Nitrogen fertilization outweighs plant species loss in shaping bacterial belowground diversity in an alpine meadow on the central Tibetan Plateau

A seven-year study on the Tibetan Plateau demonstrates that nitrogen fertilization-induced soil acidification exerts a stronger negative impact on alpine grassland bacterial diversity than plant species loss, primarily by suppressing oligotrophic taxa and stimulating copiotrophic ones.

The Gist

Imagine the Tibetan Plateau as a massive, high-altitude garden. For centuries, this garden has been home to a specific, tiny, hardy plant called Kobresia pygmaea (let's call it the "Carpet Grass"). This little plant is the garden's superstar; it grows low to the ground, forms thick mats that hold the soil together, and keeps the ecosystem running smoothly.

Underneath this garden, there is a bustling, invisible city of bacteria living in the soil. These bacteria are the garden's sanitation workers, recyclers, and engineers, keeping the soil healthy and fertile.

This study asked a big question: What happens to this underground bacterial city if we mess with the garden above? Specifically, the researchers wanted to know which of two things causes more damage:

1. Removing the plants: What if we pull out the "Carpet Grass" and other common plants?
2. Adding fertilizer: What if we dump a huge amount of nitrogen fertilizer (urea) on the garden to make it grow faster?

They ran an experiment for seven years, removing plants and adding fertilizer to different plots, then looked at what happened to the soil bacteria. Here is what they found, explained simply:

1. The Fertilizer Bomb vs. The Missing Plant

Think of the soil bacteria as a community of people who prefer a very specific, mild climate.

• The Missing Plant: When the researchers removed the dominant "Carpet Grass," the garden changed. The soil got a little more acidic (sour), but the bacterial city didn't panic too much. In fact, because the dominant grass was gone, other plants had a chance to grow, making the plant community a bit more "even." The bacteria actually seemed to like this slight shift, with their diversity staying stable or even increasing slightly.
• The Fertilizer Bomb: When they added the heavy dose of urea fertilizer, it was like dumping a bucket of vinegar and salt into the soil. The soil became extremely acidic (sour). This was a disaster for the bacteria. The "good guys" (the diverse, slow-growing specialists) died off or left, and the "bad guys" (fast-growing, opportunistic bacteria) took over.

The Verdict: The fertilizer was the real villain. While removing plants changed the garden's look, the fertilizer changed the soil's chemistry so drastically that it wiped out the bacterial diversity. Nitrogen fertilization outweighed plant species loss.

2. The "Acid Rain" Effect

Why did the fertilizer hurt so much?
Imagine the soil bacteria are like fish in a pond. They are adapted to a specific water pH (acidity level).

• The fertilizer didn't just add food; it triggered a chemical reaction that turned the soil into "acid rain."
• This acidification was the main driver of the bacterial collapse. It's like if you suddenly made the pond water too acidic for the fish to breathe. The bacteria that couldn't handle the acid vanished, and only the tough, acid-loving ones survived.

3. The Seasonal Rhythm

The study also found that the bacteria have a natural rhythm, like a clock. Their numbers and types changed naturally as the seasons went from summer to autumn.

• The Growing Season: The bacteria were most active and diverse in August (the peak of summer).
• The Fertilizer Factor: The fertilizer messed with this natural clock. It didn't just change who was there; it changed when they were there. The fertilizer made the soil so acidic that the bacterial community shifted toward "copiotrophs" (bacteria that love a feast and grow fast) and away from "oligotrophs" (bacteria that are used to slow, steady living).

4. The Big Picture: Why Should We Care?

The Tibetan Plateau is a "Third Pole," holding massive amounts of carbon (like a giant carbon sponge). The bacteria in the soil are the ones holding that carbon in place.

• If we keep adding fertilizer to these fragile grasslands (perhaps to help them recover from overgrazing), we might accidentally kill the soil bacteria.
• When the bacteria change from "slow and steady" to "fast and feast-loving," they start eating the soil carbon faster. This releases carbon back into the atmosphere, which can speed up global warming.

The Takeaway

In this high-altitude garden, losing a few types of plants is like rearranging the furniture. It changes the look of the room, but the people inside can still live there.

But dumping too much fertilizer is like setting the house on fire. It changes the fundamental environment (the acidity), forcing the residents (the bacteria) to flee or die, and replacing them with a very different, less diverse group.

The study warns us that while we might think adding fertilizer helps the grass grow, in these delicate ecosystems, it does more harm to the invisible life underground than even losing the plants themselves.

Technical Summary

1. Problem Statement and Context

The study addresses the complex interactions between two major global change drivers: plant species loss (biodiversity decline) and anthropogenic nitrogen (N) loading (fertilization/deposition). While both factors are known to alter ecosystem functioning, their interactive effects on plant communities, soil physicochemical properties, and the soil microbiome in high-altitude ecosystems remain insufficiently understood.

The research focuses on the Tibetan Plateau, a climate-relevant region ("Third Pole") hosting vast carbon-rich alpine grasslands dominated by the sedge Kobresia pygmaea. These ecosystems are vulnerable to rapid warming and increasing N inputs (from atmospheric deposition and restoration efforts). The study aims to determine:

1. Whether plant species loss or N fertilization exerts a stronger influence on soil bacterial diversity.
2. Whether these factors act primarily through direct mechanisms or indirect pathways (e.g., via changes in soil chemistry).

2. Methodology

The researchers utilized a long-term field experiment (7 years) established in 2014 at the Nagqu Alpine Grassland Ecosystem National Field Scientific Observation and Research Station (4,512 m a.s.l.).

• Experimental Design:
  • Plant Removal Treatments: Four treatments were applied to 1.5 × 1.5 m plots (8 replicates each):
    1. Control (no removal).
    2. Removal of Kobresia pygmaea (Kp).
    3. Removal of Potentilla saundersiana (Ps).
    4. Removal of both species (Kp + Ps).
  • Nitrogen Treatment: Half of the plots received annual urea application (146.7 g/plot, equivalent to 300 kg N ha⁻¹ yr⁻¹) in mid-July, representing a high fertilization regime.
• Sampling:
  • Plant Diversity: Assessed in August 2021 (peak growing season) using a grid method (400 points per plot).
  • Soil Sampling: Rhizosphere soil was collected in July, August, and October 2021. Samples were processed for physicochemical analysis and molecular biology.
• Soil Analyses:
  • Physicochemical: pH, Total Organic Carbon (TOC), Total Organic Nitrogen (TON), Ammonium (NH₄⁺), and Nitrate/Nitrite (NO₃⁻/NO₂⁻).
  • Molecular:
    • qPCR: Quantified total bacterial abundance (16S rRNA gene).
    • Amplicon Sequencing: 16S rRNA gene sequencing (Illumina MiSeq) to characterize bacterial community composition (ASVs).
• Statistical Analysis:
  • Diversity Metrics: Hill numbers (0, 1, 2) for alpha diversity; Unweighted UniFrac distance for beta diversity.
  • Modeling: Linear Mixed-Effect Models (LMM) to assess treatment effects; PERMANOVA for community composition variance; Random Forest for variable importance; and Partial Least Squares Structural Equation Modeling (PLS-SEM) to disentangle direct vs. indirect causal pathways.

3. Key Results

A. Plant Community Responses

• Species Richness: Both urea addition and plant removal significantly reduced plant species richness. Urea addition had a strong negative effect ($\beta = -0.500$). Removal of the dominant K. pygmaea caused the most severe decline in richness, while removing the subordinate P. saundersiana alone had a negligible effect.
• Community Composition: Both factors significantly shifted plant beta-diversity (composition), explaining ~21% (urea) and ~27% (removal) of the variation. No significant interaction was found between the two factors.
• Evenness: Urea addition significantly decreased plant evenness. Plant removal tended to increase evenness when the dominant species was removed.

B. Soil Property Changes

• Acidification: Urea addition caused a drastic decrease in soil pH (approx. 1 unit) during the vegetative period. Plant removal also lowered pH, but to a lesser extent.
• Nitrogen Dynamics: Urea addition increased NH₄⁺ (peaking in July) and NO₃⁻/NO₂⁻ (increasing throughout the season). Plant removal generally had a positive effect on NO₃⁻/NO₂⁻.
• Carbon/Nitrogen Ratios: TOC, TON, and TOC:TON ratios were primarily driven by the growing season (temporal dynamics) rather than the experimental treatments.

C. Bacterial Diversity and Community Structure

• Abundance: Total bacterial abundance was negatively affected by urea addition but positively influenced by the growing season duration.
• Alpha Diversity:
  • Urea Addition: Significantly decreased bacterial alpha diversity (richness and abundance).
  • Plant Removal: Surprisingly, caused a slight increase in bacterial alpha diversity, likely due to the creation of more even niche spaces after the removal of the dominant plant.
• Beta Diversity:
  • The growing season was the primary driver of bacterial community composition (explaining 35% of variance).
  • Urea addition explained 7% of the variance and significantly shifted community composition.
  • Plant removal had no significant effect on bacterial beta diversity.
• Taxonomic Shifts: Urea addition suppressed oligotrophic phyla (Acidobacteriota, Chloroflexota, Planctomycetota, Gemmatimonadota) and stimulated copiotrophic phyla (Bacillota, Bacteroidota, Pseudomonadota).

D. Mechanistic Drivers (PLS-SEM)

Structural Equation Modeling revealed the causal pathways:

• Indirect Effects: The most significant driver of bacterial diversity loss was soil acidification caused by urea addition. Acidification negatively impacted alpha diversity and shifted beta diversity.
• Nitrogen Species: Increased NO₃⁻/NO₂⁻ (driven by urea) also influenced diversity, while NH₄⁺ had a negative indirect effect.
• Direct Effects: Urea addition had a direct negative effect on bacterial alpha diversity (likely due to osmotic stress or toxicity), whereas plant removal had a minor direct effect.

4. Key Contributions

1. Quantifying Relative Impact: The study provides empirical evidence that in alpine meadows, nitrogen fertilization (via acidification) is a stronger driver of bacterial biodiversity loss than plant species loss.
2. Decoupling Direct vs. Indirect Effects: It demonstrates that while plant removal alters plant community structure significantly, its impact on the soil microbiome is largely mediated by soil chemical changes (pH) rather than direct plant-microbe feedbacks.
3. Oligotroph vs. Copiotroph Shift: It documents a specific microbial functional shift in high-altitude soils under high N input, moving from slow-growing, nutrient-efficient oligotrophs to fast-growing copiotrophs.
4. Temporal Dynamics: It highlights that seasonal variation (growing season duration) is a critical, often overlooked, driver of soil microbial dynamics, sometimes outweighing long-term treatment effects.

5. Significance and Implications

• Ecosystem Stability: The findings suggest that N-induced acidification poses a greater threat to belowground biodiversity than the loss of specific plant species in these ecosystems.
• Carbon Cycling: The shift toward copiotrophic bacteria under N fertilization may accelerate soil organic carbon turnover, potentially releasing stored carbon and creating a positive feedback loop for global climate change.
• Restoration Strategies: For degraded grasslands on the Tibetan Plateau, the study cautions that high-rate N fertilization (often used for restoration) may inadvertently degrade soil microbial health and biodiversity, even if it boosts plant biomass.
• Policy: The results underscore the need to balance N inputs in restoration projects to avoid irreversible shifts in soil microbiome structure and function.

In conclusion, the paper establishes that soil acidification driven by nitrogen fertilization is the dominant mechanism reshaping bacterial communities in Tibetan alpine meadows, outweighing the effects of plant species loss.