Understanding the role of biochar in mitigating soil water stress in simulated urban roadside soil

Biochar has been proposed as a promising amendment that may improve soil structure. However, our understanding how it mitigates extreme soil water stress in roadside soils is limited. In this study, we investigated the effects of biochar on soil properties and plant growth under extreme water stress conditions. A greenhouse experiment was conducted on two-year-old Gingko biloba saplings planted in pots with sandy soil only (CON) and with sandy soil mixed with biochar (BC). To simulate excessive water stress conditions, we increased the soil water-filled pore space up to the saturation level throughout the experimental period. We also simulated the switching water conditions by maintaining the saturation condition for 30 days, followed by no addition of water. The BC treatment significantly influenced the aggregate distribution and enhanced the proportion of macroaggregates (250 μm). The biochar itself also functioned as a macroaggregate and contributed to increased aeration under the excessive water condition. Under the switching water condition, the micropores within the biochar might have helped maintain the available water for plant roots and soil microbes. Plant growth was significantly higher in the BC than CON soils for both the excessive and switching water sets. In the BC soils, plant growth was higher in the excessive than in the switching water sets, indicating that the soil water status in our BC treatment for the excessive water set was not stressful enough to inhibit plant growth. The % optimal water condition, which is defined as the proportion of days when the soil water status is within the least limiting water range, had a very high explanatory power to explain the plant growth (r = 0.7172, p 0.0001). Our results indicate that biochar can alleviate water stresses in urban roadside soils by retaining plant available water under the wet and dry conditions.