Nitrate and ammonium absorbed through roots are the two nitrogen sources that matter most for the vast majority of plants. Nitrate dominates in well-aerated soils, while ammonium tends to be more available in acidic or waterlogged conditions. But this textbook answer only scratches the surface. Plants have evolved a surprisingly wide repertoire of strategies for obtaining nitrogen, from ancient bacterial partnerships and fungal middlemen to absorbing amino acids directly from the soil and even pulling nitrogen compounds out of the air through their leaves.
Nitrate and Ammonium From the Soil
For most terrestrial plants, the soil solution is the primary nitrogen buffet, and its two main offerings are nitrate (NO₃⁻) and ammonium (NH₄⁺). Nitrate is the dominant form of nitrogen in most aerated soils, while ammonium can become the primary form in acidic or oxygen-poor environments.1Oxford Academic (Journal of Experimental Botany). Interactions between nitrate and ammonium in their uptake, allocation, assimilation, and signaling in plants Roots take up both forms, but the two behave very differently inside the plant. Ammonium is chemically reactive and can become toxic at high concentrations, so plants typically convert it quickly. Nitrate, by contrast, can be stored in vacuoles and shuttled around before being reduced to a usable form. Many crop species grow best with a mix of both, and the optimal ratio shifts depending on soil pH, temperature, and which species you are talking about.
Researchers have focused heavily on nitrate and ammonium because they are present in natural and cropland soils at much higher concentrations than other nitrogen sources.1Oxford Academic (Journal of Experimental Botany). Interactions between nitrate and ammonium in their uptake, allocation, assimilation, and signaling in plants That said, “higher concentrations” does not mean “the only game in town.” The picture gets more interesting when you zoom out.
Organic Nitrogen Absorbed Directly by Roots
Plants do not need microbes to break every organic molecule down to nitrate or ammonium before they can use it. Root cells carry their own amino acid transporters, and the capacity of plants to take up organic nitrogen directly has been demonstrated in both lab and field conditions.2PubMed. Uptake of organic nitrogen by plants Under normal circumstances this route contributes a smaller share than inorganic uptake, but in certain ecosystems it becomes the headliner.
The best-known example comes from arctic tundra. The sedge Eriophorum vaginatum rapidly absorbs free amino acids through its roots, and amino acids account for at least 60% of the nitrogen this species takes up in the field. When researchers grew it in solution culture with amino acids instead of inorganic nitrogen, it accumulated more nitrogen and more biomass. A cereal crop grown the same way fared poorly by comparison.3Nature. Preferential use of organic nitrogen for growth by a non-mycorrhizal arctic sedge In cold soils where microbial mineralization is slow, skipping the conversion step and grabbing amino acids directly is a major competitive advantage. This is not just an arctic curiosity, though. Many boreal and alpine plant communities likely rely on organic nitrogen uptake to varying degrees, and crop scientists are increasingly interested in whether breeding for better amino acid uptake could reduce fertilizer dependence.
Symbiotic Nitrogen Fixation
The atmosphere is roughly 78% nitrogen gas, but plants cannot use it directly. A handful of microbial partners, however, can crack the extraordinarily stable triple bond in N₂ and convert it to ammonia. The best-known partnership is between legumes and rhizobia, the soil bacteria that colonize legume roots and form nodules. Inside these nodules, the bacteria fix atmospheric nitrogen using solar energy captured by the plant, producing ammonia at normal temperature and pressure.4PubMed Central. Effectiveness of nitrogen fixation in rhizobia This relationship is a cornerstone of sustainable agriculture; planting legumes can replace a meaningful portion of synthetic fertilizer needs.
Legumes get most of the attention, but they are not the only plants with nitrogen-fixing partners. Alder trees (Alnus spp.) form symbioses with the bacterium Frankia, which also builds root nodules and fixes atmospheric nitrogen.5PubMed Central. Comparative analysis of nitrogen content and its influence on actinorhizal nodule and rhizospheric microorganism diversity in three Alnus species These actinorhizal plants span more than 200 species across eight plant families, and they colonize some of the most nutrient-poor landscapes on the planet, from volcanic rubble to eroded hillsides. Their potential for restoring degraded and contaminated soils remains largely unexplored.6Trends in Plant Science. Actinorhizal symbioses: a spotlight on unexplored potential for soil restoration
An even more exotic symbiosis exists in water. The floating fern Azolla hosts a cyanobacterium, Nostoc azollae, that fixes nitrogen for the plant.7PubMed Central. Comparative genomic insights into culturable symbiotic cyanobacteria from the water fern Azolla Azolla has been used as a green manure in rice paddies across Asia for centuries precisely because of this built-in fertilizer factory.
Free-Living and Endophytic Nitrogen Fixers
Not all nitrogen-fixing bacteria need to live inside a nodule. Free-living nitrogen fixers are found throughout the soil, and they are especially active near root surfaces, where sugars leaking from roots fuel their metabolism. This free-living fixation in the rhizosphere is ubiquitous and represents a significant nitrogen source in some ecosystems.8PubMed Central. To Fix or Not To Fix: Controls on Free-Living Nitrogen Fixation in the Rhizosphere The contribution varies enormously depending on soil moisture, carbon availability, and the existing nitrogen supply, and it rarely rivals symbiotic fixation in total output. Still, in nutrient-poor grasslands and forests, these free-living bacteria can be a meaningful part of the nitrogen budget.
A more recently appreciated pathway involves bacteria living inside plant tissues themselves. Limber pine, a subalpine conifer, consistently harbors acetic acid bacteria in its needles that appear to fix nitrogen right inside the foliage. Researchers confirmed that atmospheric nitrogen diffuses readily into the needles and that nitrogenase activity is positive across the growing season.9PubMed. Evidence for foliar endophytic nitrogen fixation in a widely distributed subalpine conifer Similar endophytic nitrogen fixation has been detected in dune grasses, where bacteria living in stem and rhizome cell walls produce the enzyme that drives fixation.10FEMS Microbiology Ecology. Endophytic nitrogen fixation in dune grasses (Ammophila arenaria and Elymus mollis) from Oregon These findings are changing how ecologists think about nitrogen budgets in forests and coastal systems, because they represent inputs that traditional accounting overlooked entirely.
Mycorrhizal Fungi as Nitrogen Brokers
Most land plants form partnerships with mycorrhizal fungi, whose threadlike hyphae extend far beyond what roots alone can reach. These fungi are famous for delivering phosphorus, but they also move substantial amounts of nitrogen to their host plants. When fungal hyphae were allowed access to patches of organic matter in controlled experiments, the fungi proliferated in the patch and transferred large quantities of nitrogen and phosphorus to the plant, with a simultaneous increase in plant biomass.11PubMed Central. Resolving the ‘nitrogen paradox’ of arbuscular mycorrhizas: fertilization with organic matter brings considerable benefits for plant nutrition and growth
The practical upshot is that in natural soils, plants do not access nitrogen alone. Their fungal allies break down organic matter, intercept ammonium and nitrate that might otherwise leach away, and deliver it through the network in exchange for carbon from photosynthesis. This is particularly relevant in nutrient-poor forests, where direct root uptake alone would not support observed growth rates.
Nitrogen Through the Leaves
Roots are not the only entry point. Plants can absorb reactive nitrogen compounds directly through their leaves. This phenomenon has been studied for decades, and foliar uptake can feed nitrogen directly into plant metabolism, potentially influencing growth more readily than the same amount deposited on the soil.12PubMed. Ecological ramifications of the direct foliar uptake of nitrogen The most common gaseous form taken up this way is ammonia. Classic experiments with corn seedlings showed that at low atmospheric ammonia concentrations, plants absorbed over 40% of the ammonia they were exposed to within 24 hours.13PubMed. Air containing nitrogen-15 ammonia: foliar absorption by corn seedlings
Foliar uptake is especially important for epiphytes, plants that grow on other plants and have no contact with the ground. In montane cloud forests, epiphytes perched on small branches without accumulated canopy soil tend to have lower foliar nitrogen and isotopic signatures suggesting they get a larger share of their nitrogen directly from rain.14PubMed. Nitrogen-15 natural abundance in a montane cloud forest canopy as an indicator of nitrogen cycling and epiphyte nutrition For a fern growing on a branch high above the forest floor, rain and fog are the soil.
Carnivorous and Parasitic Plants
Some plants have taken nitrogen acquisition to dramatic extremes. Carnivorous species like pitcher plants and sundews capture insects and absorb substantial amounts of nitrogen from their prey.15PubMed Central. Insects as a Nitrogen Source for Plants They do this by absorbing amino acids derived from digested insects directly through their specialized carnivorous leaves.16Progress in Earth and Planetary Science. Carnivorous plants: unveiling trophic identity and advanced nitrogen acquisition strategies These plants evolved carnivory precisely because they inhabit nutrient-starved bogs and sandy soils where conventional root uptake barely scrapes by.
Parasitic plants take a different shortcut: they tap directly into the vascular tissue of a host plant and siphon off water, sugars, and nitrogen compounds along with various secondary metabolites.17Ecology. Parasitic Plant‐Host Interactions: Plant Performance and Indirect Effects on Parasite‐Feeding Herbivores Mistletoe is the best-known example, but thousands of parasitic plant species exist worldwide. They represent a form of nitrogen acquisition that bypasses the soil entirely.
Fertilizers and Organic Amendments in Agriculture
In managed cropland, the largest nitrogen inputs come from synthetic fertilizers. Urea, ammonium nitrate, and urea-ammonium-nitrate solutions are the workhorses of modern agriculture. But efficiency is a persistent problem. Crops use only about half of the nitrogen applied to them; the rest escapes through leaching, volatilization, and other pathways.18PubMed Central. Nitrogen use efficiency-a key to enhance crop productivity under a changing climate Controlled-release formulations like polymer-coated urea can improve this picture. In wet years, polymer-coated urea outperformed standard urea by meaningful margins in corn yield, though the advantage disappeared in dry years.19Canadian Journal of Soil Science. Urea fertilizer forms affect grain corn yield and nitrogen use efficiency
Organic amendments tell a more complicated story. Different materials release their nitrogen at very different rates. Under optimal conditions, fast-release materials like feather meal and guano can make roughly 60 to 70% of their total nitrogen available within 100 days, while stable materials like yard waste compost and vermicompost release less than 10% over the same period.20PubMed. Nitrogen mineralization from organic fertilizers and composts: Literature survey and model fitting This means a gardener using finished compost should not expect the same quick nitrogen kick as someone using blood meal. Combining organic amendments with a modest dose of synthetic fertilizer can speed up nitrogen release considerably, with studies showing ammonium release jumping by roughly 180% or more compared to the organic amendment alone.21PubMed Central. Nitrogen Mineralization of Selected Organic Materials and Their Combined Effects with Nitrogen Fertilizer on Spinach Yield
How Plants Recycle Their Own Nitrogen
Nitrogen does not just flow in one direction. Plants actively recycle nitrogen from older, senescing leaves and redistribute it to younger growth and developing seeds. As a leaf ages, its proteins are broken down and the nitrogen is shuttled to wherever the plant needs it most. Research in Arabidopsis shows that total nitrogen, amino acid, and soluble protein content all decline as leaves age, while the enzymes responsible for nitrogen remobilization ramp up.22PubMed Central. Nitrogen Recycling and Remobilization Are Differentially Controlled by Leaf Senescence and Development Stage in Arabidopsis under Low Nitrogen Nutrition In plants grown under low nitrogen conditions, the internal recycling machinery controlled up to 60% of the nitrogen that ended up in the seeds.23Journal of Experimental Botany. Nitrogen remobilization during leaf senescence: lessons from Arabidopsis to crops
This remobilization is why autumn leaves turn yellow: the green chlorophyll is dismantled so the plant can recover the nitrogen locked inside it. For farmers and gardeners, it also explains why a crop can look green and healthy early on and still run short of nitrogen later if there is not enough in the soil to support both vegetative growth and grain fill at the same time.
Why So Much Soil Nitrogen Gets Lost
The nitrogen cycle includes steps that work against plants. Nitrification, the microbial conversion of ammonium to nitrate, is essential for generating the nitrate that most crops prefer. But nitrate is also highly mobile and easily leaches below the root zone. And once nitrogen enters the nitrate pool, soil microbes can convert it to nitrous oxide or dinitrogen gas through denitrification, sending it back to the atmosphere. Together, these pathways mean that only about 30 to 50% of applied nitrogen fertilizer is actually absorbed by crops.24PubMed Central. Nitrification a Boon or Curse to the Ecosystem: Nitrification Inhibitors and their Potential for Greener Agriculture
Temperature makes the problem worse. A global analysis found that elevated temperatures shifted soil nitrogen cycling toward faster mineralization, nitrification, and denitrification, increasing nitrous oxide emissions by up to 227% regardless of whether plants were present.25PubMed. Elevated temperature shifts soil N cycling from microbial immobilization to enhanced mineralization, nitrification and denitrification across global terrestrial ecosystems In a warming world, nitrogen will cycle faster through the soil, but that does not necessarily mean more of it ends up in plants. It may instead mean more is lost to the air and groundwater before roots can grab it.
Microplastics and Nitrogen Uptake
An emerging and unwelcome complication for plant nitrogen nutrition is microplastic contamination in soil. Conventional polyethylene microplastics at high concentrations have been shown to damage root cell membranes and reduce the number of xylem vessels in peanut plants, directly inhibiting nitrogen uptake. At the same time, these particles disturbed microbial communities responsible for nitrogen cycling in the surrounding soil, reducing nitrogen availability from both directions.26PubMed. Microplastics reduce nitrogen uptake in peanut plants by damaging root cells and impairing soil nitrogen cycling
In rice, even low concentrations of polyethylene microplastics reduced the proportion of fertilizer-derived nitrogen that plants actually took up. In microplastic-free soil, rice absorbed about a quarter of the fertilizer nitrogen; with microplastic contamination, that figure dropped to as low as 13%.27Journal of Hazardous Materials. Polyethylene microplastic impairs soil gross nitrification and reduces fertilizer-derived nitrogen uptake in rice Biodegradable plastics are not necessarily better. Small particles of polylactic acid, a common biodegradable plastic, significantly reduced plant nitrogen uptake by accelerating microbial immobilization so aggressively that plants could not compete.28Soil Biology and Biochemistry. The size-variation of microplastics alters plant nitrogen uptake by regulating soil gross nitrogen transformation rates Given that microplastics are accumulating in agricultural soils worldwide through irrigation water, plastic mulch, and contaminated composts, their interference with nitrogen cycling is a question that is only going to become more pressing.
How Climate Change Reshapes Nitrogen Availability
Rising carbon dioxide and warming temperatures interact with plant nitrogen nutrition in ways that are not straightforward. Elevated CO₂ can improve nitrogen uptake efficiency in some settings, as seen in greenhouse experiments with cucumber where plants grew more fine root biomass and absorbed more nitrogen under moderately elevated CO₂.29Applied Soil Ecology. Impacts of elevated CO2 on nitrogen uptake of cucumber plants and nitrogen cycling in a greenhouse soil But the picture changes when warming enters the equation. A meta-analysis found that warming alone increased root nitrogen uptake rates by about 13%, while the combination of elevated CO₂ and warming actually decreased uptake rates by about 15%.30AoB PLANTS. A meta-analysis of the combined effects of elevated carbon dioxide and chronic warming on plant %N, protein content and N-uptake rate This counterintuitive result suggests that the fertilization effect of extra CO₂ does not simply stack with warmer temperatures. Instead, the combined conditions may alter soil microbial competition or root physiology in ways that leave plants worse off for nitrogen than either factor alone would predict. For agriculture, this means that projections of future crop yields cannot assume that more CO₂ automatically compensates for other stresses.