Biocrusts enhance non-rainfall water deposition and alter its distribution in dryland soils

Non-rainfall water deposition is an important water resource, critical for the survival of dryland vegetation and soil biota and maintaining dryland water balance. As a “living skin”, biocrusts are attracting increasing attention due to their potentially positive impacts on non-rainfall water deposition. However, the magnitude and underlying mechanisms of biocrust regulation of non-rainfall water are still unclear. In this study, we investigated the non-rainfall water deposition and distribution through continuous weighing micro-lysimeters (0–3, 3–6, and 6–10 cm depths) with bare soil and three types of biocrusts (cyanobacterial crusts, cyanobacterial-moss mixed crusts, and moss crusts) in a semiarid region of the Chinese Loess Plateau. Our results showed that the biocrusts were associated with significantly greater non-rainfall water deposition capacity (~13%–22%) in contrast to the bare soil, and biocrust type strongly influenced the daily non-rainfall water amount in the order: moss crusts > mixed crusts > cyanobacterial crusts > bare soil. Biocrusts were also associated with faster rates of non-rainfall water formation (42%; F ≥ 2.87, P ≤ 0.04), which may be linked to faster nighttime cooling in comparison to the bare soil. Atmospheric vapor condensation was the primary water source for non-rainfall water deposition at the 0–10 cm depth, as opposed to soil vapor condensation. Biocrusts had higher condensation from both sources, and had relatively more deposition from the atmosphere: atmospheric vapor condensation was greater by 114%-143% and soil vapor condensation was greater by 20%–30%. Moreover, >69% of the total non-rainfall water amount occurred in the top 3 cm of soil. The strong biocrust influence in the uppermost centimeters (F = 45.34, P < 0.001) appears to primarily drive the contrasts in non-rainfall water deposition in soils. Furthermore, all of the apparent effects of biocrusts on non-rainfall water deposition and distribution were reasonably attributed to the biocrust influences on soil physicochemical properties, especially the contents of fine particles, organic matter, high soil roughness, daily temperature difference, and moss morphology. In conclusion, biocrusts are associated with much greater non-rainfall water deposition capacity, and change non-rainfall water distribution along with soil depth, implying that they play a critical role in surface soil water balance of dryland ecosystems.