Uptake of intact amino acids by plants depends on soil amino acid concentrations

Studies in different ecosystems have shown that plants take up intact amino acids directly but little is known about the influence of free amino acid concentrations in the soil on this process. We investigated the effect of three different soil amino acid N concentrations (0.025, 0.13 and 2.5 μg N g−1 soil) on direct uptake of four dual labelled (15N, 13C) amino acids (glycine, tyrosine, lysine, valine) in a greenhouse experiment using Anthoxantum odoratum as a model plant.Our results revealed that 8–45% of applied 15N was incorporated into plant root and shoot tissue 48 h after labelling. Additional 13C enrichment showed that 2–70% of this incorporated 15N was taken up as intact amino acid. Total 15N uptake and 15N uptake as intact amino acids were significantly affected by soil amino acid N concentrations and significantly differed between the four amino acids tested.We found a positive effect of soil amino acid concentrations on uptake of mineralized 15N relative to amino acid concentrations for all amino acids which was presumably due to higher diffusion rates of mineralized tracer to the root surface. However, intact amino acid uptake relative to amino acid concentrations as well as the proportion of total 15N taken up directly decreased with increasing soil amino acid N concentrations for all amino acids, irrespective of their microbial degradability. This effect is most likely controlled by the mineral N concentration in soil and perhaps in plants which inhibits direct amino acids uptake.Overall, we conclude that plant internal regulation of amino acid uptake controlled by mineral N is the main mechanism determining direct uptake of amino acids and thus a lower contribution of intact amino acid uptake to the plants N nutrition has to be expected for higher amino acid concentrations accompanied by mineralization in soil.