Nitrogen is an essential element for growth and development of plants and microorganisms. It was thought that organic N in soil must be mineralized into in-organic form before it is used by microorganisms. However, recent studies showed that small-molecule organic N such as those in amino acids, may be taken up directly as intact molecules or de-aminated extra-cellularly followed by the uptake of ammonium (NH₄) absorbed by plants and microorganisms and termed as Direct Rout. The ratio of direct use of organic small molecule N by Soil affects the quantitative characterization of N use efficiency and is one of the new hot spot in the research on N cycling in farmland ecosystem.

Associate Professor ZHANG Lili at Institute of Applied Ecology of Chinese Academy of Sciences recently tested the effect of available C (glucose) and N (ammonium sulfate) on absorption of amino acid by soil microorganism using a laboratory experiment with glycine, NH₄ and different levels of glucose.

The results show that the when Nitrogen source (NH₄) added alone, amino acid is used mainly by the direct route. When high level carbon source-glucose was also added, initially, the major route of N absorption was direct amino acid absorption because high amount of glucose strongly promoted the growth of the microbial biomass, resulting in increased C and N requirements. With consume of Carbon source, the route gradually shifted from direct uptake to N uptake after extracellular deamination followed by uptake of NH₄ or intracellular deamination and excretion of NH₄.

This study indicates that amino acids can not only supply N but also supply C for proliferation of soil microorganisms. In the future, more work should focus on the assimilation competition of organic N between plants roots and soil microorganisms.

The study was published in Geoderma.