Testing experimentally the effect of soil resource mobility on plant competition

Aims The volume of soil beyond a plant’s roots from which that plant is able to acquire a particular nutrient depends upon the mobility of that nutrient in the soil.For this reason it has been hypothesized that the strength of competitive interactions between plants vary with soil nutrient mobility.We aimed to provide an experimental test of this hypothesis.Methods We devised two experimental systems to investigate specifically the effect of nutrient transport rates upon intraspecific competition.In the first,the exchange of rhizosphere water and dissolved nutrients between two connected pots,each containing one plant,was manipulated by alternately raising and lowering the pots.In the second experiment,the roots systems of two competing plants were separated by partitions of differing porosity,thereby varying the plants’access to water and nutrients in the other plant's rhizosphere.In this second experiment,we also applied varying amounts of nutrients to test whether higher nutrient input would reduce competition when competition for light is avoided,and applied different water levels to affect nutrient concentrations without changing nutrient supply.Important findings In both experiments,lower mobility reduced competitive effects on plant biomass and on relative growth rate(RGR),as hypothesized.In the second experiment,however,competition was more intense under high nutrient input,suggesting that low nutrient supply rates reduced the strength of the superior competitor.Competitive effects on RGR were only evident under the low water level,suggesting that under lower nutrient concentrations,competitive effects might be less pronounced.Taken together,our results provide the first direct experimental evidence that a reduction in nutrient mobility can reduce the intensity of competition between plants.