Testing for belowground resource partitioning in experimental grassland communities

Resource partitioning due to niche separation between species may explain positive diversity-productivity relationships in terrestrial plant communities. However, there is little experimental evidence for such resource partitioning. In a temperate grassland field with experimental communities of one, three, and six species, we tested (1) whether species differ in soil nitrogen uptake patterns, (2) whether species in mixtures partition soil N due to niche separation, and (3) whether realized niches of N uptake narrow with increasing species richness. Six treatment combinations consisting of three different chemical forms of ¹⁵N-labelled nitrogen (nitrate, ammonium, and dual ¹⁵N-¹³C-labelled glycine) injected at 2 depths (3 and 12 cm) were applied to each community in a split plot design. After 48 hours, aboveground plant material was harvested for determination of the ¹⁵N/¹⁴N (¹³C/¹²C) ratio. The experiment was conducted twice, each time with another pool of six species. Although species differed in their N uptake patterns in monocultures, these differences neither led to increased N uptake of mixtures compared to monocultures, nor did niche separation increase with species richness. Rather, our species behaved more similar in the 6-species mixtures than in the monocultures or in the 3-species mixtures. Furthermore, there was no ecologically significant uptake of intact glycine (organic N) by any of the species. Our results suggest that spatial and chemical partitioning of N is of minor importance for the functioning of species mixtures in temperate grasslands.