Contribution of Arbuscular Mycorrhiza and Exoenzymes to Nitrogen Acquisition of Sorghum Genotypes Under Drought

For unfertile and degraded soils of sub-Saharan Africa, nitrogen (N) is often the most limiting growth factor restricting yields. The often-suggested exploitation of advantageous rhizosphere traits remains to be validated as a strategy to overcome N limitation, specifically when N deficiency co-occurs with further abiotic stresses such as water scarcity. To identify potential beneficial rhizosphere traits, three sorghum genotypes were cultivated in ‘mesocosms’ with a root-exclusion compartment under drought and optimal conditions. N mobilization and uptake were studied by 15N application coupled with 13CO2 labeling to track plant investment into rhizosphere traits. The uptake of 15Nmin by arbuscular mycorrhizal fungi (AMF) from the root-exclusion compartment was 4-12 times higher under drought compared to well-watered conditions. In addition, water stress enhanced below-ground allocation of recently assimilated carbon (C) into microbial biomass in both compartments. Under drought conditions, potential enzymatic reaction (Vmax) of chitinase and leucine amino peptidase declined and increased, respectively. This suggests that N acquisition from protein mineralization was relatively enhanced to that of chitin following moisture limitation. Substrate affinity (Km) of LAP was strongly reduced by drought compared to that of chitinase which displayed genotype-specific shifts in rhizosphere enzyme systems. We conclude that belowground C allocation to the rhizosphere microbiome activated the AMF symbiosis and its associated bacteria. This not only led to a shift in enzyme-driven exploitation of distinct organic N sources but also induced a strong increase in AMF-based Nmin acquisition from the hyphosphere. This trait plasticity in response to drought may be harnessed for stabilizing food production from low-fertile soil under the increasingly negative impacts of droughts under climate change.