Three-dimensional pore structure and carbon distribution of macroaggregates in biochar-amended soil

Incorporation of biochar improves the physical properties of soil that are needed for plant growth; in particular, it stabilizes soil's aggregate structure. There has been little research, however, on the effects of biochar on three-dimensional (3-D) soil structure and carbon distribution within aggregates. To investigate the effects we amended two soil types (Ultisol and Vertisol) with three types of biochar (at the rate of 40 g biochar kg(-1) soil). We separated the macroaggregates (5-7 cm in diameter) into two sets to study the distributions of intra-aggregate pores and carbon. 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 the 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 of the Vertisol macroaggregates considerably. 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 of macroaggregates compared with the control. The addition of WSB to the Ultisol markedly increased the carbon concentration of macroaggregates. The heat-map analysis indicated that the effects of biochar on pore structure and carbon content depend on the type of biochar and soil. The combination of SR-mCT and scanning electron microscopy (SEM) has been shown to be powerful for quantifying the pore characteristics and carbon distribution in soil macroaggregates.