Assessment of root system dynamics of species grown in mixtures under field conditions using herbicide injection and 15N natural abundance methods:: A case study with pea, barley and mustard

Two methods were developed and used to study the root system dynamics of two species grown together or separately under field conditions. The first method, based on herbicide injection at different soil depths, was used to determine the rooting depth penetration rate of each species in pea-barley and pea-mustard mixtures. The roots absorbed the herbicide when they reached the treated zone leading to visible symptoms on the leaves which could be readily monitored. The second method used differences in N-15 natural abundance and N concentration between legume and non-legume species to quantify the contribution of each species to root biomass of a pea-barley mixture. Each contribution was calculated using N-15 abundance and N concentration of root mixtures and of subsamples of roots of individual species within mixtures. Both methods can indeed be used to distinguish roots of species in mixtures and thus to study belowground competition between associated species. The use of these methods demonstrated species differences in root system dynamics between species but also significant effects of interactions between species in mixtures. The rooting depth penetration rate was mainly species specific whereas root biomass was dependant on plant growth, allocation of dry matter between shoot and root components and growth factors such as N fertilization. Root biomass of each species may vary therefore with the level of competition between species.