Possible direct uptake of organic nitrogen from soil by chingensai (Brassica campestris L.) and carrot (Daucus carota L.)

On comparing the nitrogen uptake of four different kinds of vegetables, i.e., pimento, leaf lettuce, chingensai (a kind of Chinese cabbage), and carrot, from soil to which rapeseed cake (RC) or ammonium sulfate (AS) were applied at the same N concentration, different N uptake responses were observed. Chingensai and carrot took up more N from the soil with applied RC than with applied AS. A smaller amount of inorganic N was detected in the soil with applied RC than the one with applied AS. On the other hand, pimento and leaf lettuce grew better on the soil with applied AS than on that with applied RC. A possible explanation for the superior N uptake by chingensai and carrot in the soil with applied RC could be the direct uptake of organic N, especially a protein-like N compound with a uniform MW of 8000–9000 Da, that accumulated in the soil with applied RC. In order to support this hypothesis, two typical vegetables were examined: chingensai, which responds better to organic N, and pimento, which responds better to inorganic N. Xylem sap was collected from these plants and analyzed using size-exclusion high pressure liquid chromatography (HPLC). In xylem sap of chingensai grown in the soil with applied RC, a peak similar to that found in the soil solutions extracted with phosphate buffer was detected on the chromatogram, while this peak was absent from the chromatograms of chingensai grown in inorganic nutrients culture solution. In contrast, there were no similar peaks for the xylem sap of pimento grown in the soil with applied RC. Further, when chingensai, carrot, and pimento were cultivated in an N-free medium under aseptic conditions, the N uptake of chingensai and carrot increased with the addition of a soil solution extracted by phosphate buffer, while that of pimento did not increase. These results strongly suggest that the superior N uptake response to the application of organic material in chingensai and carrot might be related to the direct uptake of organic N from the soil.