Pore Structure Characteristics after 2 Years of Biochar Application to a Sandy Loam Field

Soil pore structure comprises the size and shape of soil pores and has a major impact on water retention and gas movement. The porous nature of biochar suggests that its application to soil can potentially alter soil pore structure characteristics, and the purpose of this study was to evaluate the effects of birch wood biochar (20, 40, and 100 Mg ha-1) applied to a sandy loam on soil total porosity and pore structure indices. Bulk and intact soil samples were collected for physicochemical analyses and water retention and gas diffusivity measurements between pF 1.0 and pF 3.0. Biochar application reduced bulk density and increased total porosity especially for soil with 100 Mg ha-1 biochar (16% and 14% reduction in bulk density and total porosity, respectively). Biochar application of more than 20 Mg ha-1 enhanced water retention, and the trend increased with increasing biochar application rate, where the maximum increment was 12% at 100 Mg ha-1 biochar treatment. A pore size distribution index, B, derived from water retention, indicated a wider soil pore size distribution in biochar-amended soil than in the reference soil, especially for the 100-Mg ha-1 application. At given matric potentials, biochar increased soil air-filled porosity by up to 25%. However, there was no difference in gas diffusivities between biochar-amended soil and the reference soil. At pF 3.0, the soil pore system became more tortuous after biochar application, with a trend that pore tortuosity increased with increasing biochar rate. Overall, addition of a wood-based biochar to a sandy loam soil resulted in a soil structure with broader pore size distribution and higher tortuosity of the interaggregate pore network. Similar studies with other types of biochars and soils are recommended toward better understanding of biochar effects on soil functions and services. © 2015 Wolters Kluwer Health, Inc. All rights reserved.