Concentration affects plant uptake of inorganic and organic forms of N

It is now recognized that plants take up N from the soil in three forms: nitrate, ammonium, and amino acids (dissolved organic N). Although scientific evidence on plant uptake of amino acids has existed in the last few decades, it is only recently that the contribution of amino acids to plant nutrition has been recognized (see Warren 2009 and literatures cited). So the traditional view that organic N has to be mineralized first into nitrate and ammonium in order to be available to the plant is not anymore valid.

Different plant species vary in their preference for N forms. For instance, early successional plant species are known to have a higher capacity for nitrate uptake than late successional species. Uptake of N in the form of ammonium and amino acids is thus more important for the latter species. In a recent study to test the hypothesis that substrate concentration affects plant preference for N forms, Warren (2009) used the herb Ocimum basilicum and the evergreen tree Eucalyptus regnans. He placed roots of intact seedlings in equimolar mixtures of nitrate, ammonium and glycine (amino acid). His results revealed that substrate concentration influenced the preference of both plants for N forms. This means that whether the plant prefers one N form over another (e.g. nitrate over ammonium and amino acid or vice versa) depends on their concentrations in the growth medium or soil.

Reference

Warren CR. 2009. Does nitrogen concentration affect relative uptake rates of nitrate, ammonium, and glycine? J. Plant Nutr. Soil Sci. 172: 224-229.