Biochar enhances soil hydraulic function but not soil aggregation in a sandy loam

Biochar has the potential to modify soil structure and soil hydraulic properties because of its small particle density, highly porous structure, grain size distribution and surface chemistry. However, knowledge of the long-term effects of biochar on soil physical properties under field conditions is limited. Using an 8-year field trial, we investigated the effects of successive additions of high-dose maize-cob-derived biochar (9.0 t ha(-1) year(-1), HB), low-dose maize-cob-derived biochar (4.5 t ha(-1) year(-1), LB), straw return (SR) and control (no biochar or straw, CK) on soil aggregate distribution, three-dimensional (3-D) pore structure, hydraulic conductivity and water retention in the upper 10 cm of a sandy loam soil from the North China Plain. Results showed that LB and HB treatments increased soil organic C content by 61.0-116.3% relative to CK. Interestingly, biochar amendment did not enhance the proportion of macroaggregates (> 2 and 0.25-2 mm) or aggregate stability, indicating limited positive effects on soil aggregation. The HB treatment decreased soil bulk density, and increased total porosity and macroporosity (> 30 mu m). The retention of soil water, including gravitational water (0-33 kPa), capillary water (33-3100 kPa) and hygroscopic water (> 3100 kPa), was improved under HB soil. The HB and LB treatments increased plant-available water content by 17.8 and 10.1%, respectively, compared with CK. In contrast, SR showed no significant increase in soil porosity and water retention capacity but improved the water stability of macroaggregates. We concluded that biochar used in the coarse-textured soil enhanced saturated hydraulic conductivity and water-holding capacity, but did not improve soil aggregation.