Three dimensional characteristics of biopores and non-biopores in the subsoil respond differently to land use and fertilization

Soil macropores consist of biopores and non-biopores, which are different in morphological features and ecological functions. We aimed to separate biopores and non-biopores and investigate the response of their three-dimensional (3D) characteristics to land use and fertilization. Intact soil cores sampled from the subsoil (20-30 cm) under a combination of two land uses, paddy and upland fields, and three fertilization treatments, no fertilizer (Control), chemical fertilizer (NPK), and chemical fertilizer plus organic manure (NPKM), were scanned with X-ray computed tomography (CT). A methodology was proposed to separate biopores and non-biopores and the 3D characteristics of the pores were quantified. Based on the proposed methodology, soil biopores were well isolated from non-biopores. The volume of the biopores contributed 30.1-58.0% and 66.3-74.1% of the volume of the total macropores in the upland and paddy subsoils, respectively. The biopores in the paddy subsoil had a lower mean pore volume (MPV) and mean branch number (MBN), but a larger branch angle than those of the biopores found in the upland subsoil. The porosity, MPV, surface area (SA) and connectivity of non-biopores were significantly higher in the upland subsoil than those in the paddy subsoil. The NPKM treatment significantly increased the volume of the large-sized biopores in the upland subsoil and the tortuosity of the biopores in the paddy subsoil, whereas the fertilization effect on non-biopores was minor. Our results demonstrate that subsoil biopores and non-biopores can be effectively distinguished according to the proposed methodology and that their 3D characteristics respond differently to land use and fertilization.