Nitrogen and phosphorus concentrations and allocation strategies among shrub organs: the effects of plant growth forms and nitrogen-fixation types

We aimed to explore the influences of plant functional groups on nutrient concentrations and allocation strategies among shrub organs, as well as to examine the effects of climate, soil and species on nutrient concentrations in shrubs of different plant functional groups. We investigated the nitrogen (N) and phosphorus (P) concentrations in roots, stems and leaves and their influencing factors of 187 shrub species in the shrublands across southern China, and we also examined the relationships between N and P among various organs using scaling analysis. The scaling relationships of N and P tended to be allometric between leaf and non-leaf organs, while they tended to be isometric among non-leaf organs. Plant functional groups affected nutrient allocation among shrub organs, where a higher proportion of nutrients were present in the stems and roots of evergreen shrubs and non-legume shrubs when compared to deciduous shrubs and legume shrubs as nutrients within a plant increased. Among organs, N and P concentrations were higher in leaves than in stems and roots. Among functional groups, evergreen shrubs and legume shrubs were more P-limited than deciduous shrubs and non-legume shrubs, respectively. The N and P concentrations in evergreen shrubs were lower and more sensitive to environmental change than in deciduous shrubs. Both N and P contents in legume shrubs were higher and more homeostatic than those of non-legume shrubs. Plant growth forms and N-fixation types exerted strong effects on nutrient concentrations and allocations among shrub organs. The influences of climate and soil on shrub N and P concentrations differed by plant functional groups.