Quantifying the water sources of Camellia oleifera during fruit growth peak period using hydrogen and oxygen isotopes

Abstract: Water is a key factor for plant growth and development. Quantifying water sources is one of the most important steps to effectively manage the irrigation of forests during the fruit peak growth period in hilly areas. However, the research on the water use of different fruit growth peaks is still lacking on Camellia oleifera. In this study, a systematic investigation was conducted to quantify the water source of Camellia oleifera forest during the fruit peak growth period using hydrogen and oxygen isotopes. The precipitation, soil, and Camellia oleifera samples were also collected from April-June (the first fruit growth peak) and July-August (the second fruit growth peak) in 2019 and 2020, where the soil sampling depth was 1 m. The soil layer was divided into three layers: 0-30, >30-60, and >60-100 cm, according to the distribution of root and soil water content. Taking Young Camellia oleifera aged 3 to 5 years as subjects, the isotopic composition of xylem and soil water was compared in the active layer of the root system at the peak of fruit growth. Linear mixed and the Bayesian hybrid (MixSIAR) models were used to quantify the water absorption source of the root system. The results indicate that the Local Meteoric Water Line (LMWL) was δD=8.29δ18O+12.99, R2=0.99 during the test period, where the soil and plant water isotope were concentrated near the LMWL. Most isotopes of xylem water were distributed in the range of soil water isotope values, where the soil water was the direct water source. The isotope value of xylem water basically intersected with that of soil water of 0-100 cm. The intersection depth mainly intersected 0-30 cm during the growth peak of the first fruit, and then increased gradually during the growth peak of the second fruit. It inferred that the water source in the first peak period came from the 0-30 cm soil layer, whereas, the water in the deeper soil layer was used in the second peak period. The model results showed that Camellia oleifera presented similar water utilization strategies in the two fruit growth peaks in 2019 and 2020. Specifically, the soil water in the 0-30 cm soil layer was mainly used at the peak of fruit growth. The contribution rates of 0-30, >30-60 and >60-100 cm soil layers were 51.3%, 28.2% and 20.5%, respectively. The water absorption depth increased in the second fruit growth peak, compared with the first growth peak. For example, the results of MixSIAR model showed that the utilization rate of 0-30 cm soil layer decreased by 19%, while the utilization rate of >30-60 and >60-100 cm two soil layers increased by 12% and 8%, respectively. The contributions of three soil layers (unit volume) to water absorption of root were 149.6, 81.1, and 58.7 mm, respectively. There was only a slight difference to simulate the contribution proportion of soil water in each soil layer using linear mixing and the MixSIAR model. This finding can provide a sound reference to formulate a suitable irrigation system for young Camellia oleifera in the southern hilly region. Correspondingly, the forest water and fertilizer construction can also be integrated for the healthy development of Camellia oleifera.