By using rotatable central composite design（RCCD） for a pot experiment with three factors,five levels,twenty runs,we determined leaf δ^13C,nitrogen content（LN） and phosphorus content（LP） of Catal- pa bungei seedling（clone 004-1） under different treatments in June and July. Mathematical model was es- tablished for analysis of main factor effects,monofactor effects, and the combined effects of W（soil water）, N（nitrogen） and P（phosphorus） on leaf δ^13C,also,LN and LP were correlated with leaf δ^13C. The results showed that： （1）monthly dynamic of leaf δ^13C of C. bungei seedlings was obviously found in the study,and the average leaf δ^13C（average measurements in June, July and August） of 20 treatments ranges from --29.12‰ to --26.60‰. （2）W had a significantly negative efirect on leaf δ^13C, and N had a significantly positive effect on leaf δ^13C, with a size sequence of N〉W, whereas P had no obvious effect on leaf δ^13C. Monofactor effects analysis results showed that the changes of leaf δ^13C with W or N increasing both showed a parabolic trend： first increasing and then decreasing. Interaction of W X N had obvious effect on leaf δ^13C of C. bungei seedling in July,but no significant effect on leaf δ^13C of C. bungei seedling in August. Moreover,the response surface on interaction between W and N indicated that leaf δ^13C increased gradually with W decreasing and N increasing. （3） Leaf δ^13C showed a significantly positive correlation with LN in July and August（P〈0.01;n=100）, whereas the correlation between leaf δ^13C and LP was not significant （P〉0.05 n= 100）. In general, leaf δ^13C of C. bungei seedling was observed to change dynamically along with the growth period,and be negatively affected by W and W）〈 N and positively affected by N, thus, it could be promoted by the rational method of irrigation and fertilizer application.
Acta Botanica Boreali-Occidentalia Sinica
coupling effects of soil water and fertilizer application
carbon isotope composition （ δ^13C）