Deep N acquisition in cultivated grasslands: Uptake of slow-release 15 N-labeled ammonium in hemiboreal monospecific leys

Aims To develop a methodology to study uptake and redistribution by plants of NH 4 + from deep soil, applying it to investigate deep root N uptake by cultivated grassland species. Methods A slow-release 15 NH 4 + label adsorbed to clinoptilolite was placed into soil (depth 42 cm) well below the densest root zone in well-established monospecific stands of five grass and two clover species. Species showing a variety of deep rooting patterns, N acquisition strategy, forage qualities, and persistence in hemiboreal conditions were chosen. The label was placed in early spring and tracked throughout one or two growing seasons in two repeated experiments. Results After two growing seasons ~ 90% of the label was tracked in the soil and harvested herbage of grasses, less in clovers. Deep N uptake was limited in spring, increased during mid-season, and was strongest in autumn in all species, despite lower herbage yield in autumn. Species differed in ability to recover and maintain 15 N in the soil–plant system. In one growing season, Lolium perenne L., Phleum pratense L., Schedonorus pratensis (Huds.) P.Beauv. and Schedonorus arundinaceus (Schreb.) Dumort herbage recovered ~ 65% of the label, Poa pratensis L. 54%, and Trifolium pratense L. and Trifolium repens L. 36–48%. Label transport to topsoil was observed, mainly attributable to plant nutrient redistribution rather than physical diffusion. Conclusions The innovative slow-release 15 N label enabled tracing species differences and seasonal changes in uptake of NH 4 + from deep soil. Among the tall-growing grasses, growth vigor appeared as important for deep N uptake as expected root depth.