Biochar Effect on Water Vapor Movement in Frozen Soils of Different Textures via Water Isotope Method

Water vapor movement in frozen soil under field condition is a common phenomenon in winter of North China. The study investigated the biochar effect on soil water content and vapor transportation in sand, loam, and clay via water ¹⁸O isotope method. Twenty-seven micro-plots were set up with 3 texture soils and 3 doses of biochar (0, 3% and 6%). Measurements were made during the winter of 2011 and 2012. By Isotope Mass Balance (IMB) method with water ¹⁸O, the amounts of water vapor-upward-moving (VUM) from the bottom layer and the sublimation occurring at the soil surface were calculated.

The results showed the upward moved vapor primarily condensed at 20-30 cm soil layer. In general, biochar significantly inhibited upward movement of vapor in sandy soil and this negative impact was highly related to its addictive amount. Specifically, 6% biochar obviously weakened VUM and improved the water content of sandy soil, while 3% biochar amendment caused less VUM for loamy and clay soils in 2011. Besides, a strong annual variation, significantly lower values of VUM in colder 2012, was observed. In conclusion, biochar addition influenced the amount of VUM in different textured soils and can be used for the management of soil moisture in the early spring.

In water-limited ecosystems, biochar increased the water holding capacity by improving soil porosity (Vugmeyster et al., 2012). Consequently, fine-textured soil held more water (Figure 1). Textures governed the soil hydraulic conductivity by absorbing more film water on the fine particles with their large specific surface area (Iwata et al., 2010).Table 1 showed additional biochar hindered the SVUM and sublimation in all tested soils. A smaller SVUM was found in colder winter (2012), which can be partially explained by hindered effect of low temperature on movement of the water molecule and vapor escaping from the liquid (Shackley and Sohi, 2010). Besides, there was more ice crystal generated in soil pores after the addition of biochar to sandy soil and prevent the vapor water redistributions (Chen et al., 2010), and therefore these negative impacts were more obvious in sandy soil. The negative values of the SVUM and sublimation were due to the water infiltration from the upper to the deeper layer.