Soil organisms in military-impacted environments: A systematic review of microbial community studies, contamination types, and methodological gaps

This systematic review of 20 studies reveals that research on soil microbial communities in military-impacted environments relies heavily on 16S rRNA sequencing for explosive and heavy metal contamination, while suffering from significant methodological gaps including inconsistent physicochemical characterization and a lack of fungal, fauna, and soil compaction data.

The Gist

Imagine the soil as a bustling, underground city. It's not just dirt; it's a metropolis teeming with life—billions of tiny residents like bacteria, fungi, and microscopic worms. These "citizens" are the city's workforce: they recycle waste, build roads (soil structure), and keep the whole ecosystem running smoothly.

Now, imagine a giant, chaotic construction crew (the military) moving in. They bring in heavy tanks, fire cannons, and leave behind toxic spills from fuel and explosives. This doesn't just mess up the surface; it shakes the foundations of the underground city, poisoning the water, crushing the buildings, and scaring off the workers.

This paper is a systematic review, which is like a detective agency hiring a team to go through thousands of old case files to answer one big question: "What do we actually know about how this underground city is faring after the military has been there?"

Here is the story of what they found, broken down into simple terms:

1. The Detective Work (The Search)

The researchers acted like librarians with a super-powerful search engine. They looked through millions of scientific papers from 2000 to 2026. They were looking for any study that checked the "health" of the soil in places where the military had trained, fought, or stored weapons.

• The Result: Out of over 6,000 papers they found, only 20 were good enough to include. It's like searching a massive library for a specific type of book and finding only a tiny handful that actually fit the criteria. This tells us that while people are studying this, it's still a very niche field.

2. The One-Track Mindset (What They Studied)

When the researchers looked at those 20 studies, they noticed a huge bias. It's as if every detective only looked at the bacteria in the soil and ignored everyone else.

• The Bacteria Obsession: 80% of the studies used a specific tool (16S rRNA sequencing) to take a "headcount" of the bacteria.
• The Missing Neighbors: They almost completely ignored the fungi (the city's decomposers) and the soil animals (like worms and beetles, the city's engineers).
• The Analogy: It's like trying to understand a city's economy by only counting the number of accountants, while ignoring the construction workers, the teachers, and the doctors. You get a picture, but it's a very incomplete one.

3. The Missing Context (The "Why" and "How")

The researchers also noticed that many studies were like a doctor diagnosing a patient without asking about their diet or lifestyle.

• The Missing Data: Many studies counted the bacteria but didn't measure the soil's "vital signs." They often forgot to check the pH (acidity), moisture, or compaction (how hard the ground is).
• The Big Gap: Shockingly, none of the studies measured soil bulk density.
• The Analogy: Military vehicles crush the soil, making it as hard as concrete. If you don't measure how hard the ground is, you can't tell if the bacteria are sick because of poison (chemicals) or because they are being suffocated by the weight of a tank. It's like trying to figure out why a plant is dying without checking if someone stepped on it.

4. The Villains (The Contaminants)

The studies focused heavily on two types of "villains":

• Explosives: Old-school bombs like TNT and RDX.
• Heavy Metals: Mostly lead from bullets and shells.
• The Blind Spot: They barely looked at fuel spills (hydrocarbons) or newer, modern explosives.
• The Analogy: It's like a detective squad that only investigates murders committed with a specific type of knife, completely ignoring poison, guns, or car accidents. As the military changes its weapons, the research is lagging behind.

5. The Geography of Knowledge (Where They Looked)

The map of these studies was very uneven.

• The Hotspots: The USA and China had the most studies.
• The Dark Zones: Areas with recent or ongoing wars (like parts of the Middle East or Eastern Europe) were almost empty.
• The Analogy: It's like having a weather map that only shows the forecast for New York and Beijing, but leaves the rest of the world blank. We know a lot about the soil in quiet military bases in the US, but we know very little about the soil in active war zones where the damage is freshest and most severe.

6. The Verdict (The "Methodological Completeness Index")

The authors created a score (0 to 1) to see how "complete" each study was.

• The Score: The average score was low (around 0.4).
• The Meaning: Most studies were "half-baked." They gave us a list of bacteria but didn't tell us enough about the environment those bacteria were living in.
• The Analogy: Imagine a restaurant review that says, "The food was spicy," but doesn't mention the ingredients, the chef, the temperature of the kitchen, or the price. It's a review, but it doesn't help you understand the meal.

The Bottom Line

This paper is a wake-up call. It tells us that while we have some cool technology to count bacteria in military soil, we are missing the big picture.

To truly understand how war and training affect the earth, scientists need to:

1. Look at the whole city: Study fungi and animals, not just bacteria.
2. Check the vital signs: Measure how hard the soil is, how wet it is, and what chemicals are there.
3. Look at the new threats: Study modern fuels and new types of bombs.
4. Go to the front lines: Do research in active conflict zones, not just in safe, quiet training grounds.

Until then, our understanding of how the military impacts the planet remains a puzzle with half the pieces missing.

Technical Summary

1. Problem Statement

Military activities (training, warfare, weapons testing) significantly alter terrestrial ecosystems through physical disturbance (compaction, cratering) and chemical contamination (explosives, heavy metals, hydrocarbons). While soil microbial communities are critical for ecosystem functioning and contaminant degradation, the current state of research regarding these communities in military-impacted soils is fragmented.

• The Gap: It remains unclear whether existing studies provide comprehensive environmental characterization or if research is biased toward specific contaminants, organisms, or geographic regions.
• The Need: A systematic synthesis is required to identify methodological trends, gaps in environmental data integration, and priorities for future investigation to better understand ecological impacts and inform remediation.

2. Methodology

The study employed a rigorous systematic review protocol following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines.

• Search Strategy:
  • Databases: Scopus, Web of Science, Dimensions, PubMed, and Google Scholar.
  • Timeframe: 2000–2026.
  • Keywords: Functional blocks covering soil/sediment environments, soil organisms/microbial communities, biological profiling methods (sequencing, culture), and military/conflict descriptors.
  • Automation: Python scripts were used for text mining, deduplication (via DOI), and initial keyword-based screening.
• Screening Process:
  • Initial Pool: 6,474 records identified.
  • Deduplication: 85 duplicates removed (6,389 remaining).
  • Screening: Automated filtering reduced the pool to 62 records for manual review. Two independent researchers verified eligibility.
  • Final Selection: 20 studies met the inclusion criteria (soil/sediment samples, military context, biological community analysis via sequencing or profiling).
• Data Extraction & Analysis:
  • Variables Extracted: Study metadata, site characteristics, soil physicochemical variables (pH, SOC, texture, bulk density, etc.), contaminant types, and biological methods.
  • Methodological Completeness Index (MCI): A novel metric was developed to quantify study quality. It scored studies (0 to 1) based on four domains:
    1. Soil physicochemical characterization.
    2. Contaminant assessment.
    3. Biological community profiling.
    4. Site context reporting.

3. Key Contributions

• First Systematic Synthesis: This is the first review to systematically aggregate literature specifically on soil biological communities in military-impacted environments using modern sequencing-based approaches.
• Development of MCI: The introduction of the Methodological Completeness Index (MCI) provides a quantitative framework to evaluate how well studies integrate environmental context with biological data, highlighting a systemic lack of holistic study designs.
• Identification of Critical Gaps: The review explicitly identifies the disconnect between biological profiling and physicochemical characterization, as well as the underrepresentation of fungi, soil fauna, and emerging contaminants.

4. Key Results

A. Study Selection and Geography

• Total Studies: 20 studies included.
• Geographic Distribution: Highly skewed. The USA accounted for 35% (7 studies), followed by China (15%) and Belgium (10%).
• Conflict Zones: Notably, very few studies originated from regions with recent or ongoing armed conflict (e.g., Ukraine, Vietnam), likely due to logistical and safety barriers.

B. Biological Methods

• Dominance of Bacteria: 16S rRNA amplicon sequencing was used in 80% (16/20) of studies.
• Understudied Groups:
  • Fungal communities (ITS sequencing): Only 1 study.
  • Soil fauna: Only 1 study.
• Methodological Mix: Many studies combined molecular methods with culture-based isolation. Advanced techniques like shotgun metagenomics were rare (2 studies), and Nanopore sequencing was absent.

C. Physicochemical Characterization

• Inconsistency: Reporting was highly variable.
  • pH: Measured in 60% of studies.
  • Organic Matter/Texture: Measured in 45%.
  • Microbial Biomass/Enzyme Activity: Reported in <20%.
  • Bulk Density: 0% of studies reported this, despite its critical importance in military landscapes (vehicle compaction).
• Implication: Biological responses are often interpreted without sufficient environmental context.

D. Contaminant Types

• Primary Focus: Explosives (45% of studies) and Heavy Metals (40%).
  • Explosives: RDX (most common), TNT, HMX. Newer compounds like DNAN were rarely studied.
  • Metals: Lead (Pb) was the most reported metal (associated with small-arms fire), followed by Cu and As.
• Secondary Focus: Hydrocarbons (20% of studies).

E. Methodological Completeness (MCI)

• Scores: MCI scores ranged from 0.175 to 0.561 (Mean: 0.405).
• Classification: Most studies fell into the "basic" category, indicating a lack of integration between soil properties, contaminant levels, and biological data. Only a few achieved "moderate" completeness.

5. Significance and Recommendations

• Ecological Interpretation: The lack of comprehensive physicochemical data (especially bulk density and enzyme activity) limits the ability to distinguish whether microbial changes are driven by chemical toxicity or physical soil disturbance.
• Future Research Priorities:
  1. Integrated Design: Future studies must combine microbial sequencing with detailed soil physicochemical and contaminant profiling.
  2. Broaden Taxonomic Scope: Include fungi and soil fauna to capture full ecosystem functioning.
  3. Emerging Contaminants: Investigate modern insensitive munitions (e.g., DNAN) and hydrocarbon mixtures.
  4. Geographic Expansion: Prioritize research in active or recently active conflict zones to understand real-time ecological impacts.
• Conclusion: While molecular tools have advanced the understanding of bacterial communities in military soils, the field suffers from methodological fragmentation and a lack of holistic environmental context. Addressing these gaps is essential for effective ecosystem management and remediation strategies.