Differential carbon allocation to nitrogen-rich patches in Poa annua precedes root proliferation but has no immediate benefit to N uptake

Nutrients are heterogeneously distributed in the soil environment. Plants have evolved a variety of mechanisms to maximise their ability to compete for limited resources, with differential root growth considered among the more important mechanisms. Despite the significant costs of root growth, little data is available regarding carbon (C) allocation to roots growing in heterogeneous conditions. Here, we investigate the allocation of recently assimilated C in Poa annua plants growing in uniform or heterogeneous nutrient conditions. In the first experiment we grew plants in split-root boxes, providing N either equally between the two chambers (0.5 mg/0.5 mg, 8 mg/8 mg) or with one side receiving more N (0.5 mg/8 mg, 8 mg/0.5 mg), and quantified C allocation and N uptake using ¹³CO₂ and K¹⁵NO₃. Where N was supplied equally to the two chambers, C was allocated equally to the roots irrespective of the total N supply. However, the ¹³C label was preferentially allocated (60:40) to high-N roots in the unequal treatments. N uptake was a function of local supply and was not affected by N supply to the roots in the other chamber. C allocation had no discernible effect on N uptake. In the second experiment, we tested whether differential N supply would lead to increased root growth in the high-N side. In this experiment, equal amounts of N were supplied to all plants as ammonium, with half receiving an equal distribution to the two root chambers (50/50), while the other half received an unequal supply (94/6). While no difference in root growth was noted in 50/50 plants, a 60:40 mass allocation was noted from day six onwards in plants receiving the 94/6 N supply. Despite increased root growth in the high-N side, the plants receiving the 94/6 treatment could not achieve the same shoot mass or N concentration as the 50/50 plants. No difference in total C allocation to the roots between treatments was noted in the first experiment, and no difference in total root mass between treatments was found in the second experiment, suggesting that root C supply was source-limited, while allocation to specific roots was strongly influenced by sink strength. Differential C allocation appears to be an important pre-requisite for increased root growth in N-rich patches.