Plant nutrients are essential for plant growth and ecosystem functioning. While Fabaceae plants are ecologically and economically significant, the phylogenetic and environmental controls on their leaf and root nutrient concentrations at a large scale have not been extensively studied.

We measured six nutrient concentrations (N, P, S, K, Ca, and Mg) in both the leaves and roots of 121 Fabaceae species across various vegetation types in China.

The scaling exponents between leaves and roots showed that N-N and P-P were significantly less than 1, Ca-Ca was significantly greater than 1, and S-S, K-K, and Mg-Mg did not significantly differ from 1, indicating divergent nutrient allocation strategies between leaves and roots. For most nutrients, phylogeny explained a larger proportion of the variation than environmental factors. However, for leaf P and K, environmental variables accounted for more variation than phylogeny, with mean annual temperature being the strongest environmental predictor for both. This suggests that these two nutrients may play a particularly important role in the environmental adaptation of Fabaceae species.

Synthesis. This study revealed contrasting nutrient allometries between leaves and roots of Fabaceae plants, clarified the relationships between nutrients and environmental variables, and highlighted the dominant role of phylogeny in explaining nutrient variation. Together, these findings enhance our understanding of the biogeographic distribution of Fabaceae species and offer insights into their adaptive responses to environmental change across broad spatial scales.

Study sites and vegetation types

We selected 88 representative natural ecosystems across China for this study. The sites span latitudes from 18.38°N to 53.3°N and longitudes from 75.47°E to 128.9°E, covering a wide range of vegetation types in the Northern Hemisphere. These include cold-temperate coniferous forests, temperate coniferous and broad-leaved mixed forests, warm temperate deciduous broad-leaved forests, subtropical evergreen broad-leaved forests, tropical rainforests, temperate steppes, temperate deserts, and the alpine vegetation of the Qinghai-Tibetan Plateau (Fig. 1).

2.2 Field design and plot establish

From 2013 to 2022, samples were collected in July and August, during peak vegetation growth. Sampling occurred in well-protected national nature reserves, ecological observatory sites, or natural vegetation areas with minimal human disturbance (Liu et al., 2023, Liu et al., 2022). In forested areas, three or four experimental plots (30 m × 40 m) were established. Within each plot, one or two shrub subplots (5 × 5 m) and two or four herb subplots (1 × 1 m) were nested. In grassland ecosystems, eight plots (1 m × 1 m) were set up at each site. For desert ecosystems, six shrub plots (10 m × 10 m) and eight herb plots (1 × 1 m) were established (Yan et al., 2023, Zhang et al., 2021). In 34 sites, we did not conduct surveys of community structure; instead, we directly collected samples of the target plants. Coordinates and elevations of sample sites were recorded using GPS on smartphones. The field surveys encompassed 331 Fabaceae species-site combinations, representing 121 species across 50 genera (Table S1-2).

2.3 Root and leaf sampling

Leaf samples from Fabaceae species were taken from fully expanded, typical, and healthy plants. To accurately sample roots from woody species, we first loosened the soil around the target plant and traced the roots to the stem. Fine roots (diameter < 2 mm) were then carefully collected, cleaned of soil, and prepared for analysis. Leaf and root samples were collected from at least three individuals per species, mixed, and dried in an oven at 60 °C until reaching a constant weight. The dried samples were then ground into a fine powder using an agate mortar grinder (RM200, Retsch, Haan, Germany) and a ball mill (MM400 Ball Mill, Retsch) for nutrient measurements.

2.4 Nutrient measurements

The concentrations of six nutrients were analyzed in both leaf and root organs: nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), and magnesium (Mg). Nitrogen concentrations were measured using an elemental analyzer (Vario Max CN Element Analyzer, Elementar, Hanau, Germany). For the other nutrients, leaf and root samples were acidified with 68% nitric acid (HNO₃) and digested using a microwave digestion system (Mars X Press Microwave Digestion System; CEM, Matthews, NC, USA). The acid solutions were heated to volatilize HNO₃, reducing their acidity, and then diluted with ultrapure water to a final volume of 15 ml. Nutrient concentrations in the solutions were measured using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 5300 DV; PerkinElmer, Waltham, MA, USA). The nutrient concentrations of leaves and roots were then calculated based on the nutrient concentrations in the solutions and the dry weight of the samples used for digestion (Liu et al., 2024, Zhang et al., 2022).