| Summary: | Light conditions and nitrogen fertilizer are crucial for plant growth, especially in the underground situations without sunlight and nitrogen deposition. In this paper, the effects of photoperiod (12 h and 16 h lighting time per day), light intensity (200, 300 and 400 μmol m<sup>−2</sup> s<sup>−1</sup>) and nitrogen addition (0, 0.15, 0.3 and 0.45 g N kg<sup>−1</sup> soil) on pakchoi growth and specific soil enzyme activity were investigated. The results demonstrated that there were strong interactive effects of light intensity and nitrogen addition on plant yield. The plant yield changed parabolically with increasing nitrogen addition when a light intensity was given between 200 and 300 μmol m<sup>−2</sup> s<sup>−1</sup>, while the yield decreased linearly with increasing nitrogen application under the light intensity of 400 μmol m<sup>−2</sup> s<sup>−1</sup>. The combination of 16 h photoperiod, 300 μmol m<sup>−2</sup> s<sup>−1</sup> light intensity and 0.3 g N kg<sup>−1</sup> soil nitrogen addition was the best for pakchoi growth. The investigation of soil enzyme showed that the activity of urease responded negatively to nitrogen addition, whereas the activity of phosphatase had positive correlation with light intensity but was not affected by nitrogen addition. Our results suggested that the toxic effect of excessive nitrogen was a better explanation for the interactive effects of light and nitrogen than the plant-microbe interaction framework. The critical toxicity level of nitrogen for pakchoi was determined and showed negative correlation with light intensity.
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