Evaluation of soil macro-aggregate characteristics in response to soil macropore characteristics investigated by X-ray computed tomography under freeze-thaw effects

As two complementary aspects of soil structure, the importance of macropore and macro-aggregate character-istics in maintaining soil functions and productivity is paramount. However, the changes in macropores and macro-aggregates and their interactions induced by the effects of freeze-thaw action have rarely been evaluated. This study aimed to evaluate the freeze-thaw effects on soil macropore and macro-aggregate characteristics and their relationships using X-ray computer tomography and dry sieving. Thirty-nine undisturbed soil columns were derived from a depth of 0-15 cm in the Mollisol area of Northeast China. Seven freeze-thaw treatments, including 0 (CK), 1 (FT.1), 3 (FT.3), 5 (FT.5), 7 (FT.7), 10 (FT.10), and 15 (FT.15) freeze-thaw cycles, and two initial soil mass water contents (30% and 40% the initial soil water content (ISWC)) were selected under controlled lab-oratory conditions. Our results showed that freeze-thaw effects significantly increased the total image porosity, mean pore diameter, porosity with branches, and connectivity density (p < 0.05). However, the total number of pores showed a "first increase - then decrease" trend with an increasing number of freeze-thaw cycles. During the freeze-thaw cycles, the macropore and macro-aggregate size distributions exhibited dynamic processes. Compared to the CK treatment, soils for 30% and 40% ISWC under the FT.15 treatment exhibited a lower > 7 mm macro-aggregates by 27.9% and 43.1%, but a higher < 3 mm macro-aggregates by 78.0% and 88.4%, respectively. The total image porosity, porosity with branches, and connectivity density were significantly positively and negatively correlated (p < 0.05) with the mean weight diameter and fractal dimension of macro -aggregates, respectively. The increase in macroporosity and pore branches easily induced the breakdown of the macro-aggregates. This study can help improve the understanding of the effects of changes in macropore characteristics and the processes of macro-aggregate breakdown and further enable the utilisation of soil structure as an indicator for assessing the sustainability and erodibility of soils.