Divergent Nutrient Resorption Strategies in C4 Desert Shrubs: Stoichiometric Evidence From Assimilative Branches

Calligonum species are widely distributed across the arid desert belt stretching from North Africa to Central Asia. Their leaves are nearly fully degraded, and photosynthetic activity is predominantly undertaken by assimilative branches (ABs). Remarkably, Calligonum represents the only known lineage within the family Polygonaceae to exhibit the C4 photosynthetic pathway. To date, on the regional scale, the nutrient resorption patterns of ABs of different Calligonum species are still unclear. We investigated three representative species from distinct taxonomic sections native to the Junggar Desert of northwestern China: C. mongolicum (CM; Sect. Medusa), C. leucocladum (CL; Sect. Pterococcus), and C. junceum (CJ; Sect. Calliphysa). Green ABs and AB litters were collected during the summer and autumn, respectively, to assess interspecific differences in nitrogen (N), phosphorus (P), and potassium (K) resorption efficiencies (NRE, PRE, and KRE, respectively), and explore their stoichiometric relationships, variation patterns, and the environmental influences. Across all species, the nutrient resorption efficiencies (NuREs) followed the order: KRE (65.03% ± 0.57%) > PRE (53.57% ± 0.48%) > NRE (23.36% ± 0.70%). Among the three taxa, CM exhibited the highest NRE (29.20% ± 1.24%) and PRE (62.44% ± 0.45%), whereas KRE was lowest in CJ (57.41% ± 1.41%). All three species exhibited a scaling relationship between NRE and PRE with slope > 2, indicating that N was resorbed more rapidly than P. The scaling relationship of PRE-KRE showed considerable interspecific variation, with CJ exhibiting a negative slope (-0.492). NuREs were positively correlated with nutrient concentrations in summer green ABs but negatively correlated with those in AB litters. Within species, the three NuREs generally exhibited similar patterns of variation across geographic, climatic, and edaphic gradients, yet marked interspecific differences persisted. Soil and climatic conditions were identified as the primary environmental determinants of NuRE variability, although species-specific responses indicated that different nutrient elements were affected by distinct interactions among environmental factors. In summary, the three Calligonum species demonstrated differentiated nutrient resorption strategies, closely tied to both their internal nutrient status and environmental contexts. These findings provide valuable insights into nutrient use strategies and adaptive mechanisms in Calligonum and other assimilative-branch shrubs inhabiting arid ecosystems.