A. According to the analysis of independent variables and dependent variables in the experiment, the purpose of this experiment is to explore the effects of soil water content and nitrogen fertilizer treatment on photosynthetic rate and chlorophyll content of paulownia. A is correct;

B, the independent variables of this experiment are soil water content and whether nitrogen fertilizer is applied, which has nothing to do with plant position. The rotation of plant position every 10 day is to control the consistency of irrelevant variables in the experiment, and B is correct;

Compared with 1 group, the independent variable is whether to apply nitrogen fertilizer, the second group applies nitrogen fertilizer, N element is the raw material of chlorophyll synthesis, and the second group applies nitrogen fertilizer, which increases chlorophyll content and enhances photoreaction, with high photosynthetic rate and correct C;

Compared with the three groups, the independent variable whether nitrogen fertilizer is applied or not, the chlorophyll content in the four groups increases, but the photosynthetic rate decreases, because the soil water content is low, and the nitrogen fertilizer application in the four groups leads to the high concentration of soil solution, which hinders the water absorption of plant root cells and affects photosynthesis, and D is wrong.

Therefore, choose: d.