. Modelling facilitation or competition within a root system: importance of the overlap of root depletion and accumulation zones. Plant and Soil, Springer Verlag, 2017, pp.1-15. 10 Aims: The concept of intra-plant, inter-root competition considers the overlap of nutrient depletion 16 zones around roots, but neglects the spatial pattern of root exudates that can increase nutrient 17 availability. We tested the hypothesis that interactions between nutrient accumulation zones due to 18 exudation by different roots can lead to intra-plant inter-root facilitation. 19 Methods:We used the PARIS model (Raynaud et al 2008) to simulate phosphorus uptake by a 20 population of roots that are able to increase phosphorus availability by exuding citrate. We carried 21 out several simulations with the same parameters but with increasing root density in order to study 22 out if changes in root densities would alter nutrient uptake per unit root. 23Results: Emerging relationships between root uptake efficiency and root length density indicated 24 cases of inter-root competition or facilitation. The sizes of the accumulation and depletion zones 25 were calculated to explain these results. Our simulations showed a continuum between cases of 26 inter-root competition and facilitation. Facilitation occurred at low exudation rates, when 27 phosphorus supply was not saturated within the phosphorus depletion zone surrounding roots. Low 28 2 exudation systems led to a lower phosphorus uptake per unit root length, but minimized phosphorus 29 losses in the process. 30Conclusions: Based on our model, we derived conditions that allowed predicting whether 31 competition, facilitation or no interaction, is the dominant interaction between roots within a root 32 system, based on the different distances to which an isolated root alters P concentration and supply. 33 34