Fig. 1.

Hypothesized interactions of RSA with rhizosphere microbial processes (represented as icons) under limitation in a mobile (nitrate, Low N) and immobile (phosphorus, Low P) soil resource. Root models of the optimal root phenotypes to overcome each limitation for a cereal and a legume are depicted. The models are based on empirical observations (Lynch, 2019). Axial roots are in red and lateral roots in blue. The number and position of icons represent our hypothesized effects of the two RSA on the predominance and location of these rhizosphere processes along the soil profile and within the root system. Soil gradients are depicted to explain the hypothesized interactions. For example, shallow roots may favor nitrification, organic matter degradation, biodegradation of root exudates, and proliferation of fungal associations. In shallow roots, we hypothesize that nitrogen fixation would be performed mostly by bacteria with structures to protect nitrogenase from oxygen, and associative nitrogen fixation by a more diverse set of bacteria might be found in intermediate root systems. Deep roots would be associated with reductive microbial processes such as denitrification and ammonification, and manganese and iron reduction. Microbial aluminum chelation would be another process that might benefit the growing root tips in deep soil domains, although aluminum toxicity is also a problem in surface soil layers in tropical soils. More examples of similar analyses with RSA traits such as number of axial roots, dimorphism, and lateral root branching density can be found in ‘Interactions of root system architecture phenotypes with soil microbial processes’.