Effects of biochar addition on soil hydraulic properties before and after freezing-thawing

Abstract

Biochar as a soil amendment has attracted wide attention worldwide. However, the study of biochar on soil hydraulic properties during freeze-thawing remains insufficient. The purpose of this study was to determine the effect of straw biochar on soil water retention and soil hydraulic conductivity during a freeze-thaw period. Specifically, the effects of different biochar application rates (0, 30, 60, 90 and 120 t·hm−2) on the soil water characteristic curve (SWRCs), saturated hydraulic conductivity (Ksat) and hydraulic characteristic parameters were analysed through field trials. The response relationship between changes in the soil pore size and hydraulic characteristics was explored. The results showed that the combination of biochar amendment and freezing-thawing significantly increased the soil micro pore size (≥0.3–5 μm), soil voids (>100 μm) and total porosity (TP) and thereby improved the soil water retention capacity during the melting period. In addition, the use of biochar can promote the Ksat before and after freezing-thawing. However, increases in the carbon added during the melting period decreased the value of Ksat by 3.89%, 8.12%, 13.02% and 18.14% (p < 0.05) due to blockage of the soil pores by fine carbon particles. Compared with the control treatment, biochar significantly increased the field capacity (FC) and available water content (AWC) in soil. The largest relative change in the FC was obtained with an applied carbon amount of 60 t·hm−2, and this effect would improve the drought resistance capacity of the soil in the spring season. The excessive application of biochar led to an imbalance between the liquid and gas phases in soil, and thus, suggested biochar should not be applied in excess amounts to avoid negative effects on the soil structure. This study revealed the response mechanism of the hydraulic characteristics of the carbon-soil mixture during the freeze-thaw period and can provide a reference for the efficient utilization of soil water resources and the prediction of soil moisture in cold and dry areas.