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

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.

稳定氢和氧同位素对土壤水分动态变化高度敏感，因此成为追踪土壤-植物-大气连续体（SPAC）中生态水文循环的关键指标。生物结皮，广泛存在于干旱生态系统，在调节SPAC中的土壤水分储存、收支和平衡方面发挥着至关重要的作用，然而其生态水文功能仍存在争议，尤其是在涉及水分同位素分馏过程，如蒸发、冷凝和雨水渗透等方面的机制。因此，同位素分析有望重新审视并阐明生物结皮在这些生态水文过程中的作用。在本研究中，对覆盖有生物结皮和无生物结皮的土壤样品进行了为期两年的采集，并分析了土壤水、雨水和露水中的稳定氢和氧同位素（2H和18O）的丰度和动态变化。我们的研究结果揭示了表层土壤（0-5厘米）水中的δ2H、δ18O和lc-excess（线条件过剩值）随降雨和气温波动而表现出变化。与裸露表层土壤相比，生物结皮覆盖增加了表层土壤中的δ2H和δ18O含量，分别提高了7.4‰（-21.3‰ vs. -28.7‰）和1.5‰（0.5‰ vs. -1.0‰），同时降低了lc-excess 5.7‰（-42.3‰ vs. -36.6‰），这表明生物结皮对强化表层土壤水分分馏具有显著影响。类似地，生物结皮引起的同位素富集现象也在0-50厘米的土壤剖面中观察到，δ2H和δ18O分别增加了3.4‰和1.0‰。生物结皮覆盖土壤中δ2H和δ18O的富集，以及lc-excess的降低，与生物结皮中增高的土壤水分和温度显著相关，这些因素作为调节生物结皮对土壤水分同位素分馏影响的潜在因子。在生物结皮覆盖的上层土壤（0-5厘米）中，未分馏水的大部分（86.5%）源自地下5-10厘米的土壤水，而最上层裸露土壤水主要源自5-10厘米（58.9%）和10-20厘米（31.3%）的土壤。我们的研究结果表明，生物结皮在增强土壤蒸发和非降雨水沉积、阻止深层雨水渗透、以及改变水蒸气扩散和吸附模式方面发挥着关键作用，这进一步加深了我们对于生物结皮在干旱生态系统生态水文过程中的作用的理解。