Table 4.

Overview of the different methods to quantify soil nutrient and water uptake by plant roots.

Root activity traits	System	Focus	Method	Assessment of niche differentiation	Short description	Reference examples
Nutrient uptake traits
Nutrient uptake kinetics (k m, I max)	Ex situ or in situ (but excavated roots)	Physiological	Tracer or measurement of nutrient depletion in solution	Yes but only indirectly	Concentration‐dependent nutrient uptake of excavated roots from an incubation solution. Uptake rate is based on fine‐root biomass, length or surface area. Natural environment of the roots (soil as a system of solid, aqueous and gaseous spaces) is changed (aqueous solution) during the assessment of uptake, mycorrhizal hyphae are destroyed	Kreuzwieser et al. (2000); Gessler et al. (2005); Lucash et al. (2007)
Short‐term net uptake rates	In situ	Ecophysiological/ecological	Tracer in natural soil	Yes	Relative tracer enrichment in plant biomass (comparison among species or functional groups). Tracer uptake from the natural soil. The uptake rate is based on plant biomass, root length or ground surface area. Processes in the soil (i.e. tracer adsorption to soil particles, tracer diffusion) and the root (transport rates) are integrated, but therefore cannot be disentangled	McKane et al. (2002); Kahmen et al. (2006); Ashton et al. (2010); Jesch et al. (2018)
Long‐term net uptake rates	In situ	Ecophysiological/ecological	Tracer in natural soil	Yes	Relative tracer enrichment in plant biomass (comparison among species or functional groups). Tracer uptake from the natural soil. Processes in the soil (i.e. tracer adsorption to soil particles, tracer diffusion) and the root (transport rates) are integrated and therefore cannot be disentangled	Chapin & Van Cleve (2000); Fotelli et al. (2004)
Water‐uptake traits
Short‐term uptake	In situ	Ecophysiological/ecological	Tracer in natural soil	Yes	Relative tracer enrichment in plant biomass (comparison between species or functional groups). Tracer uptake from the natural soil. The uptake rate is based on plant biomass or ground surface area. Allows the distinction of water uptake from different soil layers (when multiple tracers are applied) but uptake rates cannot be easily determined	Grossiord et al. (2014); Bachmann et al. (2015)