Effect of deep-rooted plant species on 15Nitrogen uptake and herbage yield in temporary agricultural grasslands

Aims: Increase of plant diversity has been suggested to enhance grassland productivity and resource use efficiency. Most studies on agricultural grasslands have focused on functional diversity of mixtures comprising legumes and non-legumes, but there is little knowledge of plant nutrient acquisition from deep- and shallow-rooted grassland plant species. To investigate whether deep-rooted (chicory: Cichorium intybus L.; Lucerne: Medicago sativa L.) and shallow-rooted (perennial ryegrass: Lolium perenne L.; white clover: Trifolium repens L.) grassland plant species differ in herbage yield and depth dependent soil N-access, we investigated in the field if 1) a mixture comprising shallow- and deep-rooted grassland plant species has greater herbage yields than a shallow-rooted binary mixture and pure stands, 2) deep-rooted grassland plant species (chicory and lucerne) are superior in terms of accessing soil N from 1.2 m soil depth compared with shallow-rooted plant species, 3) shallow-rooted grassland plant species (perennial ryegrass and white clover) are superior in terms of accessing soil N from 0.4 m soil depth compared with deep-rooted plant species and 4) a mixture of deep- and shallow-rooted plant species has access to greater amounts of soil N compared with a shallow-rooted binary mixture. Method: A 15N tracer methodology with 15N enriched ammonium-sulphate placed at three different soil depths (0.4, 0.8 and 1.2 m) was applied to determine the depth dependent soil N-access measured as plant 15N-uptake in pure stands, two-species and four-species grassland plant communities. Important findings: The study showed that herbage yield of the four-species mixture including deep- and shallow rooted grassland plant species was generally greater than both the pure stands and the two-species mixture, besides for lucerne in pure stand. This positive plant diversity effect in the four-species mixture on above-ground herbage yield could not be explained by complementary soil 15N uptake from 0.4, 0.8 and 1.2 m soil depths, even though chicory indicated deep soil 15N uptake. Perennial ryegrass demonstrated relatively deep soil 15N uptake when grown in pure stand, but showed increasing shallow 15N uptake from grown in a two-species to a four-species mixture. Total soil 15N uptake from three soil depths of a mixture 51 comprising two deep-rooted and two shallow-rooted plant species was not greater compared with a shallow-rooted two-species mixture. 15Nitrogen uptake from 1.2 m may have been too small to determine any differences. Legumes stimulated perennial ryegrass in 15N uptake from shallow soil layers, which indicated greater total 15N uptake of mixtures compared with pure stands