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Abstract: In pot experiments using a rotatable central composite design, the effects of soil water content and fertilizer application, and their connection to seedling stem height (SH), ground diameter (GD), biomass yield (BY), and leaf area (LA), were examined in seedlings of Catalpa bungei clone 004-1 using response surface methodology. Our results indicated that nitrogen application (N) and the soil water content (W) had positive effects on all four growth parameters. The size sequence of the effects was N>W; however, phosphorus application (P) exerted significantly positive effects only on BY and LA. A monofactor effects analysis indicated that changes in the four parameters with increases in W, N, and P were all parabolic. Moreover, the interactions of W×N were found to have significantly positive effects on the four growth parameters, which initially increased and then decreased with increasing N when W was fixed, and with increasing W when N was fixed. The interaction of W×P had a significantly positive effect on SH. The optimized combination was determined by establishing a multi-objective decision model based on the four parameters, when the actual W, N, and P values reached 74.605% of the field capacity, 4.710 g・plant-1, and 2.009 g・plant-1, respectively. Under these conditions, C. bungei seedlings achieved maximum growth. These recommendations will conserve water and improve production, and they provide a basic theory for seedling culture and afforestation of C. bungei. In addition, our model fitting results showed that the biomass yield of fine roots (diameter ≤2 mm) was closely correlated with SH and GD.

Key words: Soil water and fertilizer; Growth; Catalpa bungei; Root; Response surface methododogy.