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Characteristics of snowmelt transport in farmland soil in cold regions: The regulatory mechanism of biochar

As a vital water source in cold and arid areas, snowmelt alleviates spring drought problems. Based on a laboratory simulation tests, after nine freeze–thaw cycles, under the simulated conditions of different biochar application amounts (0%, 1%, 2% and 3%, denoted as H0, H1, H2 and H3, respectively) in the spring soil thawing period, the characteristics of soil structure and the process of snowmelt transport in farmland black soil were explored. The research results showed that (a) the addition of biochar increased the soil total porosity and the proportion of pores in the middle section (0.3 ~ 100 μm) and reduced the proportion of soil voids (> 100 μm) and extremely micro pores (< 0.3 μm). (b) As the biochar application amount increased, the stability of soil aggregates first increased and then decreased. The rules of the medium aggregates (0.25 ~ 2 mm) were consistent with the structural stability indexes, while the rules of the silt and clay (< 0.106 mm) were opposite. (c) With an increase in the amount of applied biochar, the saturated water conductivity of unfrozen soil increased; however, the opposite effect was observed for frozen soil. The coefficient Xi was developed to evaluate the soil infiltration performance in the thawing period based on the soil saturation water conductivity. Xi exhibited a good correlation with snowmelt transport. (d) Evaporation, infiltration and runoff also showed a trend of first improving and then producing negative effects with increasing biochar application amount in the snowmelt period. From the perspective of the utilization efficiency of spring snowmelt, the utilization rate of H0 snowmelt was 71.13% and it increased by 6.4% and 1.2% in H1 and H2, respectively, and decreased by 13.53% in the H3 treatment compared with the H0 treatment. Considering the soil structure and soil moisture demand needed for spring sowing, 1% biochar application amount is the most suitable for farmland black soil.