The authors explored the risks and benefits of repeated irrigation of Populus alba saplings with aqueous paper sludge (APS). Saplings were cultivated in pots of forest soil (3 L) in a greenhouse for 7 weeks and watered twice a week with differing concentrations of APS (0, 10, 20, 30, 50, 75, and 100 % v/v with deionized water). Plant growth and ecophysiological variables along with zinc and aluminum transfer were monitored. A stimulation of plant growth was observed with sludge treatments of 30 or 50 %, significantly correlated to APS input (r = 0.81). This may be explained by the easily available nitrogen as is shown with the positive correlation of CO2 assimilation and leaf nitrogen (r = 0.70). However, a significant reduction in plant growth was observed when treatments of 75 and 100 % of APS were administered, despite a high nutritional level (nitrogen and phosphorus). The study suggests that APS concentrations from 30 to 50 % may positively affect the growth of poplar saplings; however, the higher concentrations indicated a risk for plant growth and the environment.