Enriched stable hydrogen and oxygen isotopes in biocrusts unveil their critical roles in mediating ecohydrological processes of drylands

Description

Stable hydrogen and oxygen isotopes are highly responsive to soil moisture dynamics, making them vital indicators for tracing ecohydrological cycles within the soil-plant-atmosphere continuum (SPAC). Biocrusts, prevalent in dryland ecosystems, play a critical role in mediating soil water storage, budgets, and balances within the SPAC, yet their ecohydrological function remains controversial, especially regarding the processes contributing to water isotope fractionation such as evaporation, condensation, and rainwater infiltration. Thus, isotope analyses hold promise for revisiting and clarifying biocrust role in these ecohydrological processes. In this study, samples of biocrusts-covered and bare soils were collected over two years, and the abundance and dynamics of stable hydrogen and oxygen isotopes (2H and 18O) within soil water, rainwater, and dew were analyzed. Our results revealed that the δ2H, δ18O, and lc-excess (line-conditioned excess value) within surface soil (0–5 cm) water exhibited variations in response to rainfall and air temperature fluctuations. Compared to bare surface soil, biocrust cover enriched the δ2H and δ18O within surface soil by 7.4‰ (–21.3‰ vs. –28.7‰) and 1.5‰ (0.5‰ vs. –1.0‰), respectively, while it reduced the lc-excess by 5.7‰ (–42.3‰ vs. –36.6‰), indicating a significant effect of biocrusts on intensifying surface soil water fractionations. Similarly, biocrusts-induced enrichment of water isotopes was also observed across most of the 0–50 cm soil profile, with δ2H and δ18O being increased by 3.4‰ and 1.0‰, respectively. These enrichments of δ2H and δ18O in biocrusts-covered soil, as well as the decrease in lc-excess, were significantly correlated with the elevated soil moisture and temperature in biocrusts, serving as underlying factors mediating biocrust effects on soil water isotope fractionation. The majority (86.5%) of the unfractionated water within biocrusts-covered uppermost soil (0–5 cm) was derived from the subsurface 5–10 cm soil water, while the uppermost bare soil water was mainly derived from 5–10 cm (58.9%) and 10–20 cm (31.3%) soil. Our findings highlight the critical roles of biocrusts in intensifying soil evaporation and non-rainfall water deposition, preventing deep rainwater infiltration, and modifying the patterns of water vapor diffusion and adsorption, which advance our understating of biocrusts’ role in ecohydrological processes of dryland ecosystems.

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