring temperature, the mass cleavage of O-alkylated groups and anomeric O-C-0 carbons occurred prior to the production of fused-ring aromatic structures.2. After the application of biochar to soil, soil physical-chemical properties were effectively improved. Soil pH increased by 0.19 units with rice straw biochar (NCNPK) amendment under normal fertilization in the Jiaxing long-term location experiment, compared to fertilization without biochar, but the pH of soil from the Quzhou long-term location experiment didn’t increase. The EC in Quzhou and Jiaxing long-term location experiments were both effectively increased. Compared to the treatment without application of biochar, the treament with application of biochar had a high soil organic carbon, TN, TC and C/N (p < 0.05).3. The contents of noncrystalline Fe and Al nanominerals in the soil were determined by acid oxalic acid and sodium pyrophosphate extraction method. It was found that the contents of noncrystalline Fe and A1 nanominerals in the soil were positively correlated with soil organic matter content in both Quzhou and Jiaxing (R2> 0.67; p< 0.05). The content of SOM and noncrystalline Fe and Al nanominerals in the treatment of biochar were significantly higher than that without application of biochar.4. The results from HRTEM showed that soil mobile colloids from the BCN0, BCN180, and BCNPK treatments in the Quzhou and Jiaxing long-term location experiments contained a large amount of noncrystalline nanominerals, suggesting that biochar could promote the transformation of crystalline minerals to noncrystalline nanominerals. Main elements in noncrystalline nanominerals were Al, Si and O.5. The results from near-edge X-ray absorption fine structure (NEXAFS) spectroscopy analysis showed that organic carbon in soil colloids mainly display six characteristic peaks, including 283-286.1 eV belonging to Aromatic C 1s�'π transitions; 287.6～288.3 eV belonging to Aliphatic C 1s�'3p/σ transitions; 288.4-289.1 eV Carboxyl C 1s�'π; transitions; and 289.9-291.2 eV Carbonate, Carbonyl C=O ls�'π transitions. Compared with other treatments, the Aliphatic C increased, whereas the Carbonate, Carbonyl C decreased after the application of biochar. The Carboxyl C in N180 treament increased as compared to NO treament. C ls NEXAFS spectroscopy could characterize in situ the changes of organic carbon functional groups in soil colloids under long-term fertilization.6. XPS fitting results show that the binding energy of C ls of the soil colloids was classified as four types:Aromatic C, C-C(H), C-O and O-C=O. They were predominant chemical composition and structure on the surface of soil colloids. The content and types of chemical composition and structure on the surface of soil colloids were significantly impacted by biochar, especially the Aromatic C. The Al element and Si element on surface of soil colloids in N180 and NPK treatment were mainly Kaolinite and Montmorillonite types of Al and SiO2 types of Si, the content of noncrystalline Allophane Al2O3/Al2O3-nH2O and Allophane Si-O were increased by biochar application.Our results indicate that long-term carbon storage in soil rective minerals occurs via retention with aromatic carbon from dissolved biochar carbon.