Abstract: Aims The research on dynamic niche partitioning of soil resource uptake is crucial for the understanding of plant coexistence mechanisms. However, there are still knowledge gaps in the interpretation of this field. Methods In the growing season of 2019, a mature mixed forest of Populus tomentosa and Robinia pseudoacacia on the North China Plain was repeatedly sampled for water isotopes and soil water content, and fine root sampling was performed at the end of the growing season. Seasonal water uptake patterns of trees were determined by the hydrogen-oxygen stable isotope method and Bayesian mixture model (MixSIAR). The degree of niche overlap between P. tomentosa and R. pseudoacacia was assessed by Pianka’s normalized overlap value. Important findings Both tree species have deep root systems. Nevertheless, P. tomentosa tended to de-velop horizontal lateral roots and a higher proportion of fine roots were distributed in the shallow soil layers (0–30 cm). In contrast, R. pseudoacacia inclined to develop vertical taproots and a high proportion of fine roots were distributed in the deep soil layers (100–600 cm). In terms of the mean values of the whole growing season, the primary water sources for P. tomentosa and R. pseudoacacia were water from the middle (30–100 cm) and deep soil horizons. However, the contribution of water from shallow and middle soil horizons to the water uptake of P. tomentosa was higher than that of R. pseudoacacia, while the opposite was true for deep soil water and groundwater for the two species. P. tomentosa and R. pseu-doacacia showed completely opposite water uptake strategies in response to drought and heavy summer rainfall. During the dry season, P. tomentosa enhanced the water uptake contribution of the middle soil layer, while R. pseudoacacia pomoted the relative water uptake from the groundwater. When heavy rainfall events occurred, P. tomentosa increased the water uptake contribution of the shallow soil layer, while R. pseudoacacia increased the water uptake contribution of the deep soil layer. In conclusion, there was niche complementarity in the fine root and water uptake between P. tomentosa and R. pseudoacacia. Furthermore, the degree of niche complementation in water uptake varied with seasons, and the wa-ter-uptake niche complementation degree in the dry season was relatively higher than that in other seasons. In addition, the study also showed that the root niche partitioning was not representative of the water uptake partitioning of trees. This study provides support for further understanding of plant coexistence mecha-nisms, and will provide an important reference for the formulation of future mixed forest management strategies to cope with climate change.

Key words: dynamic water uptake niche, fine root niche, Populus tomentosa, Robinia pseudoacacia, mixed forest, the North China Plain