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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.

Subsurface drip irrigation(SDI) is an intensive silvicultural practice which has the potential to increase productivity of triploid Populus tomentosa pulpwood plantations.In order to efficiently apply SDI in the cultivation of P.tomentosa pulpwood plantations,it is necessary to determine irrigation timing.A field experiment was conducted to investigate the effect of soil water potential(ψsoil) on the growth and physiological characteristics of 6—7 year old trees under SDI in a P.tomentosa pulpwood plantation.The experiment included three ψsoil treatments,which initiated irrigation when the ψsoil at 20 cm soil depth and 10 cm distance from a drip emitter reached-25,-50,and-75 kPa,respectively.A control non-irrigation treatment(CK) was also included.Meteorological factors,ψsoil,groundwater level,and tree growth were monitored hourly,daily,every 1—10 days,and monthly for two years,respectively.Soil water content at 20 and 50 cm depth,pre-dawn leaf water potential(ψpd),and trunk sap flow rate were measured in selected periods.Results showed that relative to CK,SDI could improve diameter,height,and individual volume increment of P.tomentosa.Irrespective of SDI treatment,SDI on average increased annual volume growth of the 6-and 7-year-old plantations by 24% and 28%,respectively over the CK treatment.Annual volume growth of the 6-year-old plantation following the-25 kPa treatment reached 39.90 m3 hm-2a-1,which was 43.5% higher than the CK treatment(P0.01).Relative to the-50 and-75 kPa treatments,annual volume growth in the-25 kPa treatment was 20% and 31% higher(P0.01) in the 6-year-old plantation,respectively,and 13% and 14% higher(P0.05) in the 7-year-old plantation,respectively.Relative to CK during the fast growing period(April—July) of P.tomentosa,SDI increased the soil water content at 20 and 50 cm depth by 35% and 27%,respectively;increased average daily trunk sap flow rate(SFmean) by 46%;and increased ψpd by 41%.The ψsoil threshold for initiating irrigation had no significant effect(P0.05) on the SFmean and ψpd of P.tomentosa.However,among the three ψsoil treatments,the average SFmean and ψpd values were highest following the-25 kPa treatment.The increase of soil water availability in the 0—20 cm soil layer(i.e.,active rooting zone) was also greatest in the-25 kPa treatment.These results may explain why the-25 kPa treatment had the highest tree growth rate among the three ψsoil treatments.Overall,the observed increase in soil water content,SFmean,and ψpd indicate that these are the mechanisms by which SDI significantly improves P.tomentosa tree growth.In conclusion,our findings suggest that SDI should be promoted in the cultivation of P.tomentosa pulpwood plantations.To realize optimum tree growth,irrigation should be initiated when the ψsoil at 20 cm depth and 10 cm distance from a drip emitter reaches-25 kPa.Furthermore,it is recommended that irrigation should be applied between April and July,and terminated between August and October when planting P.tomentosa in regions similar to our experimental plantation.