Nitrates and phosphates enter the environment from a surprisingly wide range of sources, both natural and human-made. In nature, nitrates form mainly through microbial processes that convert atmospheric nitrogen gas into reactive forms, while phosphates come primarily from the slow weathering of ancient rocks. Human activity has massively amplified both nutrient flows: synthetic fertilizer production, phosphate mining, livestock farming, and industrial discharge now dwarf natural inputs in most populated watersheds. Understanding where these nutrients originate matters because their overabundance drives water pollution, algal blooms, and health risks that affect billions of people.
Natural Sources of Nitrate
The atmosphere is roughly 78 percent nitrogen gas, but most living things cannot use nitrogen in that form. It has to be “fixed” first, meaning converted into compounds like ammonia or nitrate that plants and microbes can absorb. In nature, this conversion happens two main ways: lightning strikes that fuse nitrogen and oxygen into nitrogen oxides, and specialized bacteria that pull nitrogen gas out of the air and turn it into ammonia. That ammonia then gets oxidized by other soil and water bacteria into nitrite and then nitrate, a two-step process called nitrification.
Research in environments as extreme as coastal Antarctica has confirmed that microbial nitrogen fixation, nitrification, and denitrification all operate actively even in ice-free polar soils and lake sediments, demonstrating how fundamental and widespread these biological processes are.1bioRxiv. Unveiling unique microbial nitrogen cycling and novel nitrification drivers in coastal Antarctica Wherever soil, water, and microbes coexist, some amount of nitrate is being produced naturally.
Geological sources also contribute. Certain sedimentary rock formations contain nitrogen-bearing minerals, and as groundwater flows through them, it can pick up nitrate. A study of volcanic and lacustrine deposits in a river system found nitrate-nitrogen concentrations of 0.11 to 0.33 mg/L originating from shallow aquifers that intersected nitrogen-bearing geologic materials.2Heliyon. Geologically-derived nitrogen and phosphorus as a source of riverine nutrients These concentrations are modest compared to agricultural inputs, but they establish a natural baseline that has always existed in many watersheds.
Natural Sources of Phosphate
Phosphorus is locked inside rocks. Unlike nitrogen, it has no significant gaseous phase that cycles through the atmosphere. The primary natural mechanism that frees phosphorus is weathering: rain, frost, and chemical reactions slowly dissolve phosphorus-bearing minerals and release phosphate ions into soil and water. The most common of these minerals is apatite, a calcium phosphate found in igneous and sedimentary rock worldwide.
A long-term study in the Rocky Mountains found that dissolution of bedrock fluorapatite was the principal phosphorus source to local waters, releasing about 48 mol of phosphorus per hectare per year, a rate enhanced by frost-cracking in the periglacial landscape.3Chemical Geology. Decoupling between silicate and phosphate mineral weathering in the granitic periglacial weathering-limited landscape of the Loch Vale watershed, Rocky Mountain National Park, Colorado, USA Volcanic deposits contribute as well. The same river study that measured geologic nitrogen also attributed phosphate-phosphorus concentrations of 0.132 to 0.255 mg/L to rapid release from weathering volcanic deposits.2Heliyon. Geologically-derived nitrogen and phosphorus as a source of riverine nutrients
Biological sources matter too. Seabird and bat colonies concentrate phosphorus in their guano, which accumulates over centuries and can form substantial phosphate deposits. Under rainy conditions, soluble compounds wash out of guano, and if this happens on a limestone or coral substrate, the phosphates react with calcium to form dense, apatite-rich “rock guano.”4IntechOpen. Guano: The White Gold of the Seabirds Some of the phosphate deposits mined commercially today started this way millions of years ago.
Synthetic Fertilizers and the Haber-Bosch Process
Natural nitrogen fixation was never fast enough to feed a growing global population. The invention of the Haber-Bosch process in the early twentieth century changed that by enabling industrial-scale conversion of atmospheric nitrogen into ammonia, which is then processed into ammonium nitrate, urea, and other fertilizers. This single technology has been feeding humanity for more than a hundred years, but it depends on fossil fuels and is both energy- and carbon-intensive.5Environmental Research Letters. Energy and food security implications of transitioning synthetic nitrogen fertilizers to net-zero emissions
The sheer scale of synthetic nitrogen production is staggering. Roughly half of the nitrogen atoms in the protein in your body likely trace back to the Haber-Bosch process. But most of the fertilizer applied to fields does not stay there. Plants take up a fraction, and the rest enters streams, rivers, and groundwater as nitrate through leaching and runoff. This is the single largest reason nitrate levels in waterways around the world have climbed over the past century.
Phosphate Mining
Phosphorus fertilizers come from a fundamentally different supply chain. Because phosphorus does not cycle through the atmosphere, it must be mined from sedimentary phosphate rock deposits. Global production runs in the range of 170 million tons of phosphate rock per year.6Heliyon. Comprehensive Recovery and Sustainable Development of Phosphate Resources The rock is processed into phosphoric acid, which is then used to make fertilizers like diammonium phosphate and triple superphosphate.
Phosphate rock is a non-renewable resource. Deposits are concentrated in a handful of countries, with Morocco and Western Sahara holding the largest known reserves. The ore also contains other elements, including uranium and rare earths. Uranium in phosphate accounts for more than 80 percent of the world’s unconventional uranium resources, and rare earth elements in the annual phosphate rock production total nearly 100,000 tons. If these elements are not recovered during processing, they end up in fertilizers and are spread across farmland, making future recovery essentially impossible.6Heliyon. Comprehensive Recovery and Sustainable Development of Phosphate Resources This gives phosphate mining a resource-depletion dimension that nitrogen fertilizer production does not share.
How Agriculture Moves Nitrate and Phosphate Into Water
Farming is the dominant human pathway for both nutrients entering waterways, but the mechanisms differ for nitrate and phosphate. Nitrate is highly soluble and moves easily through soil with percolating rainwater, reaching groundwater or emerging in tile drains. Phosphate binds tightly to soil particles, so it travels primarily by surface runoff, attached to eroded sediment or dissolved in water flowing over the land surface after heavy rain.
Animal manure adds both nutrients simultaneously. Research on cattle slurry applied to arable land found that applications increased ammonium-nitrogen losses in surface water compared to plots receiving inorganic fertilizer only or no treatment, and that rainfall events immediately after manure application were especially likely to generate nutrient runoff.7Environmental Pollution. Nutrient losses by surface run-off following the application of organic manures to arable land. 1. Nitrogen Studies of beef cattle manure and compost applied to fields with crop residues found similar patterns for phosphorus runoff, with losses depending on whether the manure was left on the surface or incorporated into the soil by disking.8Journal of Environmental Quality. Phosphorus and Nitrogen in Runoff following Beef Cattle Manure or Compost Application
Timing is everything. A heavy storm within a day or two of manure spreading can wash a large slug of nutrients off the field in a single event. This is why many jurisdictions now restrict when and how manure can be applied, requiring soil incorporation or banning spreading on frozen ground or before forecasted rain.
Industrial and Urban Sources
Agriculture gets most of the attention, but cities and factories contribute nitrates and phosphates too. Municipal wastewater is rich in both nutrients. Human waste contains nitrogen from protein metabolism and phosphorus from both diet and detergents. Even after treatment, effluent discharged into rivers and coastal waters carries measurable nutrient loads.
Certain industries produce nitrate-heavy waste streams. Metal-finishing operations, for instance, use nitric acid for etching and pickling, generating wastewater with nitrate concentrations far above what natural systems can absorb. One study of a metal-finishing plant documented wastewater nitrate concentrations between 141 and 210 mg nitrate-nitrogen per liter, orders of magnitude higher than what is found in natural surface water.9Heliyon. Biological nitrate removal from wastewater of a metal-finishing industry Explosives manufacturing, food processing, and pharmaceutical production also generate nitrate-laden effluents.
Urban stormwater is an underappreciated phosphate source. Lawn fertilizers, pet waste, leaf litter decomposing on pavement, and even car wash runoff all carry phosphorus into storm drains. Unlike agricultural runoff, which can be intercepted by vegetated buffers, stormwater from paved surfaces often flows directly into streams through pipe networks.
Atmospheric Deposition
Nitrate also falls from the sky. Nitrogen oxides released by vehicle exhaust, power plants, and industrial combustion react in the atmosphere with ozone and hydroxyl radicals to form nitric acid, which returns to the surface as acid rain or dry deposition. Research comparing urban and rural sites in Northeast China found that the chemical pathways forming atmospheric nitrate shift between seasons and locations: in summer, the reaction of nitrogen dioxide with hydroxyl radicals dominates, while in winter, ozone-related pathways contribute a much larger share of atmospheric nitrate formation.10Heliyon. Atmospheric nitrate formation pathways in urban and rural atmosphere of Northeast China
Atmospheric nitrogen deposition is particularly important in ecosystems far from farms. Mountain lakes, remote forests, and coastal waters that receive little direct agricultural runoff can still accumulate excess nitrogen from the air. In some sensitive alpine and Arctic environments, atmospheric deposition has become the primary driver of nitrogen enrichment.
Internal Loading From Lake and Ocean Sediments
One counterintuitive source of phosphate is the very sediment at the bottom of a lake or estuary. Phosphorus that settled out of the water column years or decades ago can be re-released when conditions change, a phenomenon called internal loading. The trigger is usually oxygen depletion. When bottom waters go anoxic, iron compounds in the sediment that had been holding phosphate in place dissolve, and the phosphate diffuses back into the water above.
Lab experiments have confirmed that phosphorus is released from sediments under both anoxic and oxygenated conditions, but release is substantially greater when oxygen is absent. The transformation between different phosphorus forms in the sediment shifts during these episodes, with iron- and aluminum-bound phosphorus behaving differently under each regime.11PubMed. Effects of oxygen on the release and distribution of phosphorus in the sediments under the light condition High-resolution measurements at the water-sediment interface in a eutrophic lake showed that during anaerobic intervals, concentrations of dissolved iron and phosphorus in pore water surged upward into the overlying water, confirming that re-dissolution of iron-bound phosphorus pools is the most important source of mobile phosphorus and worsens the overall phosphorus budget of the lake.12Science of The Total Environment. In-situ, high-resolution evidence from water-sediment interface for significant role of iron bound phosphorus in eutrophic lake
This creates a frustrating feedback loop. Excess phosphorus causes algal blooms, the dead algae sink and decompose, consuming oxygen, and the resulting anoxia releases even more phosphorus from the sediment. Research on shallow lake areas estimated that these zones contributed about half of the total phosphorus flux caused by sediment anoxia.13Aquatic Sciences. Internal phosphorus loading due to sediment anoxia in shallow areas: implications for lake aeration treatments This is why some lakes remain nutrient-rich long after external pollution sources have been cleaned up: decades of phosphorus stored in their sediments keeps recycling back.
What Happens When Too Much Arrives
When nitrate and phosphate levels climb in a body of water, the result is eutrophication: explosive growth of algae and aquatic plants that chokes out other life. Harmful algal blooms are one of the most visible consequences. A scientific consensus statement noted that degraded water quality from increased nutrient pollution promotes the development and persistence of many harmful algal blooms, and that the composition of the nutrient pool, not just the total quantity, affects which species dominate.14PubMed Central. Eutrophication and Harmful Algal Blooms: A Scientific Consensus
The downstream effects cascade. Agricultural, urban, and industrial activities have dramatically increased nitrogen and phosphorus pollution from headwater streams to coastal areas worldwide, creating hypoxic “dead zones” that reduce fish and shellfish production, generating taste and odor problems in drinking water, stimulating greenhouse gas releases, and degrading cultural and recreational values of waterways.15WIREs Water. Nutrients, eutrophication and harmful algal blooms along the freshwater to marine continuum The Gulf of Mexico dead zone, fed by the Mississippi River’s nutrient load, is one of the most studied examples, but similar zones exist in the Baltic Sea, the East China Sea, and hundreds of smaller water bodies.
Health Risks From Nitrate in Drinking Water
Nitrate contamination of groundwater is a direct health concern for people who depend on wells. Regulatory limits for nitrate in drinking water were originally set to protect against methemoglobinemia in infants, a condition where nitrite (converted from nitrate in the gut) interferes with the blood’s ability to carry oxygen. But the health risks extend further. A comprehensive review found that the strongest evidence for adverse effects beyond methemoglobinemia includes colorectal cancer, thyroid disease, and neural tube defects, with many studies observing increased risk at nitrate levels below current regulatory limits.16PubMed Central. Drinking Water Nitrate and Human Health: An Updated Review
The problem is widespread. An assessment of groundwater across India’s Indo-Gangetic Plain, one of the most intensively farmed regions on Earth, found that a substantial share of sampled wells posed a potential health risk, with children under six facing higher risk than adults.17Groundwater for Sustainable Development. Nitrate contamination in groundwater and associated health risk assessment for Indo-Gangetic Plain, India Nitrate contamination in groundwater is largely due to fertilizer and manure applications in agricultural areas combined with improper waste disposal. Unlike bacteria or many chemical contaminants, nitrate is not removed by standard household water filters or boiling.
Where Nitrate Disappears Naturally
Nature has its own way of removing excess nitrate. Denitrification, carried out by soil and sediment bacteria, converts nitrate back into nitrogen gas, which escapes to the atmosphere and is effectively lost from the ecosystem. This process is the dominant pathway by which fixed nitrogen leaves forest ecosystems, restricting the availability of a key nutrient for plant productivity.18PubMed Central. Microbial denitrification dominates nitrate losses from forest ecosystems A second microbial pathway, called dissimilatory nitrate reduction to ammonium, retains the nitrogen in the soil but in a different form. Both of these nitrate-reducing processes also produce nitrous oxide, a potent greenhouse gas, which means even the natural removal of nitrate has climate implications.19PubMed Central. Soil nitrate reducing processes – drivers, mechanisms for spatial variation, and significance for nitrous oxide production
Phosphorus has no comparable atmospheric escape route. Once phosphorus enters a watershed, it can only leave by being buried in deep sediments, taken up by organisms that are physically removed (like harvested fish or crops), or flushed downstream to the ocean where it eventually settles on the seafloor. This asymmetry is why phosphorus pollution in lakes can be so persistent. Reducing nitrogen inputs often produces faster water quality improvements than reducing phosphorus, because denitrification keeps working in the background.
Reducing Nutrient Runoff
Vegetated buffer strips along streams and rivers are one of the most studied tools for intercepting nutrients before they reach water. A subtropical field study found that all three types of buffer zones tested (forest, grassland, and mixed) delayed surface runoff by roughly twofold compared to bare cropland and effectively retained phosphate loads in both surface and subsurface flow. Forest buffers achieved the highest surface water phosphorus reduction at about 80 percent, while grassland buffers cut surface nitrate loads by roughly 50 percent.20Ecological Engineering. Potential of different buffer zones as nature-based solutions to mitigate agricultural runoff nutrients in the subtropics The catch is that buffers can sometimes become nutrient sources themselves if they accumulate too much over time, so the type of vegetation and management matter.
Short-rotation willow plantings along waterways offer a dual benefit. A life cycle assessment of willow riparian buffer strips found that while the system received some phosphate inputs, the nutrients removed via regular willow harvest far exceeded those inputs, making the system a net phosphorus sink over its lifetime rather than a burden.21Renewable and Sustainable Energy Reviews. Life cycle assessment of a short-rotation coppice willow riparian buffer strip for farm nutrient mitigation and renewable energy production The harvested willow biomass can also be used for bioenergy, turning a pollution problem into a fuel source.
At the wastewater treatment end, technologies for recovering phosphorus rather than just removing it are gaining traction. Controlled crystallization can recover phosphorus as struvite, a slow-release fertilizer mineral, at rates of 80 to 90 percent from concentrated waste streams. The process also reduces ammonia concentrations by about 29 percent.22Heliyon. Struvite precipitation within wastewater treatment: A problem or a circular economy opportunity? Recovering phosphorus from wastewater closes a loop that currently runs in one direction: mine to field to drain to sea.
Global Trade and Virtual Nutrient Flows
Nitrates and phosphates do not just flow through rivers. They also flow through global supply chains, embedded invisibly in traded food and feed. When a country exports soybeans, it effectively exports the nitrogen and phosphorus that went into growing them, depleting its own soils and concentrating those nutrients wherever the crops are consumed or processed.
Between 1997 and 2016, the total physical flow of nutrients in globally traded agricultural products increased from about 10 Tg nitrogen and 1.4 Tg phosphorus to 27 Tg nitrogen and 3.5 Tg phosphorus, roughly a 2.8-fold increase. Virtual nutrient flows, which account for all the nutrients used in production rather than just those in the final product, grew from 13.4 to 36.6 Tg nitrogen and from 8.9 to 24.5 Tg phosphorus over the same period.23Nature Communications. Physical and virtual nutrient flows in global telecoupled agricultural trade networks In 2014, total global agricultural phosphorus trade was about 2.78 Tg per year, with roughly 95 percent embodied in crops and 5 percent in livestock products. More than half of this traded crop phosphorus was exported by just the United States, Brazil, and the European Union.24Global Environmental Change. Influences of international agricultural trade on the global phosphorus cycle and its associated issues
This pattern means that phosphorus depletion and phosphorus pollution are happening in different places. Exporting nations mine their phosphate rock and draw down their soil reserves, while importing nations concentrate nutrients in feed lots and wastewater systems that were never designed to handle them. The geography of where nitrates and phosphates “come from” is increasingly decoupled from the geography of where they cause environmental damage, a disconnect that makes the problem harder to manage through local regulation alone.