Nutrient foraging behaviour of four co-occurring perennial grassland plant species alone does not predict behaviour with neighbours

1. The spatial arrangement of nutrients and neighbours in soil influences plant growth and reproduction. Plants often respond to such stimuli through plasticity in root proliferation (root mass per soil volume), or the breadth of their root system. 2. Here, we asked how plants adjust nutrient foraging strategies when grown alone or with neighbours. We asked (i) Does root proliferation into nutrient-rich patches when plants are grown alone predict root proliferation when plants are grown with neighbours? (ii) What factors (nutrients or neighbours) best predict the probability of root placement at different soil locations? (iii) How does the spatial distribution of nutrients alter the degree to which neighbours suppress plant growth? 3. To answer these questions, we grew four grassland species either as individual plants or in competition, in patchy or patch-free soil, in a factorial design. We used genomic DNA to identify the spatial distribution of roots of each species when plants were grown in mixtures. 4. The root foraging behaviour of individuals grown alone did not consistently predict behaviour in mixture. Specifically, (i) the behaviour of individually grown plants predicted behaviour of competing plants inside patches, but not in background soil. We observed over-proliferation of roots in background soil relative to what was expected from plants grown alone. (ii) Neighbours were consistently the most important variable for predicting the placement of roots in soil and caused either an increase in root system breadth, or no change relative to alone. (iii) If a species experienced growth suppression when grown in competition, individuals experienced this more severely in patchy soil compared to patch-free soil. 5. Synthesis. Game theoretic models have predicted that under interspecific competition, over-proliferation of roots in the presence of neighbours might occur for some species but not others. Our data are consistent with these predictions but more work is needed. Nutrient foraging studies have primarily focused on plants grown alone or assumed that plants do not respond separately to neighbours and nutrients. Our data call these practices into question and contribute to a growing understanding that plants integrate information about both nutrients and neighbours when placing roots in soil.