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.稳定氢氧同位素对土壤水分动态变化具有高度响应性，可作为追踪土壤-植物-大气连续体（soil-plant-atmosphere continuum, SPAC）内生态水文循环的关键指标。生物结皮（biocrusts）广泛分布于旱地生态系统中，在调控SPAC内土壤储水、水量收支与水分平衡方面发挥着关键作用，但其生态水文功能仍存在争议，尤其是在与蒸发、凝结、雨水入渗等引发水同位素分馏的相关过程中。 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。 本研究历时两年采集了生物结皮覆盖土壤与裸土样品，并对土壤水、雨水以及露水中的稳定氢氧同位素（²H和¹⁸O）的丰度与动态变化进行了分析。 研究结果显示，表层土壤（0~5 cm）水中的δ²H、δ¹⁸O及线条件过剩值（line-conditioned excess, lc-excess）会随降雨与气温波动发生变化。与裸土表层相比，生物结皮覆盖使表层土壤的δ²H和δ¹⁸O分别富集7.4‰（-21.3‰ 相较于 -28.7‰）和1.5‰（0.5‰ 相较于 -1.0‰），同时使线条件过剩值降低5.7‰（-42.3‰ 相较于 -36.6‰），表明生物结皮可显著加剧表层土壤的水同位素分馏作用。同样，在0~50 cm的绝大多数土壤剖面中均观察到生物结皮诱导的水同位素富集现象，δ²H和δ¹⁸O分别升高3.4‰和1.0‰。 生物结皮覆盖土壤中δ²H与δ¹⁸O的富集以及线条件过剩值的降低，与生物结皮层内升高的土壤水分含量和温度显著相关，这正是生物结皮调控土壤水同位素分馏的潜在机制。生物结皮覆盖的最表层土壤（0~5 cm）中86.5%的未分馏水来自5~10 cm的深层土壤水，而裸土最表层土壤水主要来自5~10 cm（58.9%）和10~20 cm（31.3%）的土层。 本研究结果凸显了生物结皮在加剧土壤蒸发、非降雨水分沉积、阻碍雨水深层入渗以及改变水汽扩散与吸附模式方面的关键作用，从而提升了我们对旱地生态系统生态水文过程中生物结皮功能的认知。