Table 1.

Contrasting states of root architectural traits and the hypothesized effect on microbial habitat conditions

Root trait	Trait states	Hypothesized effect on root exudation, physicochemical conditions, and habitat space for microbes	Hypothesized effect on microbial processes in the rhizosphere	Supporting evidence
Rooting angle and depth	Steep—deep roots versus shallow—shallow roots	Increase in percentage of fixed carbon allocated to deeper soil layers. Increase of microaerophilic or anaerobic pockets around the rhizosphere of root tips. Soil compaction affecting the rhizosphere.	Increased number of root tips under microaerophilic conditions, favoring reduction of nitrogen compounds to ammonia or to N2. Nitrification may be favored in the rhizosphere of plants containing aerenchyma through which oxygen can pass into deeper soil domains. Nitrogen fixation may occur in the rhizosphere where air is brought to deep soil layers. The resulting ammonia might be quickly taken up by the plants rather than entering the dissimilatory N2 production. Associations with microbes tolerant to hypoxia, reduced rates of microbial consumption of root-derived carbon. Reduced predation by protozoa. Reduced exposure to soil-borne pathogens. Increased iron reduction and aluminum chelation.	Not reported in the literature—nitrogen cycle in paddy rice as it relates to soil depth: Ishii et al. (2011). Soil compaction effects on rhizosphere microbiota in crops: Longepierre et al. (2021, 2022)
Number of axial roots	Many versus few	Increased amounts of root exudates. Increased surface for microbial attachment. Larger amounts of substrate to be degraded from root decay. Steeper gradients of nutrient concentration in the depletion zone in the horizontal axis.	Overall increase in microbial abundance and diversity. Diversification in metabolism under more pronounced nutrient and water gradients in the depletion zones around the roots. Endophytic colonization increased. Reduced ammonia availability due to plant uptake might favor competence with microbes. Oxygen brought with the roots (either during or after root growth) might trigger nitrification of carbon sources such as root exudates or other sources such as debris or organic matter.	Not reported in the literature—review on plant–microbe competence for nitrogen: Moreau et al. (2015)
Lateral root density	High versus low	Greater number of exudation points from lateral roots. Greater number of attachment points for microbes. Steeper gradients of nutrient concentration in the depletion zone in the horizontal axis.	Overall increase in microbial abundance and diversity. Diversification in metabolism under more pronounced nutrient and water gradients in the depletion zones around lateral roots. Endophytic colonization increased. Reduced ammonia availability due to plant uptake might favor competition with microbes. Increased mycorrhizal colonization.	Schmidt et al. (2018); review on plant–microbe competence for nitrogen: Moreau et al. (2015) Studies on lateral root versus axial root: Saleem et al. (2016; Zai et al. (2021)
Lateral root length	Long versus short	Root exudates allocated further from the axial root, or from roots of lower branching orders. Overall increase of exudates per plant.	Differences in composition of the lateral root microbiome given the different horizontal gradients of resources between the two phenotypes. Longer roots may recruit a more diverse microbiome due to the increased soil volume explored. Attenuated effect of intra-root competition for nitrogen, which may lead to reduced plant–microbe competition for nitrogen	Not reported in the literature. Studies on lateral root versus axial root: Saleem et al. (2016); Zai et al. (2021); Kawasaki et al. (2021)

Modified from Galindo-Castañeda et al. (2022).