J Plant Ecol ›› 2019, Vol. 12 ›› Issue (1): 78-88.

• Research Articles •

### Size-asymmetric root competition in deep, nutrient-poor soil

Camilla Ruø Rasmussen1,*, Anne Nygaard Weisbach2, Kristian Thorup-Kristensen1 and Jacob Weiner2

1. 1 Department of Plant and Environmental Sciences, University of Copenhagen, H?jbakkegaard Allé 13, Taastrup DK-2630,
Denmark
2 Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, Frederiksberg DK-1871, Denmark
*Correspondence address. Department of Plant and Environmental Sciences, University of Copenhagen, H?jbakkegaard Allé 13, Taastrup DK-2630, Denmark. Tel: +45-35320871; E-mail: crr@plen.ku.dk
• Received:2017-01-01 Revised:2017-11-07 Accepted:2017-11-18 Online:2017-11-20 Published:2019-02-01

Abstract:

Aims

There is much evidence that plant competition below ground is size symmetric, i.e. that competing plants share contested resources in proportion to their sizes. Several researchers have hypothesized that a patchy distribution of soil nutrients could result in size-asymmetric root competition. We tested this hypothesis.

Methods

In a greenhouse experiment, Triticum aestivum (wheat) individuals of different initial sizes were grown alone or with below-ground competition from one neighbour, in 1 m tall, narrow containers in a nitrogen-poor field soil with (i) no added nitrogen, (ii) nitrogen fertilizer mixed into the upper 50 cm, and (iii) the same amount of fertilizer mixed into a 20–30 cm deep layer. We measured total leaf length throughout the experiment, and above-ground biomass and nitrogen concentration at harvest. We also measured root depth and frequency over time in a subset of containers.

Important Findings

Competing plants were half the size of non-competing plants, meaning that root competition was very strong. Root competition was size-asymmetric to some degree in all soil treatments. Neighbours larger than the target plant showed a greater per-unit-size effect on target growth than neighbours smaller than the target. Size variation increased over time for competing individuals, but decreased for non-competing pairs. Contrary to expectations, the presence of a high-nutrient patch reduced the strength and size asymmetry of competition temporarily. Size asymmetry in poor, deep soils may result from directionality in resource interception as roots compete for limited nutrients by growing deeper into soil layers that have not yet been exploited. Root competition can be size asymmetric, but not to the same degree as competition for light.