Nitrogen uptake and reallocation from roots drive the regrowth of a dominant plant in temperate grassland after low defoliation

The ability of plants to efficiently supply nitrogen (N) to their shoots is critical for reconstructing photosynthetic tissues after defoliation events. We conducted an indoor defoliation experiment and used 13 C and 15 N labeling to investigate the photosynthetic carbon (C) allocation, the relationships between C allocation and N uptake, and the contribution of N reallocation for regrowth . Leymus chinensis increased the amount of belowground 13 C to the low defoliation (LD) treatment, but allocated less 13 C belowground in response to the medium (MD) and high defoliation (HD) treatments, these changes were related to photosynthetic C fixation. Moreover, there was no evidence of a trade-off in C allocation between root exudates and roots, because their direct sources of C differed. The uptake rate of 15 N was the greatest in the LD treatment, when the C invested belowground was maximal. MD treatment increased specific root length and specific root area, which augmented the N uptake. Crucially, defoliation increased the N reallocation from roots to shoots, likely due to the greater N demand of shoot. Altogether, our results show that N supply was governed by N uptake and reallocation for Leymus chinensis after low defoliation.