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

Abstract

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 μm) 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 biochar was 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 μm and >1800 μm, while both the “occupying effect” and “expansion effect” were observed at the size of 1200–1800 μm. For CSB and BB treatments, the “expansion effect” predominate only at the size of 600–1200 μm; whereas both the “occupying effect” and “expansion effect” were observed at the size of <600 μm and >1800 μm. 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.