Nitrogen-fixing plants are essential players in ecosystems worldwide. By forming partnerships with bacteria, these plants convert atmospheric nitrogen into forms that enrich the soil, supporting the growth of other plants and maintaining ecological balance.
Despite their importance, the factors that determine where these plants thrive—especially in temperate regions—have remained unclear. A recent study published in Global Ecology and Biogeography (May 2022) sheds new light on this puzzle.
Led by Joshua Doby and a team of ecologists, the research reveals that aridity, or dryness, is the most critical factor shaping the diversity and evolutionary history of nitrogen-fixing plants across North America. This finding challenges the long-held belief that soil nitrogen levels are the primary driver.
Understanding Nitrogen-Fixing Plants
Nitrogen is a building block of life. It is essential for DNA, proteins, and photosynthesis, the process by which plants convert sunlight into energy. While most plants rely on nitrogen present in the soil, nitrogen-fixing plants have a unique ability.
They form partnerships with bacteria in their root nodules, allowing them to “fix” nitrogen directly from the air. Symbiotic nitrogen fixation refers to this mutually beneficial relationship: the plant provides sugars to the bacteria, and the bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), a form the plant can use.
This adaptation lets them grow in poor or disturbed soils where other plants struggle. Examples include legumes like beans, clover, and acacia trees, as well as actinorhizal plants like alders and ceanothus.
These plants not only sustain ecosystems but also play vital roles in agriculture and reforestation. Understanding what limits their diversity is crucial for conservation and managing invasive species.
Research Methods: NEON Data Analysis
The research team analyzed data from the National Ecological Observatory Network (NEON), a large-scale project funded by the U.S. National Science Foundation to collect standardized ecological data across North America.
NEON operates 81 field sites, including Alaska, Hawaii, and Puerto Rico, and uses consistent methods to monitor plants, soils, climate, and other variables. For this study, scientists examined 1,520 plant plots within NEON sites, focusing on three main aspects of biodiversity:
1. Species Richness (SR): The total number of different species in a given area. Higher species richness often indicates a healthier, more resilient ecosystem.
2. Phylogenetic Diversity (PD): A measure of how evolutionarily distinct the species in a community are. Calculated as the total length of evolutionary branches connecting all species in a plot, PD reflects the breadth of evolutionary history present. For example, a community with species from distantly related lineages (e.g., a pine tree and a daisy) has higher PD than one with closely related species (e.g., different grasses).
3. Mean Pairwise Distance (MPD): The average evolutionary distance between all pairs of species in a community. High MPD means species are distantly related, suggesting diverse evolutionary strategies, while low MPD indicates closely related species with similar traits. Each plant species was classified into one of four groups:
- Native nitrogen-fixers: Plants like wild lupines (Lupinus) and redbuds (Cercis) that naturally grow in North America and form root nodules with nitrogen-fixing bacteria.
- Native non-fixers: Plants such as grasses, oaks, and pines that do not fix nitrogen and rely solely on soil nutrients.
- Exotic nitrogen-fixers: Invasive species like kudzu (Pueraria montana), introduced from other regions, which form nodules and alter soil chemistry.
- Exotic non-fixers: Non-native plants like cheatgrass (Bromus tectorum) that do not fix nitrogen and often outcompete native species.
The team also collected data on environmental factors, including:
- Soil Nitrogen (%): The percentage of nitrogen in the soil, measured through chemical analysis. Low nitrogen levels typically limit plant growth, but nitrogen-fixers bypass this constraint.
- Aridity Index: A ratio of annual precipitation to potential evapotranspiration (how much water evaporates under ideal conditions). Values below 0.65 indicate arid or semi-arid regions.
- Fire Frequency: The number of large fires (>500–1,000 acres) recorded at each site since 1984, sourced from the Monitoring Trends in Burn Severity (MTBS) database.
Advanced statistical models, including generalized linear mixed models (GLMMs), were used to determine how these factors influenced the diversity of nitrogen-fixing plants.
GLMMs are tools that account for both fixed effects (e.g., soil nitrogen) and random effects (e.g., variation between NEON sites), providing robust insights into ecological patterns.
Aridity Key to Plant Diversity
The study’s most significant discovery was the dominant role of aridity. While soil nitrogen and other factors were important, dryness emerged as the strongest predictor of nitrogen-fixing plant diversity. Here’s a closer look at the results:
1. Evolutionary Diversity Thrives in Dry Regions
Arid environments, such as the deserts of the southwestern United States, hosted nitrogen-fixing plants from distantly related evolutionary lineages.
For example, plants like mesquite (Prosopis) and indigo bush (Psorothamnus)—which have deep roots and high leaf nitrogen to conserve water—were common in these areas.
The study found that aridity alone explained 14.4% of the variation in phylogenetic diversity among nitrogen-fixers. In contrast, soil nitrogen and fire history had minimal effects.
Why Aridity Matters: Nitrogen-fixing plants often have higher leaf nitrogen content, which enhances photosynthesis and water-use efficiency. In dry environments, this trait helps minimize water loss while maximizing growth—a critical advantage over non-fixing plants.
For instance, mesquite trees in the Sonoran Desert use their deep taproots to access groundwater, while their high nitrogen leaves optimize carbon fixation under scorching conditions.
2. Native vs. Exotic Plants: A Tale of Two Strategies
Native nitrogen-fixing plants flourished in arid, low-nitrogen soils. Their species richness decreased in wetter regions, but not as sharply as non-fixing plants.
For instance, in warm, arid areas like Arizona, native nitrogen-fixers made up 30–40% of plant communities, compared to just 10–15% in humid regions. Exotic nitrogen-fixers, however, told a different story.
Species like kudzu thrived in moderate conditions but struggled in extremely dry or high-nitrogen soils. This suggests that invasive species may lack the specialized adaptations of their native counterparts.
The Role of Soil Nitrogen: While nitrogen-fixing plants are known to thrive in low-nitrogen soils, the study revealed a paradox: both nitrogen-fixing and non-fixing plants were more diverse in nitrogen-poor environments.
Native nitrogen-fixers saw an 18% increase in species richness for every 1% drop in soil nitrogen. Non-fixers also benefited, though less dramatically, with an 8% rise in species richness under the same conditions.
This implies that low nitrogen levels create opportunities for diverse plant strategies, not just nitrogen fixation. For example, non-fixing plants in these areas may rely on partnerships with fungi (mycorrhizae) to scavenge nutrients or adopt slow-growth strategies to conserve resources.
3. Latitude and Fire: Minor Players
The study found that nitrogen-fixing plants become less common at higher latitudes, such as in Alaska compared to Texas.
However, this pattern was driven by aridity, not latitude itself. Similarly, historical fire frequency—a factor thought to promote nitrogen-fixers by clearing competitors—had no significant impact on their diversity. This contrasts with findings from tropical regions, where fire often benefits nitrogen-fixing legumes by creating open habitats and enriching soils with ash.
Nitrogen-Fixers in Desert Ecosystems
To illustrate these findings, the study highlighted specific regions and species:
1. The Sonoran Desert: A Hotspot for Evolutionary Innovation
In the Sonoran Desert, nitrogen-fixing plants like mesquite and indigo bush dominate.
These plants belong to ancient evolutionary lineages with traits like deep root systems and high leaf nitrogen content, which help them survive extreme dryness.
The study found that these desert communities had 150–200% higher evolutionary diversity (MPD) compared to non-fixing plants in the same area.
For instance, mesquite (a member of the pea family) coexists with Psorothamnus (a distant relative in the bean family), showcasing a wide evolutionary spread. This diversity suggests that multiple lineages independently evolved strategies to thrive in arid conditions, providing a genetic reservoir for drought tolerance.
2. The Kudzu Invasion: Limits in Fertile Soils
Kudzu, an invasive nitrogen-fixer from Asia, thrives in the southeastern United States.
However, the study revealed that it struggles in high-nitrogen soils. In fertile areas, kudzu’s diversity dropped by 30%, as fast-growing non-fixing plants outcompeted it.
This suggests that managing soil nutrients could help control invasive species. For example, reducing nitrogen runoff from farms might limit kudzu’s spread while favoring native plants adapted to low-nutrient conditions.
Climate Change and Plant Conservation
The study’s findings have far-reaching implications for how we manage ecosystems in a changing climate:
1. Protecting Dryland Ecosystems
As global temperatures rise, arid regions are expected to expand.
Native nitrogen-fixing plants, with their drought-tolerant traits, could become critical for preventing soil erosion and maintaining biodiversity in these areas. Conservation efforts should prioritize protecting these species and their habitats.
For example, the Mojave Desert’s Astragalus species (a nitrogen-fixing legume) stabilize soils and provide food for pollinators, making them keystone species in this fragile ecosystem.
2. Managing Invasive Species
Invasive nitrogen-fixers like kudzu and saltcedar (Tamarix) pose significant threats.
The study suggests that these species may be less problematic in high-nitrogen soils, offering a potential management strategy. For instance, planting fast-growing non-fixing plants in fertile areas could suppress invasives.
Conversely, their spread in arid regions could worsen water scarcity, requiring careful monitoring. In Arizona, saltcedar trees consume large amounts of groundwater, displacing native willows and cottonwoods.
3. Agricultural and Restoration Strategies
Farmers and land managers can leverage nitrogen-fixing plants to improve soil health.
For example, planting native legumes like lupines (Lupinus) in degraded drylands could enhance soil fertility without the need for synthetic fertilizers.
Similarly, actinorhizal plants like alders (Alnus) could boost carbon storage in reforestation projects. In the Pacific Northwest, red alder trees enrich soils with nitrogen, supporting the growth of conifers like Douglas fir.
Future Nitrogen-Fixer Research Directions
While the study answers many questions, it also opens new avenues for exploration:
1. Symbiotic Relationships: How do the bacteria in root nodules influence the spread of exotic nitrogen-fixers? Are some species limited by a lack of compatible bacteria in new environments? For example, invasive Acacia species in South Africa often fail to form nodules with local bacteria, limiting their spread.
2. Global Patterns: Do these findings apply to tropical rainforests or Arctic tundra? Comparing results across biomes could reveal universal principles or regional exceptions. In the Amazon, nitrogen-fixing trees dominate in young, nutrient-poor soils, but their diversity declines in older forests—a pattern not seen in temperate zones.
3. Long-Term Monitoring: How will decades of climate change affect nitrogen-fixing plant communities? Continuous data collection, like that done by NEON, will be essential for tracking these changes. For instance, prolonged droughts in the U.S. Southwest could favor nitrogen-fixers over non-fixers, altering ecosystem dynamics.
Conclusion
This study revolutionizes our understanding of nitrogen-fixing plants. By showing that aridity—not soil nitrogen—is the key driver of their diversity, it challenges old assumptions and provides a fresh framework for ecological research.
The findings highlight the resilience of native nitrogen-fixers in dry environments and the complex challenges faced by invasive species. As droughts become more frequent due to climate change, protecting these arid-adapted plants will be crucial for sustaining ecosystems and supporting human livelihoods. By integrating evolutionary history, field data, and climate science, this research offers a roadmap for conserving biodiversity in an uncertain future.
Key Terms and Concepts
What is Nitrogen Fixation: Nitrogen fixation is the process where certain plants and bacteria convert atmospheric nitrogen gas (N₂) into ammonia (NH₃), a form usable by plants. This is crucial because most plants cannot use atmospheric nitrogen directly, relying instead on soil nutrients. Nitrogen-fixing plants, like beans or clover, form partnerships with bacteria in their root nodules to perform this conversion. This process enriches soil fertility, supports plant growth in nutrient-poor areas, and reduces the need for synthetic fertilizers in agriculture. For example, farmers plant legumes like soybeans to naturally boost soil nitrogen. Without nitrogen fixation, ecosystems would struggle to sustain plant life, especially in poor soils.
What is Aridity: Aridity refers to the dryness of a region, determined by low rainfall and high evaporation. It is measured using the Aridity Index, calculated as annual precipitation divided by potential evapotranspiration (how much water could evaporate). Values below 0.65 indicate arid regions, like deserts. Aridity shapes plant survival by limiting water availability. In the study, arid regions favored nitrogen-fixing plants like mesquite, which have deep roots and high leaf nitrogen to conserve water. Understanding aridity helps predict how plants adapt to droughts and climate change.
What is Phylogenetic Diversity (PD): Phylogenetic diversity measures the evolutionary relationships among species in an ecosystem. It calculates the total branch length of a “family tree” connecting all species in a community. High PD means species are distantly related, indicating diverse evolutionary strategies. For example, a desert with mesquite (a legume) and indigo bush (a distant relative) has high PD. The study found arid regions had higher PD for nitrogen-fixers, suggesting they host unique lineages. PD helps scientists assess ecosystem resilience, as diverse lineages may better withstand environmental changes.
What is Species Richness (SR): Species richness is the number of different species in a specific area. A forest with 50 tree species has higher SR than one with 10. In the study, nitrogen-fixing plants showed higher SR in arid, low-nitrogen soils. High SR often signals a healthy ecosystem, as more species can support complex food webs. However, invasive species can artificially inflate SR while harming native biodiversity. Monitoring SR helps conservationists identify areas needing protection.
What is Mean Pairwise Distance (MPD): Mean Pairwise Distance (MPD) measures the average evolutionary difference between all pairs of species in a community. High MPD means species are distantly related, like comparing a pine tree to a daisy. Low MPD indicates closely related species, like different grasses. In the study, arid regions had higher MPD for nitrogen-fixers, showing they evolved diverse strategies to survive dryness. MPD helps scientists understand how environmental pressures shape evolutionary traits.
What is NEON: The National Ecological Observatory Network (NEON) is a U.S.-funded project collecting long-term ecological data across 81 sites, including forests, deserts, and wetlands. NEON standardizes measurements of plants, soils, climate, and animals to track environmental changes. In the study, NEON data helped link aridity to nitrogen-fixer diversity. NEON’s open-access data supports global research on climate change, invasive species, and biodiversity loss.
What is a Symbiotic Relationship: A symbiotic relationship is a close interaction between two species where both benefit. For nitrogen-fixing plants, this involves bacteria (e.g., Rhizobia) living in root nodules. The plant provides sugars to the bacteria, and the bacteria convert nitrogen gas into usable ammonia. This mutualism allows plants to thrive in poor soils. Examples include clover and soybeans. Symbiosis is vital for ecosystem health, enabling nutrient cycling and plant growth.
What are Root Nodules: Root nodules are small, round structures on plant roots where nitrogen-fixing bacteria live. They form through a chemical dialogue between the plant and bacteria. Inside nodules, bacteria convert atmospheric nitrogen into ammonia. Legumes like peas and beans have prominent nodules. These structures are key to sustainable agriculture, reducing fertilizer use by naturally enriching soil.
What are Legumes: Legumes are plants in the pea family (Fabaceae) that form nitrogen-fixing root nodules with bacteria. Examples include beans, lentils, and acacia trees. Legumes improve soil fertility and are used in crop rotation to replenish nitrogen. In the study, desert legumes like mesquite showed high drought tolerance. Globally, legumes provide protein-rich food for humans and livestock.
What are Actinorhizal Plants: Actinorhizal plants, like alders and ceanothus, form nitrogen-fixing partnerships with Frankia bacteria. Unlike legumes, they belong to various plant families. These plants thrive in harsh soils, such as mining sites or cold climates, and help rehabilitate degraded land. In the study, their evolutionary diversity contributed to high PD in arid regions.
What is Soil Nitrogen: Soil nitrogen is the nitrogen content in soil, essential for plant growth. Most plants absorb nitrogen as nitrate (NO₃⁻) or ammonium (NH₄⁺). Low nitrogen limits growth, but nitrogen-fixers bypass this. The study found both nitrogen-fixers and non-fixers were diverse in low-nitrogen soils, suggesting multiple survival strategies. Excessive soil nitrogen from fertilizers can pollute waterways and harm ecosystems.
What is the Aridity Index: The Aridity Index measures regional dryness by dividing annual precipitation by potential evapotranspiration. Values below 0.65 indicate arid zones (e.g., deserts). The study used this index to show nitrogen-fixers thrive in dry areas. This index helps predict crop viability, water scarcity, and desertification risks.
What are Native Species: Native species naturally occur in a region without human introduction. Examples include wild lupines in North America. They are adapted to local conditions and support ecosystem balance. In the study, native nitrogen-fixers dominated arid regions, highlighting their ecological importance. Protecting native species prevents biodiversity loss.
What are Exotic Species: Exotic species are introduced to a region by humans, accidentally or intentionally. While some are harmless, others become invasive. The study noted exotic nitrogen-fixers like kudzu struggle in high-nitrogen soils. Exotic species can disrupt food chains and outcompete natives, requiring careful management.
What are Invasive Species: Invasive species are exotics that spread aggressively, harming ecosystems or economies. Examples include kudzu and cheatgrass. The study found invasive nitrogen-fixers alter soil chemistry and reduce native diversity. Controlling invasives often involves habitat restoration or biological controls.
What is an Evolutionary Lineage: An evolutionary lineage is a sequence of species descended from a common ancestor. For example, birds evolved from theropod dinosaurs. In the study, nitrogen-fixers in arid regions belonged to ancient lineages like Prosopis, showcasing long-term adaptation to dryness. Studying lineages helps trace trait evolution.
What is Climate Change: Climate change refers to long-term shifts in temperature, rainfall, and weather patterns, primarily driven by human activities like burning fossil fuels. It intensifies aridity, affecting plant distributions. The study suggests nitrogen-fixers may expand as deserts grow, impacting ecosystems and agriculture. Mitigating climate change requires reducing greenhouse gas emissions.
What is Soil pH: Soil pH measures acidity or alkalinity on a scale of 0 (acidic) to 14 (alkaline). Most plants prefer neutral pH (6–7). Acidic soils (pH <6) limit nutrient availability, while alkaline soils (pH >7) may cause toxicity. The study found soil pH influenced exotic non-fixers but not nitrogen-fixers. Adjusting pH with lime or sulfur optimizes crop growth.
What is Fire Frequency: Fire frequency is how often fires occur in an area. The study found no link between fire history and nitrogen-fixer diversity, unlike tropical studies where fire promotes legumes. Controlled burns can manage invasive species but may harm fire-sensitive ecosystems.
What are Generalised Linear Mixed Models (GLMMs): GLMMs are statistical tools analyzing data with both fixed effects (e.g., soil nitrogen) and random effects (e.g., site variations). The study used GLMMs to isolate aridity’s impact on diversity. GLMMs help ecologists account for complex variables in natural systems.
What is the Akaike Information Criterion (AIC): AIC evaluates statistical model quality, balancing fit and complexity. Lower AIC values indicate better models. The study used AIC to confirm aridity’s dominance over other factors. AIC helps researchers choose the most accurate models without overfitting.
What is Biodiversity: Biodiversity is the variety of life in an ecosystem, including species, genes, and habitats. High biodiversity, as seen in nitrogen-fixer communities, supports ecosystem services like pollination and soil health. The study highlights protecting biodiversity in arid zones to combat climate impacts.
What is Photosynthesis: Photosynthesis is the process plants use to convert sunlight, water, and CO₂ into glucose and oxygen. Nitrogen-fixers invest extra nitrogen into photosynthesis proteins, enhancing efficiency. This adaptation helps them thrive in arid regions, as seen in mesquite’s high-leaf nitrogen content.
What are Mycorrhizae: Mycorrhizae are symbiotic fungi that colonize plant roots, helping them absorb water and nutrients. Non-nitrogen-fixing plants in poor soils may rely on mycorrhizae instead of nodules. This partnership is crucial for forest ecosystems and crop productivity.
What is a Keystone Species: A keystone species has a disproportionate impact on its ecosystem. In deserts, nitrogen-fixers like mesquite are keystone species—they stabilize soils, provide food, and support other species. Protecting keystone species is vital for maintaining ecological balance.
Reference:
Doby, J. R., Li, D., Folk, R. A., Siniscalchi, C. M., & Guralnick, R. P. (2022). Aridity drives phylogenetic diversity and species richness patterns of nitrogen‐fixing plants in North America. Global Ecology and Biogeography, 31(8), 1630-1642. https://doi.org/10.1111/geb.13535
