Beneath the Drought: Rhizosphere Processes Shape Genotype-Specific Sorghum Responses to Drought-Induced N Limitation.

Droughts increasingly threaten crop productivity in nutrient-depleted tropical soils. We investigated how water limitation influences rhizomicrobial traits (microbial biomass and functional community composition, enzyme activities) and nitrogen (N) uptake in three sorghum genotypes Makueni local (Mkl), Gadam (Gd), and IESH 22012 (IESH) under well-watered and drought conditions. Depth-specific (0-30 and 30-60 cm) 15N labelling traced N uptake at flowering and grain-filling. Drought reduced grain N content across all genotypes but shoot N only in IESH. Gd enhanced 15N recovery in grains via post-anthesis uptake from both depths under drought, whereas Mkl and IESH rather reallocated N within the plants towards the grain. Compared to well-watered conditions, rhizosphere chitinase activity declined in Mkl under drought, while leucine aminopeptidase (LAP) activity remained unchanged. Under drought, microbial biomass decreased at flowering but recovered at grain-filling, coinciding with enhanced arbuscular mycorrhiza fungi (AMF) colonisation. At grain-filling, AMF associations with specific bacterial taxa aligned with grain N recovery, N-utilisation efficiency, and LAP activity, displaying complementary roles within the plant-AMF-bacterial functional consortia in sustaining N acquisition. Overall, N acquisition in sorghum shifted in a genotype-specific manner from reliance on microbial activity under well-watered conditions to selective AMF-bacterial partnership at grain-filling under drought. These responses highlight the significant role of rhizosphere functional dynamics in sorghum N nutrition during reproductive stages under concomitant drought and nutrient limitation.