Temporal variation of water-use efficiency and water sources of Phragmites australis in the coastal wetland: Based on stable isotopic composition (d(13)C and d???????O-18)

Water-use characteristics are important aspects of plant adaptation to environmental factors. While water-use efficiency (WUE) and water sources used by plants in arid habitats are commonly studied, water-use strategies of plants in coastal wetlands are poorly studied. Although ample water is available in coastal wetlands, water conditions vary among seasons, suggesting that the plant in coastal wetlands may need to adapt to seasonal changes in water availability. To uncover temporal variation in plant water-use strategies in the coastal wetland, we investigated WUE and water sources used by Phragmites australis in the tidal marsh of the Yellow River Delta employing stable isotopic composition indicators. Temporal variation of delta C-13 and delta O-18 in leaf organic matter were measured in two years (2018 and 2019) along with plant traits and environmental factors, to determine the factors affecting delta C-13 and delta O-18 in leaf organic matter. Water sources (rainfall, soil water in different depths, tidal water, and groundwater) were sampled for delta O-18 composition in the early growth season (2019 May, June, and July) to determine water sources using the Bayesian mixing model MixSIAR. We found that P. australis had high WUE in spring and autumn, but WUE declined during summer. Changes in WUE were explained by variation in plant height, leaf nutrient concentrations (C: N, total N, total S), leaf traits (leaf length and leaf number), soil electronic conductivity (EC), soil pH, soil nutrients (NO3-N at 0-30 cm soil depth, total S at 0-30 cm soil depth, available P at the soil depths of 0-10 cm and 40-50 cm, and total C at the 30-40 cm soil depth). Spring drought may enhance WUE through increasing soil salinity (i.e., EC) and lowering pH. Water uptake in the early growth season was composed mainly of rainfall, soil water, and groundwater. However, tidal water (i.e., ocean water) was not a main water source. To sum up, leaf WUE varied with seasons in the coastal wetland, which may be related to plant traits and soil factors, particularly salinity and pH. The plant in the tidal marsh may not take up ocean water, even though it is really abundant. Precipitation is still an important water source promoting growth of these plants in the coastal wetland, therefore, to conserve them, supplementing fresh water in the drought season is suggested