Differential soil moisture pulse uptake by coexisting plants in an alpine Achnatherum splendens grassland community

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 Qinghai-Tibetan 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 delta D 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