Quantifying resource use complementarity in grassland species: A comparison of different nutrient tracers

Abstract Stable isotopes (e.g. 15 N) and rare elements (e.g. Li + , Rb + , Sr 2+ ) have been applied to trace nutrient uptake in plants. Tracer methods are increasingly used to quantify soil resource niche partitioning in multi-species communities. Niche partitioning allows for complementarity in nutrient uptake. Spatial complementarity is most frequently measured on separate plots, which bears a risk of between plot variations. This could be avoided with a method that allows for quantification of nutrient partitioning within the same plot. However, there is uncertainty whether uptake is sufficiently similar among different tracers to allow for direct comparison. Therefore we tested uptake similarity between Li + and Rb + to determine if they can serve as analogues to quantify nutrient uptake from different soil depths. We found a strong overall correlation between Li + and Rb + accumulation, irrespective of the duration of tracer exposition and plant species identity. However, the slope of the regression between both elements was different in roots and shoots and between different functional groups, pointing to the need of correction factors. Comparisons with other tracers showed that Li + and Rb + accumulation is clearly more similar to each other than to Sr 2+ and 15 N accumulation. We therefore conclude: Li + and Rb + have a strong potential to be used as tracers for quantifying spatial complementarity within one given plot under field conditions.