Difference in Acquisition of Soil Organic Nitrogen between Pak-Choi and Tomato

This study was to elucidate the factors contributing to organic nitrogen (N) acquisition in different plant species, pak-choi (Brassica rapa L. var. chinensis) and tomato (Solanum lycopersicum L.). Two hypotheses were proposed and examined: 1) higher protease activity in rhizosphere contributes to the ability to utilize organic N in soils; 2) the differences in ability to absorb amino acids are responsible for the different characteristics of organic N utilization. Seedlings were grown in soils mixed with vermiculite with different N sources (no N (-N), ammonium sulfate (AS), and cattle farmyard manure (CM)). Plant growth and N accumulation were the best in the AS treatment in both species. When comparing the growth and N accumulation between -N and CM treatments, those of pak-choi were significantly enhanced in the CM treatment, but no difference was found in tomato. In the rhizosphere soils, the highest protease activities were detected in the CM treatment irrespective of plant species. The correlation analysis between rhizospheric protease activity and plant total N accumulation in the -N and CM treatments showed that a significant positive correlation only in pak-choi. The determination of amino acid absorption rate in excised roots indicated that glycine was taken up at significantly higher rate in pak-choi than in tomato. Taken together, two possible factors for the acquisition of organic N was suggested from this study: protease activity in rhizosphere and ability to absorb amino acids