The effect of biochar on the water-soil environmental system in freezing-thawing farmland soil: The perspective of complexity

The moisture content of farmland soils is closely related to the farmland soil environment. Although biochar has been widely studied for farmland soil amelioration in tropical and temperate farmland soils, its application in areas of seasonally frozen soil is rare. In this study, field experiments were conducted to explore the effect of biochar on soil temperature and soil liquid moisture content in seasonally frozen soils and its corresponding mechanism. Biochar was applied to the soil at different rates (3 kg·m-2, 6 kg·m-2, 9 kg·m-2, and 12 kg·m-2) in autumn and spring. Daily monitoring data from the 20 cm soil-layer recorded for one year from the biochar application date were analyzed. The approximate entropy was introduced to explore the complex changes in soil temperature and soil liquid moisture content under biochar application in seasonally frozen soils. According to the calculation of approximate entropy, the application of biochar increased the complexity for most treatments. In the case of ignoring the heterogeneity of snowfall and uneven land tillage to the soil, we infer that this change was caused by changes in properties of the biochar due to the freeze-thaw cycle. The treatment under mixed biochar application in spring and autumn of 9 kg·m-2 had the smallest change in water and heat complexity. And the approximate entropy of this treatment is the smallest. Moreover, in the freezing period, the soil liquid moisture content is positively correlated with the biochar amount applied and negatively correlated with the biochar and soil mixing time. In the melting period, the opposite correlations occur. The changes in soil moisture conditions caused by freezing and thawing restrict the affinity of biochar for water. Therefore, the effect of biochar addition on the soil liquid moisture content varies among different freezing and thawing periods.