Junru Chen, Shenglong Li, Behzad Ghanbarian, Bo Xiao
Abstract
As an important biological community at the soil-atmosphere boundary in drylands, biocrusts are crucial in water-heat coupling processes, such as water vapor transport in topsoil, and thus have a significant impact on dryland hydrological activities. However, current conventional measurements are incapable of comprehensively and fully capturing hydrothermal dynamics and water evaporation processes in the fast-changing top soil layer. To address this knowledge gap and investigate the effect of biocrusts on top soil evaporation processes at the fine scale (millimeter scale), we, therefore, conducted a year-long field observation experiment using 11-needle heat pulse sensors. We continuously monitored water evaporation processes in near-surface soils with and without biocrusts. Results showed that biocrusts significantly increased the annual mean soil water content (θ), sensible heat flux (G) and evaporation rate (E) of topsoil (1.5–10 mm) compared to bare soil (p < 0.05). In comparison with the bare soil, the average annual θ of the biocrusts significantly increased by 25.0 % (p < 0.001) in the 1.5-mm soil layer, and its average annual G by 57.8 % (p = 0.005). Furthermore, we found that the average annual E of the biocrusts within the 1.5–4 mm soil layer significantly increased by 31.2 % (p < 0.001). This was mainly attributed to the higher fine particle density and organic matter content in the biocrusts and the retention of water in the soil surface and to the lower surface albedo in the biocrusts, which resulted in more energy adsorption and higher evaporation. In addition, the peak E in both biocrusts and bare soil showed a time lag due to differences in the timing of solar heat arrival at the surface. Interestingly, we also found that as rainfall infiltrated into the biocrust layer, the θ in the lower depths of the biocrust layer could be lower than that of the bare soil, caused a lower E than that in the bare soil. In summary, biocrusts altered the soil water distribution due to their profound effects on the physicochemical properties of the surface soil, which ultimately led to vertical variability of evaporation in the soil profile. Biocrusts may accelerate dryland water loss due to their larger E, and, therefore, generate complex feedbacks on dryland soil hydrological activity.
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