Soil structure evolution after gypsum, structure lime and pulp mill sludge applications: The amendment type matters

Various soil amendments are used to improve soil structural stability against water erosion. Gypsum, structure lime and pulp mill sludges have all received interest in northern conditions and shown positive results in this respect. However, only a few studies have focused on the impacts of amendments on soil pore systems, particularly on their temporal dynamics under field conditions. In this study, we used X-ray tomography for time-lapse imaging of field-incubated soil samples to follow the structural dynamics of clay soil treated with gypsum, structure lime, or pulp mill sludge. Our results showed that gypsum and structure lime did not notably influence the macropore structure dynamics as compared to the non-treated control. In contrast, pulp mill sludge altered the macropore system significantly in comparison to control and two inorganic amendments. Pulp mill sludge increased the share of large macropores (>1200 µm), whereas the smaller macroporosity (<300 µm) temporarily decreased after the initiation of field incubation. After the first winter, the differences in this pore size region between the treatments levelled off. Our results show that, in addition to soil structural stability, soil amendments can influence the soil macropore system, but the magnitude and nature of these effects strongly depend on amendment type.