Plant traits and species interactions along gradients of N, P and K availabilities

Plant performance is driven by nutrient availability. So far, studies on plant nutrient relationships mostly focused on nitrogen (N) and phosphorus (P), whereas potassium (K) received less attention. We evaluated whether plant responses to variation in nutrient availability are similar or different for the nutrients N, P and K and whether the same plant traits are decisive for the competitive ability of species along gradients of N, P and K availabilities. We studied plant trait responses of three temperate perennial grass species (Alopecurus pratensis, Agrostis capillaris and Anthoxanthum odoratum, each dominant under N limitation, P limitation and K limitation, respectively). Each gradient consisted of eight treatments (N1–N8, P1–P8, K1–K8), with N:P:K supply rates being 1:0.8:6 (N1), 128:0.8:6 (N8), 12:0.075:6 (P1), 12:9.6:6 (P8), 12:0.8:0.56 (K1) and 12:0.8:72 (K8) (total supply per plant individual in mg). Plant individuals were kept either in single‐species pots (intraspecific competition) or in mixed‐species pots (interspecific competition). After 2 months, plant biomass, leaf and root traits and root enzyme activity were measured. Most below‐ground traits (e.g. total root length, root diameter) responded most strongly to competition as predictor variable and only secondly to the type of nutrient or their supply rates (unlike RGRTₒₜₐₗ and SLA). Leaf chlorophyll content and root enzyme activity responded most strongly to supply rate of nutrients and the interaction of nutrient type and supply rate, respectively. In line with our prediction, A. pratensis was the superior competitor under N limitation and A. odoratum was the superior competitor under K limitation (assessed by relative dominance [RD] and relative competitive strength [RCS]). Both species showed high investment in total root length. Opposite to our expectation, A. capillaris was a bad competitor under P limitation, with low root length but higher root phosphatase activity. Our results indicate that below‐ground plant traits are decisive in nutrient‐related competition between plant species. Further, competitive strength is determined by a high total root length per soil volume both under N and K limitation. Under P limitation, the model species A. capillaris showed a weak competitive performance either because it was lacking colonization with mycorrhiza and/or the high investment in phosphatase activity posed a trade‐off with higher N investment in phosphatase production. A free Plain Language Summary can be found within the Supporting Information of this article.