DOI: 10.5061/dryad.hqbzkh1sh
title: Nitrogen addition drives local extinction of legumes in meadow steppe

description: Nitrogen (N) enrichment poses a critical threat to legume diversity
 through three interlinked mechanisms: stimulating growth of non-leguminous
 competitors, enhancing community canopy coverage, and inducing light
 limitation. Canopy management practices such as mowing may partially
 mitigate these impacts by reducing species dominance and restoring light
 availability. Through a decadal field experiment in a meadow steppe, we
 systematically investigated the interactive effects of chronic N addition
 and mowing regimes on legume biomass dynamics and species persistence. Our
 findings demonstrate a time-dependent relationship between N enrichment
 and legume species richness. Most legume species, particularly rare and
 subordinate ones, were eliminated at intermediate-to-late stages when N
 addition rates exceeded 10 g N m−2 yr−1. This collapse correlated with the
 progressive suppression of legumes by biomass accumulation in the dominant
 non-legume species, Leymus chinensis. Beyond this ecological threshold,
 only Thermopsis lanceolata persisted in the community, though its biomass
 ultimately declined with increasing N inputs and associated metal element
 uptake. Greenhouse experiments revealed this species' survival
 strategy involves rapid downregulation of N fixation capacity − a
 potentially critical functional trait distinguishing it from other legumes
 in the study site. Mowing partially counteracted N induced biodiversity
 loss at sub-threshold enrichment levels (i.e., ˂ 10 g N m−2 yr−1) by
 weakening the competitive dominance of nitrophilous plants and maintaining
 light penetration. However, this management intervention proved
 ineffective against legume extirpation under prolonged N exposure or
 supra-threshold addition rates. Our mechanistic analysis identified three
 cascading drivers of legume extinction: (1) competitive exclusion by
 N-responsive species, (2) light limitation from canopy closure, and (3)
 phytotoxic effects of metal element accumulation. Synthesis. This study
 predicts an ecosystem regime shift toward simplified N₂-fixing
 associations and dominance of non-symbiotic plant species in temperate
 grasslands under chronic N deposition. Such compositional changes could
 fundamentally alter terrestrial N cycling patterns, with cascading
 consequences for ecosystem functioning. While mowing provides limited
 buffering capacity at moderate N loads, our results emphasize the
 imperative for stricter atmospheric N emission controls to preserve legume
 biodiversity and associated ecological services.
description: Description of the data and file structure The dataset is related to the
 paper "Nitrogen Addition Drives Local Extinction of Legumes in Meadow
 Steppe". The excel file contains four sheets. The “field experiment”
 (contains data from 2014-2023), “light transmittance” (2023) and “soil
 data” (2023) sheets contain data collected from the study site (field),
 and “greenhouse” sheet contains data collected in 2019 from the greenhouse
 of *Thermopsis lanceolata*. **Description:**  *Variables* * plot: The
 nitrogen (N) treatment plot. The study site contains 480 plots, of which
 384 are relevant to our study. * block: Study block. Each block contains
 60 plots and total 8 blocks.   * type: N fertilizer types. This study
 includes (NH₄)₂SO₄, NH₄HCO₃, urea, slow-release urea. * dose: the N
 addition rates. Six rates are included, which are 0, 2, 5,10, 20 and 50 g
 N m⁻² yr⁻¹. *  mowing: The mowing treatment. * year: The study year. *
 ANPP: Aboveground net primary productivity in each plot (g m⁻² yr⁻¹). *
 legume biomass: The aboveground biomass of the legume functional groups in
 each plot (g m⁻² yr⁻¹). * abundance: The legume richness in each plot,
 which refers to the total number of legume species. * T.l biomass: The
 aboveground biomass of the dominant legume, *Thermopsis lanceolata*, in
 each plot (g m⁻² yr⁻¹). * L.c biomass: The aboveground biomass of the
 dominant grass, *Leymus chinensis*, in each plot (g m⁻² yr⁻¹). * relative
 biomass of L.c: The relative biomass of *Leymus chinensis*, which is ratio
 of L.c biomass to ANPP. * relative biomass of legumes: The relative
 biomass of the legume group, which is referred to the ratio of legume
 biomass to ANPP. * relative biomass of T.l: The relative biomass of
 *Thermopsis lanceolata*, and is calculated as its biomass divided by ANPP.
 * relative biomass of T.l in legumes: The percentage of *Thermopsis
 lanceolata* in the legume group, which is the ratio of *Thermopsis
 lanceolata* biomass to legume biomass. * precipitation: The precipitation
 (mm) in the growing season (from June to August). NA means not available
 data. * AGB: The aboveground biomass ( g plant⁻¹). * BGB: The belowground
 biomass ( g plant⁻¹). * total biomass: The sum of AGB and BGB ( g
 plant⁻¹). * nodule biomass: The biomass of nodules in each plant ( mg
 nodule g plant⁻¹). * investment: The investment to nodulation, which is
 based on the nodule biomass divided by the total biomass. * Ramet: The
 ramet number of each plant. * light0%: The light transmittance at the
 litter layer. * light20%: The light transmittance at 20 cm above the
 litter layer. * element: Measured metal element in plants and soil, which
 includes Fe, Mn, Al and Ca. * content: The content of each element ( mg
 kg⁻¹). * species: The studied items.