Diurnal dynamics can modify plant–microbial competition for N uptake via C allocation

Plant–microbial competition for N in the rhizosphere affects net primary production and N cycling, and depends on plant physiological factors, including photosynthesis and transpiration. Here, we studied the diurnal competition for N between plants and microorganisms and quantified plant C allocation by maize (Zea mays) and wheat (Triticum aestivum) in a nutrient-poor soil. In situ concurrent pulse 15NO3− and 13CO2 labeling was carried out to trace the N uptake from the soil and C allocation to plant–microorganisms–soil system in the first 12 and 24 h after labeling, corresponding to daytime and nighttime. Plants outcompeted microorganisms for NO3− uptake from the rhizosphere during daytime and nighttime. Diurnal dynamics had a low effect on microbial NO3− uptake, which varied between 7 and 15% of added N for maize and between 3 and 6% of added N for wheat after the first 12 and 24 hours. NO3− uptake by maize was not affected by diurnal dynamics, but NO3− uptake by wheat was higher during daytime than that during nighttime. The 13C allocation to plants and microorganisms was similar between daytime and nighttime. N utilization was closely related to photosynthate partitioning in plant shoots and roots, indicating coupling between C allocation and N utilization. These findings indicate that plants grown in nutrient-poor soils (as used in this study) dominate over microorganisms for N uptake and release newly photosynthesized C for rhizosphere microorganisms. The diurnal dynamics of these plant–microbial interactions are plant species–specific: important for wheat but irrelevant for maize.