Dynamics of water uptake by maize on sloping farmland in a shallow Entisol in Southwest China

•Recent rainwater didn't well mix with the “old” soil water.•Immobile soil water suffered strong evaporation.•Maize seedlings acquired immobile soil water at 0-10 cm.•Maize is capable of using more deep soil water as roots grow.•Maize didn't equally use the same ratio of immobile and mobile soil water.

The water use patterns of maize grown in shallow soils remain poorly understood. To explore the water uptake dynamics of maize from a loamy Entisol with an average thickness of 40 cm, excavation, isotopic tracer analysis and soil water potential measurements were combined to study the source of water used by maize. The differences between the δD of the water used by maize and the δD of the mobile soil water (MSW, the fraction of soil water with high mobility which can be easily replaced by the infiltrating rainwater) indicated that maize did not take up much MSW. The local meteoric evaporation line, the δD of the bulk soil water (BSW, total soil water including mobile and immobile soil water) and the soil water collected using a lysimeter were used in a model to calculate the isotopic compositions of different fractions of soil water and the proportion of immobile soil water (ImSW, the fraction of soil water with little mobility which was tightly bound to the soil particles). ImSW resulted from several heavy rains that occurred before the sampling. The primary water sources for maize varied temporally and spatially. Maize seedlings at the one-leaf stage used ImSW from 0 to 10 cm soil depth; however, maize plants generally used more BSW from deeper soil layers when the roots reached greater depths. The ratios of MSW to ImSW were not equal between the maize stems and soil, with more ImSW in the maize stems, particularly during the seedling stage. This result invalidates the core concept of most watershed hydrology models and classical hypothesis in the isotopic models of general atmospheric circulation. The difference between the MSW and ImSW in the water cycle of the soil-plant-atmosphere continuum should be considered in the future studies of identifying the plant water sources and modeling hydrological processes.

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