Background and aims We carried out field experiments to investigate if an agricultural grassland mixture comprising shallow- (perennial ryegrass: Lolium perenne L.; white clover: Trifolium repens L.) and deep- (chicory: Cichorium intybus L.; Lucerne: Medicago sativa L.) rooting grassland species has greater herbage yields than a shallow-rooting two-species mixture and pure stands, if deep-rooting grassland species are superior in accessing soil 15N from 1.2 m soil depth compared with shallow-rooting plant species and vice versa, if a mixture of deep- and shallow-rooting plant species has access to greater amounts of soil 15N compared with a shallow-rooting binary mixture, and if leguminous plants affect herbage yield and soil 15N-access. Methods 15N-enriched ammonium-sulphate was placed at three different soil depths (0.4, 0.8 and 1.2 m) to determine the depth dependent soil 15N-access of pure stands, two-species and four-species grassland communities. Results Herbage yield and soil 15N-access of the mixture including deep- and shallow-rooting grassland species were generally greater than the pure stands and the two-species mixture, except for herbage yield in pure stand lucerne. This positive plant diversity effect could not be explained by complementary soil 15N-access of the different plant species from 0.4, 0.8 and 1.2 m soil depths, even though deep-rooting chicory acquired relatively large amounts of deep soil 15N and shallow-rooting perennial ryegrass when grown in a mixture relatively large amounts of shallow soil 15N. Legumes fixed large amounts of N2, added and spared N for non-leguminous plants, which especially stimulated the growth of perennial ryegrass. Conclusions Our study showed that increased plant diversity in agricultural grasslands can have positive effects on the environment (improved N use may lead to reduced N leaching) and agricultural production (increased herbage yield). A complementary effect between legumes and non-leguminous plants and increasing plant diversity had a greater positive impact on herbage yield compared with complementary vertical soil 15N-access.
Plant and Soil, 2013, Vol 371, Issue 1-2, p. 313-325