Uptake of organic nitrogen in the field by four agriculturally important plant species

Uptake of glycine was studied in four plants commonly used in grasslands in northern Europe (Phleum pratense, Trifolium hybridum, T. pratense, and Ranunculus acris) and compared to uptake of ammonium and nitrate. The experiment was conducted in the field, but with plants transferred to pots with soil 8-10 d before the start of the experiment. Plant uptake of U-(C2N)-C-13-N-15 glycine, (NH4+)-N-15, and (NO3-)-N-15 was studied by injecting dilute (1 mmol/L) solutions of respectively labeled N source into the pots and harvesting plants 21 h later. Measurements of C-13 and N-15 in roots showed that, in all plants, part of the glycine N was taken up in the form of intact amino acid. Hence, regressions of plots of excess C-13 against excess N-15 showed that a minimum of 19-23% of the glycine-derived N was taken up as intact amino acid; possible losses of labeled C atoms of glycine during its metabolism in the plants implies that these estimates are conservative. Uptake of the different N sources was similar in the two Trifolium species, while rates of nitrate uptake were comparably high in P. pratense, and rates of glycine uptake were comparably low in R. acris. N-15 labeling of shoots was detected in all species, whereas significant levels of C-13 tracer was only found in shoots of P. pratense. It is concluded that a capacity for uptake of organic N exists also in an agricultural setting, despite the rapid turnover of organic N usually found under such conditions. This adds to the growing knowledge of plant utilization of organic N sources in natural ecosystems and stresses the need for reexamining this step in the biogeochemical N cycle.