Legume Shrubs Are More Nitrogen-Homeostatic than Non-legume Shrubs.

Legumes are characterized as keeping stable nutrient supply under nutrient-limited conditions. However, few studies examined the legumes' stoichiometric advantages over other plants across various taxa in natural ecosystems. We explored differences in nitrogen (N) and phosphorus (P) stoichiometry of different tissue types (leaf, stem, and root) between N2-fixing legume shrubs and non-N2-fixing shrubs from 299 broadleaved deciduous shrubland sites in northern China. After excluding effects of taxonomy and environmental variables, these two functional groups differed considerably in nutrient regulation. N concentrations and N:P ratios were higher in legume shrubs than in non-N2-fixing shrubs. N concentrations were positively correlated between the plants and soil for non-N2-fixing shrubs, but not for legume shrubs, indicating a stronger stoichiometric homeostasis in legume shrubs than in non-N2-fixing shrubs. N concentrations were positively correlated among three tissue types for non-N2-fixing shrubs, but not between leaves and non-leaf tissues for legume shrubs, demonstrating that N concentrations were more dependent among tissues for non-N2-fixing shrubs than for legume shrubs. N and P concentrations were correlated within all tissues for both functional groups, but the regression slopes were flatter for legume shrubs than non-N2-fixing shrubs, implying that legume shrubs were more P limited than non-N2-fixing shrubs. These results address significant differences in stoichiometry between legume shrubs and non-N2-fixing shrubs, and indicate the influence of symbiotic nitrogen fixation (SNF) on plant stoichiometry. Overall, N2-fixing legume shrubs are higher and more stoichiometrically homeostatic in N concentrations. However, due to excess uptake of N, legumes may suffer from potential P limitation. With their N advantage, legume shrubs could be good nurse plants in restoration sites with degraded soil, but their P supply should be taken care of during management according to our results.

豆科植物的特点是在养分有限的条件下能保持稳定的养分供应。然而，在自然生态系统中，很少有研究检验豆科植物相对于其他不同分类群植物在化学计量方面的优势。我们研究了中国北方299个阔叶落叶灌丛样地中固氮豆科灌木和非固氮灌木不同组织类型（叶、茎和根）的氮（N）和磷（P）化学计量差异。在排除分类和环境变量的影响后，这两个功能群在养分调节方面存在显著差异。豆科灌木的氮浓度和氮磷比高于非固氮灌木。对于非固氮灌木，植物和土壤之间的氮浓度呈正相关，但对于豆科灌木则不存在这种相关性，这表明豆科灌木比非固氮灌木具有更强的化学计量内稳态。对于非固氮灌木，三种组织类型之间的氮浓度呈正相关，但对于豆科灌木，叶片和非叶片组织之间不存在这种相关性，这表明非固氮灌木组织间氮浓度的相关性更强。两个功能群所有组织内的氮和磷浓度都存在相关性，但豆科灌木的回归斜率比非固氮灌木更平缓，这意味着豆科灌木比非固氮灌木更容易受到磷的限制。这些结果揭示了豆科灌木和非固氮灌木在化学计量方面的显著差异，并表明共生固氮作用（SNF）对植物化学计量的影响。总体而言，固氮豆科灌木的氮浓度更高，且在化学计量上更具内稳态。然而，由于过量吸收氮，豆科植物可能会受到潜在的磷限制。凭借其氮优势，豆科灌木在土壤退化的恢复地中可以作为良好的保育植物，但根据我们的研究结果，在管理过程中应注意其磷的供应。