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The niche differentiation of resources among coexisting plants commonly reflects the fundamental functions of plant coexistence in water-limited ecosystems. However, the dynamics of water use patterns by coexisting plants that respond to soil moisture pulses are scarcely known in the semiarid alpine ecosystems of the QinghaiTibetan Plateau, particularly for deep-rooted grasses such as Achnatherum splendens in the Qinghai Lake watershed. Hence, we used the stable deuterium isotope method to detect the potential water sources for A. splendens during two growing seasons of 2013 and 2014. Our results indicate that SWC and dD values of shallow soil layer (0-10 cm) showed the highest variations in comparison with those in other soil layers during the growing seasons due to the combined effect of evaporation and precipitation inputs. A. splendens depended largely on shallow soil water availability at the early growing season. At the peak of the growing season, this deep-rooted grass A. splendens and shallow-rooted grass Leymus chinensis showed a high degree of response in the water use source to the changes in soil moisture pulses and shifted their water source from shallow to deep soil layer because water in the shallow soil layer became less available due to a long-time rainless days with strong evaporation effect on the shallow soil layer. In contrast, shallow soil water was utilized by all coexisting plants owing to an abrupt increase in shallow SWC with large events or long-lasting small events. At the late growing season, A. splendens used water from the shallow (0-10 cm) and middle soil layer (10-30 cm), while L. chinensis mainly relied on the shallow soil layer water. Comparatively, shallow-rooted herbs (Heteropappus altaicus and Allium tanguticum) predominantly used water from the shallow soil layer (0-10 cm) over the entire growing seasons regardless of soil water availability. Overall, the contrasting water use patterns by coexisting plants demonstrate their adaptations to the fluctuations of soil moisture pulses in water-limited ecosystems.

The complex interactions between shrub traits, soil structure, and soil water dynamics are not well understood yet. This study investigated rainfall partition by C. microphylla L., spatial soil water pattern, soil hydraulic conductivity, and soil macropores to ascertain preferential water flow to deep soil layer by shrub. Results indicated that high variability in throughfall existed within individual shrub stand: average coefficient of variation was 0.36 ± 0.13 for shrub and 0.15 ± 0.13 for interspace grass. Throughfall was less at the center of the shrub patch (30–60% of rainfall) than the outward positions at the edges of the canopy (70–90% of rainfall). Soil water responded differently to rainfall, soil depth, and vegetation type and showed high variability within shrub patches and on the slope. Greater and deeper infiltration was observed beneath C. microphylla L. canopy than interspaces grass after rainfall with large amount and high intensity, suggesting that macropore flow dominated in shrub patches. X‐ray CT showed that macroporosity was over six times greater in soil under C. microphylla L. than interspace grass. Soil hydraulic conductivity for shrub at saturation and the pressure heads of −30, −60, and −150 mm were 3, 2, 2.5, and 2 times than those of grass, respectively. Shrub patches had a significant lower bulk density and higher porosity than grass patches at the top 0‐ to 30‐cm depth. Soil hydraulic conductivity was significantly correlated to organic matter content, total N, bulk density, and porosity. This study suggests that rainfall partition by shrub's canopy and subsurface soil macropores induced by root architecture results in preferential water flow into deep soil layer, which might favor competitive advantages for water by shrubs under arid conditions.