Divergent nutrient resorption strategies in C4 desert shrubs: Stoichiometric evidence from assimilative branches

Abstract: 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), 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.

摘要：沙拐枣属（Calligonum）物种广泛分布于北非至中亚的干旱荒漠带。其叶片近乎完全退化，光合活动主要由同化枝（assimilative branches, ABs）完成。值得注意的是，沙拐枣属是蓼科（Polygonaceae）中目前已知唯一具备C4光合途径的类群。截至目前，区域尺度下不同沙拐枣属物种同化枝的养分回收模式仍不明晰。
本研究针对中国西北准噶尔荒漠的3个来自不同分类组的代表性物种开展调查：蒙古沙拐枣（C. mongolicum, CM；美头组Sect. Medusa）、白枝沙拐枣（C. leucocladum, CL；翅果组Sect. Pterococcus）以及绳枝沙拐枣（C. junceum, CJ；沙拐枣组Sect. Calliphysa）。分别于夏季和秋季采集绿色同化枝与同化枝凋落物，以评估氮（N）、磷（P）、钾（K）回收效率（NRE、PRE、KRE）的种间差异，并探究其化学计量关系、变化模式及环境影响。
所有受试物种的养分回收效率（NuREs）依次为：KRE（65.03 ± 0.57%）> PRE（53.57 ± 0.48%）> NRE（23.36 ± 0.70%）。3个类群中，蒙古沙拐枣的NRE（29.20 ± 1.24%）与PRE（62.44 ± 0.45%）均为最高，而绳枝沙拐枣的KRE最低（57.41 ± 1.41%）。3个物种的NRE与PRE均呈现斜率大于2的缩放关系，表明氮的回收速率快于磷。PRE与KRE的缩放关系存在显著种间变异，其中绳枝沙拐枣呈现负斜率（−0.492）。
养分回收效率与夏季绿色同化枝中的养分浓度呈正相关，而与同化枝凋落物中的养分浓度呈负相关。在物种内部，3种养分回收效率在地理、气候及土壤梯度上通常呈现相似的变化模式，但种间差异显著。研究发现，土壤与气候条件是养分回收效率变异的主要环境驱动因子，尽管物种特异性响应表明不同养分元素受环境因子间不同互作关系的影响。
综上，3种沙拐枣属物种展现出差异化的养分回收策略，这与其内部养分状况及环境背景密切相关。本研究结果为理解干旱生态系统中沙拐枣属及其他同化枝灌木的养分利用策略与适应机制提供了重要参考。