| Strongly acidic red soils widely cover the tropical and subtropical regions of southern China and are considered to be degraded soil. Biochar is a porous and low density carbon rich material produced through the heating of natural organic materials under low oxygen condition. Because biochar contains larger amounts of nutrients, the addition of biochar to soil can increase the nutrient content of low fertility soils. Our objectives were to describe the effect of biochar on selected soil physical characteristics of red soil, thereby evaluating the possible benefits of biochar in the improvement of red soils, and to understand the mechanisms involved in the improvement of physical properties of soils.The morphology, pore structure, chemical composition of three biochars made from the straw (SB), woodchips (WCB), and wastewater sludge (WSB) were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with X-ray energy-dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), and mercury intrusion porosimetry (MIP). FESEM images indicated that structure of the biochar was highly heterogeneous with larger macropores in the several to tens micron scale. The EDS analysis indicated that the biochar particles consisted of higher carbon content with small amount of Si, Al, and Ca. XRD pattern of biochar revealed the presence of quartz and calcite in the biochar. The cumulative pore volume (CPV) curves of biochars indicate that the SB has the greatest amount of pore volume (2.910cm3·g-1), while the WSB have the least (0.413cm3·g-1)-The CPV curves of SB and WCB become essentially flat at diameter less than1μm, suggesting almost all of the pores in SB and WCB have a diameter larger than1μm The IR spectra of biochars included several adsorption bands associated to aromatic C-H, C=C, C=O stretching, aliphatic C-H stretching, and O-H stretching.Three biochars (SB, WCB, and WSB) were applied at the rate of0,20,40, and60g biochar kg-1soil and were incubated for180days at field water capacity in the glasshouse. The effect of biochar on the chemical and physical quality of a red soil was evaluated by pH, exchangeable acid, nutrient contents, water retention, and mechanical strength of biochar-amended soils. The WCB-and WSB-amended soils had a significantly (p<0.05) higher soil pH and lower exchangeable acidity than the control treatment. The effect of biochar on soil pH and acidity increased with increased addition level of biochar, especially for WCB-amended soils. The application of biochars significantly increases the total C, available P and K contents after biochar application. The increases in available P and K with increasing levels of biochar are most likely due to the presence of these nutrients in the biochar itself. SB contains much higher K content than WCB and WSB, which leads to much higher increased K in SB-amended soils. Compared with the control, the exchangeable K, Ca, and Mg contents also significantly increased in the biochar-amended soils.The saturated water contents (SWC) of biochar-amended soils were significantly higher than those of control treatments. The tensile strength of original Ultisol is466kPa, which is reduced to233,164, and175kPa at the rate of6%WCB, SB, and WSB application. The reduction in tensile strength was greater in WSB-amended soils than SB-and WCB-amended soils. The c of soil amended with WCB had significant decreases as compared with the control, whereas no significant difference was found in the SB-and WSB-amended soils. No significant effects of biochars on internal friction angle (cp) were found. These different mechanical behaviours of soils amended with different biochars might be due to different initial pore structure and degree of pore water saturation of the soils. |
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