By using rotatable central composite design(RCCD) for a pot experiment with three factors,five levels,twenty runs,we determined leaf δ¹³C,nitrogen content(LN) and phosphorus content(LP) of Catalpa bungei seedling(clone 004-1) under different treatments in June and July.Mathematical model was established for analysis of main factor effects,monofactor effects,and the combined effects of W(soil water),N(nitrogen) and P(phosphorus) on leaf δ¹³C,also,LN and LP were correlated with leaf δ¹³C.The results showed that:(1)monthly dynamic of leaf δ¹³C of C.bungei seedlings was obviously found in the study,and the average leaf δ¹³C(average measurements in June,July and August) of 20 treatments ranges from -29.12‰ to -26.60‰.(2)W had a significantly negative effect on leaf δ¹³C,and N had a significantly positive effect on leaf δ¹³C,with a size sequence of N>W,whereas P had no obvious effect on leaf δ¹³C.Monofactor effects analysis results showed that the changes of leaf δ¹³C with W or N increasing both showed a parabolic trend:first increasing and then decreasing.Interaction of W×N had obvious effect on leaf δ¹³C of C.bungei seedling in July,but no significant effect on leaf δ¹³C of C.bungei seedling in August.Moreover,the response surface on interaction between W and N indicated that leaf δ¹³C increased gradually with W decreasing and N increasing.(3) Leaf δ¹³C showed a significantly positive correlation with LN in July and August(P<0.01;n=100),whereas the correlation between leaf δ¹³C and LP was not significant(P>0.05;n=100).In general,leaf δ¹³C 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.