| The pore characteristics of soils affect many functions and processes of soils, such as transport of water, gas and solute, biological activity, carbon sequestration, mechanical property and aggregate stability. However, the pore characteristics, possible aggregation mechanism, response of pore structure to biochar incorporation, and the pore structure for soil microorganism inhabit are still not fully understood at the aggregate level.The aims of this research were (1) visualize and quantify the pore structures in different types of soil aggregates and nodules based on advanced imaging techniques developed on third-generation Synchrotron Radiation Facility, the digital image process techniques, and the multi-dimension data fusion method, (2) investigate the pore structure response to environmental impact and anthropogenic activity. The primary were concluded as follows:(1) Two types of soil (red soil and black soil) with different cementing substance were investigated in this study. The characteristics of pore structures and cementing substances were investigated by combing SR-mCT and SEM-EDS techniques. Data fusion combined SR-mCT and SEM/EDS method was used to establish the relationship between pore strocture and cementing substance within the aggregate. SR-mCT data indicates that black aggregate has higher total porosity and DA value than red soil aggregate. Different size aggregates of red soil (5-10,2-5,and 0.5-2 mm in diameter) have similar pore size distribution (PSD), whereas different size aggregates of black soil have different PSD pattern. The SEM-EDS mapping reveal that the iron for red soil aggregate distribute relatively connective within the aggregates. The carbon distribution in black soil aggregates is highly heterogeneous, with a large number of isolated hotspots within the aggregate. Results of wavelet analysis suggest that the red soil aggregate is formed by global and enormous interaction among micro-aggregates and iron oxide; the iron oxides within the entire aggregate was mobilized and deposited in the wet/dry cycles, which resulted the different size aggregates in the red soil exhibited a similar pore size distribution pattern. The black soil aggregate is formed by the localized interaction among micro-aggregates and main cementing agent (carbon), which was accompanied by dynamic composition and decomposition of SOM. The cementing process was mainly influenced by the composition of SOM. The composition of SOM were local process, which dependent on pore size. Therefore, the pore size distribution pattern of different sized aggregates were different. The Heatmap analysis demonstrates that the iron oxide not only acts as the binding agent, but also influences the pore size as well as the pore shapes of the red soil aggregates, whereas the carbon only functions as binding agent and has little influence on pore structure of the black soil aggregates.(2)The micro-habit conditions for microbes in two different sized red soil aggregates (0.3-0.5 mm and 3-5 mm) were investigated by a synergistic use of SR-mCT, advanced visualize techniques and microbes incubation experiment. SR-mCT revealed that the Pore structures in soil aggregates provided proper micro-habit conditions for soil microbes. However, different size of soil aggregates have totally different pore characteristics. The total porosity in 3-5 mm aggregate was about 103 higher than that of 0.3-0.5 mm aggregate. The physical capacity for bacteria, actinomyces and fungus were 104,102 and 104 in 3-5 mm aggregate than that of 0.3-0.5 mm aggregate. Moreover, in 3-5 mm soil aggregate, highly complicated pore network systems with large number of mesopores and macropores were observed, which provided large enough survival space and activity scope for soil microbes just as bulk soil. The pore networks provided channels and pathways for air, water and nutrition exchange, which also provided ideal microenvironment for microbe inhabitation. The anaerobic microorganism in soil may even migrate with the water follow through the pore system within the aggregate. In 0.3-0.5 mm aggregate, large numbers of individual pores observed inside the aggregates, while only small-scale pore networks composed by only few numbers of multi-connected pores were also observed. Therefore, the pathways for air, water and nutrition exchange were limited, and the microenvironment in 0.3-0.5 mm soil aggregate was not appropriate for microbe inhabitation. The potential spatial distribution of soil microbes revealed that the fungus may possibly distributed at the outer part of the aggregate, because most of the macropores distributed at the outer part.(3) A 180 days’incubation experiment were conducted to investigate the effects of biochar (straw biochar, woodchips biochar and waste-water sludge biochar) on three-dimensional (3-D) soil structure within macroaggregates of two typical low yield soils (Ultisol and Vertisol). Synchrotron-based X-ray micro-computed tomography (SR-mCT) indicated that application of woodchip biochar (WCB) and waste-water sludge biochar (WSB) increased significantly the porosities of macroaggregates, whereas straw biochar (SB) was less effective. The increased porosity of macroaggregates in the biochar-amended soil is attributed to both the original porosity in the biochar and newly formed pores resulting from the interaction between soil and the biochar. The addition of straw biochar has no obvious effect on the spatial distribution of pores in macroaggregates of either soil, whereas WCB and WSB changed the directional pore distribution considerably of the Vertisol macroaggregates. In the WCB- and WSB-amended Ultisol, the number of pores increased in the outer parts of the macroaggregates, thereby greatly improving the exchange of air, water and nutrients with the adjacent soil matrix. Carbon mapping of the macroaggregates showed that the application of SB did not greatly increase the carbon content in macroaggregates compared with the control.The addition of WSB to the Ultisol markedly increased the carbontration of macroaggregates. The heat-map analysis indicated that the application of SB mainly influence the pore shape of macroaggregates, which have limited impact on porosity. The application of WCB and WSB markedly increased the porosity of both Ultisol and Vertisol macroaggregates. However, the WCB and WSB performed better in improving the pore structures of Ultisol macroaggregates than that of Vertisol, which may due to the different chemical properties.(4) Different size Fe-Mn nodules from a Plinthudult in the subtropical region of China were collected, and their studied internal structure, formation process and environmental fingerprints were investigated by three synchrotron-based X-ray microprobe techniques:X-ray micro-computed tomography (SR-mCT), X-ray micro-fluorescence (μ-XRF), and micro X-ray absorption near edge structures (μ-XANES). The SR-mCT images reveal that the large nodules (2-3,3-5, and 5-8 mm in diameter) have distinct Fe and Mn concentric ring structure, while the small nodule (1-2 mm) exhibits a homogeneous fabric. The μ-XRF maps show that Mn is concentrated near the nucleus of the nodule, while Fe is more concentrated in the exterior of the nodule. The 1-2 mm nodule has significantly higher total porosity than the >2 mm nodules; however, the spatial distribution of pores within the nodules indicates that the >2 mm nodules have smaller >30 mm pores and show more anisotropy in the outer part than in the inner part. The porosity and pore anisotropy of the nodules were considered to affect the size and shape of Fe-Mn nodules in soil. Higher macroporosity in the outer region of the nodule increases the exchange of air, moisture, and amorphous Fe and Mn oxides with the adjacent soil matrix, which enables the nodule to grow. Lower macroporosity in the outer part of the nodule decreases the exchange of air and moisture, which may gradually restrict the growth of the nodule. Quantification of the ring structure for the 3- to 5-mm nodule reconstructed the formation process of the nodule, showing that the nodule was formed through two growth stages, with each stage consisting of two and five redox processes. The internal structure of Fe-Mn nodules not only records pedo-environmental changes during the formation process but also provides ample opportunity for the adsorption and sequestration of trace elements from the soil. |
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