Quadratic relationships between the evaporation loss fraction and the root water uptake ratio in a wheat-maize rotation system

Quantifying crop water uses and non-productive water loss under irrigation conditions plays an important role in the rational distribution of water resources. The hydrogen and oxygen stable isotope methods have been used in agricultural studies to quantify root water uptake (RWU) patterns and soil evaporation losses (f). However, their relationships and mechanisms are still unclear. In this study, we used the Bayesian stable isotope mixing (MixSIAR) model to quantify RWU patterns, and the Craig-Gordon model to quantify the evaporation loss of the winter wheat-summer maize rotation system under different irrigation conditions. Results of the MixSIAR model showed that 0–40 cm soil water was the main source of the two crops, contributing more than 50 % of soil water uptake. The crop water source changes were regulated by both root development and water availability, and this combined effect was enhanced by different irrigation treatments. The root water uptake ratio at 0–20 cm showed quadratic relationships with evaporation loss fraction. The f threshold point marks the breakpoint where root water uptake is affected by relative humidity or soil water content. The variation in f between wheat and maize is related to the physiological regulation of water in different species. It could be used to monitor plant water deficit status, and to distinguish root-distribution-dependent and soil–water-availability-dependent root water uptake patterns. Our research investigates the mechanisms behind changes in crop water use patterns, which will aid in developing more rational crop rotation systems.