Double effects of biochar in affecting the macropore systemof paddy soils identified by high-resolution X-ray tomography

Biochars are widely used to improve soil macropore structures. However, the size-dependent effects of biochars in affecting macropore structure still remain unclear. In this study, the modification of soil macropore structure following biochar addition was investigated by high-resolution X-ray tomography (CT) and advanced data analytical methods. Quantification of soil macropore structure (>52 mu m) was based on the intact soil cores (5 cm in diameter and 7 cm in height) collected from biochar-amended paddy soil. The treatments were: (a) control (CK), (b) rice straw biochar (RSB), (c) corn straw biochar (CSB), and (d) bamboo biochar (BB). The application rate of biocharwas 25 Mg ha(-1) (w/w). Results revealed that the biochars affected the soil macropores through both occupying effect and expansion effect. The occupying effect means that the biochar particles occupy the original pore space of the soil and reduce the soil porosity, while the expansion effect means that the biochar particles produce additional pore space in the soil matrix and increase the soil porosity. For all the biochar treatments in our study, the occupying effect was dominated. Therefore, the connected- and isolated porosity of biochar-treated soils were significantly lower than those of CK. The size-dependent effects of biochar in modifying soil macropores were observed. For RSB treatment, the expansion effect predominated at the size of <1200 mu m and >1800 mu m, while both the occupying effect and expansion effect were observed at the size of 1200-1800 mu m. For CSB and BB treatments, the expansion effect predominate only at the size of 600-1200 mu m; whereas both the occupying effect and expansion effect were observed at the size of <600 mu m and >1800 mu m. Our results indicated that the effects of biochar on soil macroporosity were dependent on the combined effects the occupying effect and expansion effect. In conclusion, all the biochar types in this study have adverse effects in increasing the soil macroporosity. (C) 2020 Elsevier B.V. All rights reserved.