Accumulation And Distribution Of Black Carbon In Soils And Its Simulated Degradation

Due to its special physical and chemical properties, black carbon can be used to enhance soil fertility, adsorb contaminants and sequester atmospheric C in terrestrial systems to offset C emissions and combat global climate change. Investigating the accumulation and stability of black carbon can provide a scientific basis to better understanding the effect of human activities on the accumulation of black carbon, the stability mechanism of black carbon, the contribution of black carbon on the global climate change and soil management.This dissertation, which is supported by the Natural Science Foundation of China (N0.40771090;4047164), aimed to understand the effect of human activity on the accumulation, the changes in chemical composition and surface chemical properties of black carbon in soils due to long-term natural oxidation, the contribution of biotic and abiotic oxidation on the degradation of black carbon, the effects of different intensity of abiotically oxidation conditions (hot air, hydrogen peroxide and nitric acid) on black carbon stability, and the impacts of short-and long-term degradation on the surface properties of black carbon and its potential implications on soil. The main conclusions are as follows:(1) The surface soil and profile soil samples have been influenced by human activities to varying degrees in Zhejiang Province, and deterimined by chemical analysis method. The resuts showed that the mean content of black carbon in litter, surface layer and subsurface layer in the forest soils were3.96,6.91, and3.39g kg-1in that order, accounting for0.96%,8.07%, and11.82%of the soil total organic carbon (TOC), respectively. From40sites experiencing fire events within the last40years, black carbon in the litter and0-10cm soils was significantly higher than soils without a record of fire (p<0.05), whereas in the10-20cm soils fire events had no major impact on black carbon content (p<0.05). In the hilly area, accumulation of black carbon in the soils was generally higher on the low part compared to the top and upper part. The contents of black carbon in urban soils ranged from0to15.22g kg-1, and averaged in3.83g kg-1. Mean contents of black carbon in urban soils for different land uses were in the order manufacture area> vegetable land in suburban area> road in urban area> green space in urban area. Proportion of black carbon in total organic carbon ranged from0%to53.2%with the mean of24.83%. The proportion decreased in the sequence of manufacture area> road in urban area> vegetable land in suburban area> urban green space. In fluvimarine plain, total organic carbon pool in1m soil profile was lower in silty-clayey yellow mottled paddy soil on fluvialmarine than those of blue clayey paddy soil and gleyed blue clayey paddy soil in typical stagnic anthrosols. Black carbon, accounted for4.2%-24.6%of the total soil organic carbon, decreased with increasing depth for silty-clayey yellow mottled paddy soil on fluvialmarine and blue clayey paddy soil, and was even for gleyed blue clayey paddy soil. However, black carbon tended to be richened in clay component. The size of black carbon decreased with increasing profile depth.(2) Black carbon samples were collected from several historical fire-impacted sites from Zhejiang Province and were investigated the changes in chemical composition and surface chemical properties. The results showed that compared with new formed black carbon, soil black carbon exhibited a much lower surface area and alkaline groups, which were in the range of48-132m2g-1and2.65-18.93cmol kg-1, respectively. In contrast, carboxyl, acid groups and total group increased, their contents were16.59～63.24,38.66～124.17and50.80-1298.39cmol kg-1, respectively. Additionally, infrared spectroscopy identification of representative samples also showed that O-containing functional groups increased distinctly with the increase of time. Therefore, above results suggest that soil environments, land use, and formation time of black carbon may all have an impact on soil carbon biogeochemical cycles and soil biological chemistry.(3) In order to compare the importance of biotic and abiotic degradation on the properties changes of black carbon, the laboratoty simulated studies were used. The results showed that the labile fractions and stability of black carbon depend on the type of black carbon. In the condition of incubation at30℃, additional microbe can promote the mineralization of black carbon caused more CO2is released than abiotic degradation. However, in this condition, there is no significant changes in the surface oxygen-containing functional groups, surface area, and pore distribution after biotic degradation compared to abiotic degradation. The amount and rate of released CO2of70℃abiotic oxidation is more than that of30℃biotic oxidation. Additionally, surface oxygen-containing functional groups, surface area, pore distribution, and adsorption properties changed significantly after70℃abiotic oxidation. These results indicated that the severe oxidation provide more contribution to the black carbon degradation than microbe oxidation.(4) We used chemical oxidation method to investigate the effects of oxidative intensity on the changes of black carbon properties. The results showed that the changes of black carbon properties significantly depended on oxidative intensity. Hot air oxidation could cause blockage in pores of the original black carbon, which may explain the decrease in surface area and total pore volumes. While these parameters increased dramatically after nitric acid and hydrogen peroxide oxidation, as the structure of the original black carbon was severely destroyed, and the smaller size particles formed the slit-like mesopores. The data of FTIR spectra and zeta potentials indicated that nitric acid fixed the most oxygen-containing functional groups. These oxidation led to changes in elemental composition of black carbon. The nitric acid oxidized black carbon was shown to have highest oxygen content. These findings suggest that the changes of properties of black carbon oxidized by abiotic processes may affect black carbon environmental behavior.(5) A simulated oxidation technique was used to examine the impacts of degradation on the surface properties of biochar and the potential implications of the changes in biochar properties were discussed. To simulate the short-and long-term environmental degradation, mild and harsh degradation were employed. Results showed that after mild degradation, the black carbon samples showed significant reductions in surface area and pore volumes. After harsh degradation, the black carbon samples revealed dramatic variations in their surface chemistry, surface area, pore volumes, morphology and adsorption properties. The results clearly indicate that changes of black carbon surface properties were affected by black carbon types and oxidative conditions. It is suggested that black carbon surface properties are likely to be gradually altered during environmental exposure. This implies that these changes have potential effects for altering the physicochemical properties of black carbon amended soils.