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The pulp and paper industry is a very large consumer of fresh water,and almost all the input reappears as effluent requiring treatment and disposal.As poplar can recycle organic residues from wastewater due to its high water and nutrient use,paper mill effluent(PME) could serve as a source of water and nutrients for poplar plantations.Consequently,irrigating poplar plantations with PME will reduce the need for other inputs of irrigation and fertilization.However,some elements(e.g.Na) which commonly occur in PME also represent a potential risk to poplar growth,since they have only moderate tolerance to salinity.Improper irrigation can result in toxicities and nutrient imbalance in plants.We hypothesized that diluting PME with pure water to a suitable concentration could avoid negative effects and would have better irrigation and fertilization value.In this study,a pot experiment was conducted from June to September in 2009 to investigate the effect of irrigation with different dilutions of PME on(1) the growth and nutrient status of one-year-old triploid Populus tomentosa seedlings and(2) soil chemical properties.The experiment included five PME irrigation treatments which diluted the concentration of PME to 12.5%(1F7Q),16.7%(1F5Q),25%(1F3Q),33.3%(1F2Q) and 50%(1F1Q),respectively.A control pure water irrigation treatment(CK) was also included.At the end of the experiment,height,ground-level diameter,and biomass of the seedlings in all treatments were measured as indicators of plant growth,total N and P of stems,leaves,and roots were analyzed to indicate plant nutrient status,and soil chemical properties(pH,organic matter,total N,available N and P) were also measured.PME irrigation had no significant effect on the pH and available P content of the soil(P0.05),but percentages of organic matter,total N,and available N increased significantly with increasing concentration of PME(P0.05).With appropriate dilution,PME irrigation significantly increased the N content in roots and leaves and the P content in stems(P0.05),but had no significant influence on the P content in leaves and roots and the N content in stems(P0.05).The growth of P.tomentosa seedlings increased markedly when the concentration of PME increased from 12.5%(1F7Q) to 16.7%(1F5Q),but when the concentration continued to increase(1F3Q,1F2Q,1F1Q),increments in ground-level diameter and height decreased gradually.Relative to the CK treatment,the increment of ground-level diameter and height of P.tomentosa seedlings following only the 1F5Q treatment were significantly higher(102% and 47% respectively,P0.05).Biomass of P.tomentosa seedlings in the 1F5Q and 1F3Q treatments reached 247 and 230 g,respectively,which were significantly higher than the CK treatment(19.3% and 11.1%,P0.05).However,no significant difference was detected between the other PME treatments and CK(P0.05).In conclusion,PME irrigation promoted the growth of P.tomentosa seedlings,and increased plant nutrient status and soil fertility.When the dilution concentration of PME was appropriate,the growth of P.tomentosa seedlings was significantly improved by PME irrigation.Consequently,our findings suggest that PME can be used to irrigate P.tomentosa seedlings,but it should be diluted to an appropriate concentration.From our results,it is recommended that the dilution concentration of PME should be 16%—25%,and trials are needed to validate these conclusions under field conditions.