Microbial mechanisms of rhizosphere nitrogen cycling mediated by cotton root exudates under nitrogen reduction: roles of thiamine and L-lysine.

backgroundRoot exudates play a crucial role in regulating nitrogen (N) availability in the rhizosphere. However, how specific exudate components mediate soil N cycling processes via microbial communities remains poorly understood.

resultsIn a cotton field experiment with four N application rates (N1: 330, N2: 281, N3: 231, N4: 0 kg·ha⁻¹), we employed in-situ collection and metabolomic analysis of root exudates and identified thiamine and L-lysine as key components induced under N reduction. Using a rhizosphere microcosm system, we demonstrated that in response to mild N reduction (N2), thiamine recruited functional microbes such as Alphaproteobacteria and Cephalotrichum, thereby enhancing β-glucosidase activity. This 'C-N coupling' strategy coordinates soil C decomposition and N transformation via recruited functional microbes, increased soil dissolved inorganic N (DIN) by 27.1% and the net N mineralization rate (NMR) by 18.8%. Under more moderate N reduction (N3), L-lysine stimulated N-cycling bacteria including Methylomirabilota and Myxococcota, synergistically activated β-1,4-N-acetylglucosaminidase and urease, and enhanced organic N mineralization. This 'direct N activation' strategy directly hydrolyzes organic N substrates without relying on C coupling, markedly increased soil DIN by 73.8% and NMR by 12.4%.

conclusionOur findings reveal that specific root-derived metabolites, thiamine and L-lysine, orchestrate distinct microbial mechanisms to sustain N availability under different N reduction scenarios. This study provides novel insights into plant-microbe-nutrient feedbacks and identifies potential metabolic targets for improving N use efficiency in agroecosystems.