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

• We compared uptake of lithium and rubidium in different grassland species to test if they are suitable to investigate nutrient uptake from different soil depth within a given plot.
• We found a strong correlation between both tracers, irrespective of duration of uptake and plant species identity.
• Therefore we conclude, that these tracers are potentially useful for such an approach.

Stable isotopes (e.g. 15N) and rare elements (e.g. Li+, Rb+, Sr2+) 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 Sr2+ and 15N 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.