Effects of pore structure characterized by synchrotron-based micro-computed tomography on aggregate stability of black soil under freeze-thaw cycles

Abstract

Soil structure affects soil quality, which in turn influences agricultural productivity. In north-eastern China, periodic melting of the frozen layer is very common in late autumn and early spring. Such freeze-thaw cycles affect soil structure, aggravating soil erosion and affecting soil quality. The objective of this study was to evaluate the effects of aggregate pore characteristics on the aggregate stability of black soil under freeze-thaw cycles. Undisturbed topsoil samples were submitted to 0, 1, 3, 5, 7, 10, 15, and 20 freeze-thaw cycles under controlled laboratory conditions. Then, 5–7 mm aggregates were collected from each treatment for synchrotron-based X-ray micro-computed tomography (SR-μCT) scanning with 3.25 μm voxel resolution at the Shanghai Synchrotron Radiation Facility (SSRF); 3D micro tomography images were then reconstructed using ImageJ software. Aggregate stabilities against slaking (MWDFW) and mechanical breakdown (MWDWS) were also measured after each treatment. The relationships between pore parameters and aggregate stability were then analyzed using the method of partial least squares regression (PLSR). 2D and 3D image visualization showed that the aggregate microstructures developed increasingly porous and connected pore networks following repeated freeze-thaw cycles. Quantitative analysis showed that total porosity, porosity of elongated pores (EP), and porosity of equivalent pore size >100 μm (P>100) significantly increased with an increasing number of freeze-thaw cycles. Such cycles decreased the aggregate stability, which was closely associated with pore characteristics. According to PLSR, the pore characteristics accounted for 86 % and 81 % of the variation in MWDFW and MWDWS, respectively. Total porosity, EP, and P>100 were identified as the primary factors controlling the air content, water entrance, and failure stress of interior aggregates, all of which could weaken aggregate stability. These results improve the understanding of pore characteristics and aggregate stability, which are key factors influencing soil erosion and soil quality during the seasonal freeze-thaw period.