Report on the Impact of Anthropogenic Nitrogen Deposition on Plant Diversity and Ecosystem Functions

Abstract

Anthropogenic nitrogen (N) deposition poses a significant global threat to ecosystem functions, particularly affecting terrestrial ecosystems. While N deposition is generally predicted to increase plant productivity and reduce biodiversity by enhancing dominant plant species, herbivores may counterbalance these effects by consuming the surplus biomass. This report presents findings from a multifactorial field experiment conducted in a semi-arid grassland ecosystem in northeast China, examining the effects of nitrogen addition on herbivore control and plant diversity. Results indicate that nitrogen addition disrupts top-down herbivore control by reducing herbivore abundance through changes in plant chemistry and architecture, leading to a 21% loss in plant Shannon diversity. These findings highlight the complex interactions between nitrogen deposition, herbivory, and plant community dynamics, with implications for achieving Sustainable Development Goals (SDGs) related to life on land and ecosystem sustainability.

Introduction

Anthropogenic nitrogen deposition significantly alters ecosystem functions, often increasing plant productivity while decreasing plant diversity due to enhanced dominance of few species. Herbivores are hypothesized to mitigate these effects by consuming excess biomass, exerting top-down control that supports biodiversity. However, the net effect of nitrogen deposition on plant communities is context-dependent, influenced by complex interactions among plants, herbivores, and predators.

Relevance to Sustainable Development Goals (SDGs)

• SDG 15: Life on Land – Understanding nitrogen deposition impacts is critical for conserving terrestrial ecosystems and biodiversity.
• SDG 13: Climate Action – Managing nitrogen inputs contributes to mitigating climate change effects on ecosystems.
• SDG 12: Responsible Consumption and Production – Insights into nutrient cycling inform sustainable agricultural and environmental practices.

Methods

Study System and Experimental Design

The study was conducted at the Grassland Ecosystem National Observation and Research Station of Northeast Normal University in Jilin Province, China, characterized by semi-arid grassland dominated by the perennial grass Leymus chinensis. Two grasshopper species (Euchorthippus unicolor and E. cheui) served as primary herbivores, and the orb-weaving spider Argiope bruennichi as a dominant predator.

A 3-year field experiment (2016–2018) involved nitrogen addition (10 g m⁻² year⁻¹) and grasshopper herbivory treatments using mesocosms to manipulate herbivore presence and density. Plant biomass, diversity, herbivore and predator abundance, plant traits, and abiotic conditions were measured to assess interactive effects.

Results and Discussion

Interactive Effects of Nitrogen and Herbivores on Plant Communities

1. At ambient nitrogen levels, grasshoppers exerted strong top-down control by suppressing dominant grass biomass, increasing forb biomass and plant diversity (Shannon index).
2. Nitrogen addition eliminated this top-down control by reducing herbivore abundance, leading to increased total plant biomass and dominance of L. chinensis, and a 21% reduction in plant diversity when herbivores were present.
3. In the absence of grasshoppers, nitrogen addition increased total biomass but did not significantly affect plant diversity.

Herbivore Density Effects

• Nitrogen addition caused a 53% decline in grasshopper abundance, disrupting herbivore-mediated top-down control.
• Experimental manipulation confirmed that maintaining grasshopper densities at ambient levels preserved top-down effects despite nitrogen addition.

Potential Causal Pathways

1. Plant Chemistry Alterations: Nitrogen addition nearly doubled leaf nitrogen content and slightly increased secondary compounds (phenols, tannins), altering plant stoichiometry and reducing herbivore fitness, particularly affecting the dominant grasshopper species.
2. Vegetation Structure and Predator Dynamics: Enhanced plant biomass and height provided more substrate for orb-weaving spiders, doubling spider densities and increasing prey capture rates, thereby elevating predation pressure on grasshoppers.
3. Abiotic Conditions: Minor decreases in temperature and slight increases in soil moisture were observed but did not significantly correlate with grasshopper abundance.

Implications for Sustainable Development Goals (SDGs)

• SDG 15: Life on Land – The disruption of herbivore control by nitrogen deposition threatens plant biodiversity and ecosystem balance, emphasizing the need for sustainable nitrogen management to conserve terrestrial ecosystems.
• SDG 13: Climate Action – Understanding nutrient impacts aids in mitigating adverse effects of climate change on ecosystem functions.
• SDG 12: Responsible Consumption and Production – Findings support the development of nutrient management strategies that minimize ecological harm while maintaining productivity.

Conclusions

This study challenges the prevailing assumption that herbivores will offset nitrogen deposition effects by increasing consumption. Instead, nitrogen inputs can suppress herbivore abundance through changes in plant chemistry and architecture, overriding consumer control and leading to increased plant biomass dominance and reduced diversity. These findings underscore the complexity of trophic interactions under nutrient enrichment and highlight the importance of integrated ecosystem management to achieve Sustainable Development Goals related to biodiversity conservation and ecosystem sustainability.

Data Availability

All data used in this study are publicly available in the Dryad database at https://doi.org/10.5061/dryad.kkwh70sb8.

1. Sustainable Development Goals (SDGs) Addressed or Connected to the Issues Highlighted in the Article

1. SDG 15: Life on Land
   • The article focuses on terrestrial ecosystems, specifically grassland ecosystems, and the impact of anthropogenic nitrogen deposition on plant diversity and ecosystem functions.
   • It addresses biodiversity loss due to nitrogen deposition and altered trophic interactions involving herbivores and predators.
2. SDG 13: Climate Action
   • The article discusses anthropogenic nitrogen deposition, which is linked to human activities affecting climate and atmospheric composition.
   • Understanding nitrogen deposition effects contributes to climate change mitigation and ecosystem resilience.
3. SDG 2: Zero Hunger
   • By studying plant productivity and biomass in grasslands, the article indirectly relates to sustainable land management and food security.
   • Maintaining plant diversity and productivity supports ecosystem services relevant to agriculture and grazing.

2. Specific Targets Under Those SDGs Identified Based on the Article’s Content

1. SDG 15: Life on Land
   • Target 15.1: By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services.
   • Target 15.5: Take urgent and significant action to reduce the degradation of natural habitats, halt the loss of biodiversity, and protect and prevent the extinction of threatened species.
2. SDG 13: Climate Action
   • Target 13.3: Improve education, awareness-raising and human and institutional capacity on climate change mitigation, adaptation, impact reduction, and early warning.
   • This research enhances understanding of nitrogen deposition impacts, contributing to informed climate action.
3. SDG 2: Zero Hunger
   • Target 2.4: By 2030, ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production, help maintain ecosystems, and strengthen capacity for adaptation to climate change.

3. Indicators Mentioned or Implied in the Article to Measure Progress Towards the Identified Targets

1. Plant Diversity and Biomass Indicators
   • Shannon Diversity Index: Used to measure plant species diversity changes under nitrogen deposition and herbivory treatments.
   • Total Plant Biomass: Quantified to assess productivity changes in response to nitrogen addition and herbivore presence.
   • Species Richness: Number of plant species recorded in quadrats to monitor biodiversity.
2. Herbivore and Predator Abundance Indicators
   • Grasshopper Abundance: Measured to understand top-down control effects and responses to nitrogen deposition.
   • Spider (Predator) Density and Web Characteristics: Used to assess predator-prey interactions and ecosystem engineering effects.
3. Plant Chemical and Structural Traits
   • Leaf Nitrogen Content and Secondary Compounds (Phenols, Tannins): Indicators of plant chemistry changes affecting herbivore fitness.
   • Plant Height and Cover: Structural traits influencing predator habitat and trophic interactions.
4. Abiotic Factors
   • Soil Moisture and Temperature: Measured to evaluate habitat conditions affecting herbivore fitness.

4. Table of SDGs, Targets, and Indicators Relevant to the Article

Source: nature.com