Does nitrogen and phosphorus addition alter water use traits of Moso bamboo (Phyllostachys edulis (Carrière) J. Houz.) forest in the fast-growing period in subtropical China?

Nitrogen (N) and phosphorus (P) fertilizers are widely applied in Moso bamboo (Phyllostachys edulis) forest to enhance productivity; however, their roles in regulating bamboo water-use traits remain poorly understood, particularly the interactive effects of N and P across phenological stages during the fast-growing period, including the shoot-culm coexistence stage (stage I), height development (stage II), and crown development (stage III). We conducted a completely randomized block experiment with four fertilization treatments (control, N, P, and combined NP) in a Moso bamboo forest in Zhejiang Province, China. We quantified sap flow rate (SF), diurnal and nocturnal water use (Qday and Qnight), stand transpiration (Tr), instantaneous water-use efficiency (WUEleaf), and their environmental and physiological controls. Water-use responses to nutrient addition were strongly stage-dependent and mediated by meteorological conditions. Nitrogen or phosphorus addition alone did not significantly affect SF at any stage; however, combined NP addition significantly increased SF during stage II. Phosphorus additional alone did not significantly influence Tr at any stage, whereas NP addition significantly increased Tr during stage II and III, indicating synergistic N × P effects. Nutrient addition significantly reduced WUEleaf during stages II and III, while NP addition increased LAI during stage III. Solar radiation, air temperature, and vapor pressure deficit were major regulators of stand transpiration, with stronger effects observed during stage III. These findings highlight the interactive roles of nutrient availability and environmental drivers in regulating bamboo water use and suggest that long-term effects of fertilization on bamboo hydraulic functioning warrant further investigation.