DOI: 10.5061/dryad.m0cfxpph0
title: Data from: Nitrogen enrichment promotes isopods through reduced carbon and nitrogen stoichiometric mismatch with understory plants

description: As key detritivores and fungal grazers, terrestrial isopods (Isopoda:
 Oniscidea) play crucial roles in mediating ecosystem processes. Although
 nitrogen enrichment represents a major global change driver known to
 modify soil food webs, its long-term effects on the abundance of these
 keystone detritivore remain largely unknown. For this study, we conducted
 a 10-year nitrogen enrichment experiment to monitor the active density and
 biomass of terrestrial isopods across eight seasons over two years during
 2020 to 2022 in 13- and 17-year-old poplar plantations, respectively. Our
 results revealed that nitrogen enrichment increased the abundance and
 biomass of terrestrial isopods. For nitrogen enrichment levels of 5, 10,
 15, and 30 g N m⁻² yr⁻¹, the corresponding increases in isopods active
 density were estimated to be 11.6%, 24.5%, 38.9%, and 92.9 % higher,
 respectively, relative to the ambient N level. Furthermore, nitrogen
 enrichment did not alter the carbon to nitrogen ratio (C:N) of the body of
 isopods, but reduced the C:N of poplar leaf litter, understory plants, and
 detritus. Nitrogen enrichment also reduced the understory plant diversity
 and altered understory plant species composition. The structural equation
 models revealed that the responses of isopod abundance to nitrogen
 enrichment were always inversely related to the C:N of the understory
 community, while showing no correlation with poplar leaf litter quality.
 This study reveals a new mechanism by which nitrogen enrichment promotes
 soil invertebrates by affecting the quality of the understory rather than
 the overstory. Our results suggest that long-term nitrogen enrichment
 affects soil detritivores through seasonally dependent pathways mediated
 by the understory plant community. This reveals a previously unrecognized
 mechanism through which nitrogen deposition influences soil food web
 structure.
description: This dataset originates from a <strong>long‐term nitrogen
 (N) enrichment experiment</strong> conducted in coastal poplar
 (<em>Populus deltoides</em> cv. “I-35”) plantations at the
 Dongtai Forest Farm, Jiangsu Province, eastern China (32°52′ N, 120°49′
 E). The region has a temperate monsoon climate with a mean annual
 temperature of 14.9 °C and precipitation of ~1,050 mm.
 The experiment was established in <strong>May
 2012</strong> in two stand ages (13- and 17-year-old plantations at
 the start of the experiment). A randomized block design included
 <strong>five N enrichment levels</strong> (0, 5, 10, 15, and
 30 g N m⁻² yr⁻¹ as NH₄NO₃) with <strong>four replicate
 blocks</strong> per level. Each treatment subplot measured 25 × 30 m
 with 10 m buffers, and blocks were separated by ≥500 m. Nitrogen solutions
 were applied six times annually during the growing season (May–October) to
 simulate chronic atmospheric deposition; control plots received equal
 volumes of water only. <strong>Isopod
 sampling</strong> was conducted across <strong>eight seasonal
 time points</strong> from August 2020 to June 2022 using pitfall
 traps (Ø7 cm, 14 cm depth) filled with saturated NaCl solution. Six traps
 per plot were deployed for seven days per sampling event. All terrestrial
 isopods (adults and juveniles) were counted and oven-dried (60 °C) to
 determine active density (individuals m⁻² day⁻¹) and dry biomass (g m⁻²
 day⁻¹). <strong>Understory
 vegetation</strong> was surveyed concurrently in five 2 × 2 m
 quadrats per plot to quantify species richness, functional group cover
 (herbs, grasses, legumes), and biomass (oven-dried at 60 °C). Poplar leaf
 litterfall was collected in six 0.5 × 0.5 m traps per plot; forest floor
 detritus was collected from five 0.5 × 0.5 m frames per plot. All plant
 and detritus samples were dried to constant weight. For
 <strong>C:N stoichiometry</strong>, isopod tissue (composite
 of 20 individuals per plot), poplar litter, understory vegetation, and
 detritus were ground and analyzed for total carbon and nitrogen using an
 Elementar Vario EL III elemental analyzer. The
 accompanying R scripts include code for data cleaning, generalized linear
 mixed models (GLMMs), and piecewise structural equation modeling (SEM)
 used to evaluate the effects of N enrichment, stand age, and season on
 isopod density/biomass, plant diversity, and C:N ratios.
description: --- # Nitrogen Enrichment and Isopods: Open Source Data --- We provide the
 raw data (**eN_Isopod.csv**), isopod tissue stoichiometry data
 (**eN_Isopod_Stoichiometry.csv**), and the R analysis scripts used in the
 study *Nitrogen enrichment promotes terrestrial isopods through reduced
 carbon and nitrogen stoichiometric mismatch with understory plants* (Ni et
 al., 2025). These datasets come from a 10-year nitrogen enrichment
 experiment in poplar plantations (Dongtai Forest Farm, Jiangsu, China) and
 include repeated measurements of isopod abundance/biomass, plant community
 composition, and C:N stoichiometry across eight seasonal sampling events
 (2020–2022). ## Descriptions The data (.csv) and code (.R) are provided in
 `csv-data-r-code.zip`, hosted on Dryad. ### **eN_Isopod.csv** *
 *TimeLine*: Sampling date (MM/DD/YYYY) * *Year*: Sampling year * *Month*:
 Sampling month * *Season*: Sampling season (Summer, Autumn, Winter,
 Spring) * *Age*: Stand age of poplar plantation (“13a” = 13-year-old
 stand; “17a” = 17-year-old stand) * *Nitrogen*: Nitrogen enrichment
 treatment level code (e.g., N0, N5…) * *NitrogenRate*: Annual nitrogen
 addition rate (g N m⁻² yr⁻¹; 0, 5, 10, 15, 30) * *PlotID*: Plot identifier
 * *SubPlotID*: Subplot identifier * *TotalBiomass*: Total dry biomass of
 isopods (g m⁻² day⁻¹) * *TotalCount*: Total isopod active density
 (individuals m⁻² day⁻¹) * *UnderstoryRichness*: Species richness of
 understory plants (number of species per quadrat) * *FracLegumeCover*:
 Fractional cover of legumes in understory vegetation *
 *FracLegumeAbundance*: Fractional abundance of legumes * *LitterMass*:
 Poplar leaf litter mass (g m⁻²) * *LitterCN*: Carbon-to-nitrogen ratio
 (C:N) of poplar leaf litter * *UnderstoryCN*: C:N ratio of understory
 vegetation * *UnderstoryBiomass*: Dry biomass of understory vegetation (g
 m⁻²) * *DetritusMass*: Forest floor detritus mass (g m⁻²) * *DetritusCN*:
 C:N ratio of detritus * *FracLegumeCv*: Legume cover * *FracGrassCv*:
 Grass cover * *FracForbCv*: Forb cover ### **eN_Isopod_Stoichiometry.csv**
 * *Age*: Stand age (“Y” = 13-year-old, “M” = 17-year-old) *
 *NitrogenRate*: Nitrogen enrichment rate (g N m⁻² yr⁻¹) * *PlotID*: Plot
 identifier * *TissueCN*: Carbon-to-nitrogen ratio of isopod body tissue
 (mean of ground composite samples) ### **R Scripts** R script used for: 1.
 Loading and cleaning datasets 2. Statistical analyses (GLMMs, SEM)
 examining nitrogen effects on isopod density/biomass and plant C:N ratios
 and visualizations. ## Code/Software * **R version 4.2.3** is required to
 run *eN_Isopod_Script.R*. * Key packages: `dplyr`, `glmmTMB`, `emmeans`,
 `piecewiseSEM`, `ggplot2` etc. * The script is annotated to indicate data
 import, cleaning, model fitting (GLMM, SEM), and figure creation. --- This
 README accompanies the raw data and code to ensure reproducibility of all
 results reported in **Ni et al. 2025, Functional Ecology**, *Nitrogen
 enrichment promotes isopods through reduced carbon and nitrogen
 stoichiometric mismatch with understory plants*. --- ### Changelog:
 October 6, 2025: Updated README