Antibiotics hidden in soil are becoming an increasingly important environmental concern, and new research suggests that the problem may be especially difficult to control in saline–alkali farmland. A study published in Environmental Geochemistry and Health has examined how two widely discussed soil amendments—pig manure and biochar—change the chemical forms and biological availability of oxytetracycline and sulfamethoxazole in salt-affected soils. The findings indicate that carefully selected amendment doses can substantially reduce the fraction of antibiotics available to plants, microorganisms, and soil fauna. But the results also reveal a critical warning: more amendment does not always mean better remediation. In particular, excessive biochar increased the short-term availability of oxytetracycline, demonstrating that the environmental behavior of antibiotics depends on a complex interaction between pollutant chemistry, soil salinity, organic matter, pH, and amendment properties.
The research focused on oxytetracycline, commonly abbreviated as OTC, and sulfamethoxazole, or SMX. These compounds belong to different antibiotic families and behave differently once they enter soil. Oxytetracycline is a tetracycline antibiotic with several functional groups capable of binding minerals, organic matter, and metal ions. Its mobility is strongly influenced by pH and by the presence of calcium, magnesium, iron, and other cations that can form bridges between the antibiotic and soil surfaces. Sulfamethoxazole is a sulfonamide antibiotic whose charge changes with environmental pH. Depending on conditions, it may exist in neutral, positively charged, or negatively charged forms, each with different sorption and transport characteristics. These chemical differences help explain why the two antibiotics responded differently to the same amendment and why saline–alkali soils can create a particularly unpredictable setting for contamination control.
Saline–alkali soils contain elevated concentrations of soluble salts and, in many cases, excessive exchangeable sodium and high pH. Such conditions can alter soil structure, reduce water infiltration, affect microbial activity, and modify the electrical forces operating at mineral surfaces. Salt ions may compete with antibiotics for adsorption sites, compress the electrical double layer around soil particles, and change the strength of interactions between charged contaminants and clay or organic matter. The study found that increasing soil salinity generally increased antibiotic bioavailability. This means that even when the total concentration of an antibiotic remains unchanged, a larger fraction may become chemically accessible to organisms or more readily available for movement into soil water. That distinction is important because total residue measurements alone do not necessarily indicate ecological risk.
To measure bioavailability, the researchers examined the fraction of each antibiotic that could be mobilized from the soil and potentially interact with living organisms. Bioavailability is not identical to total concentration. A contaminant may be present in soil but trapped inside mineral structures, strongly bound to aged organic matter, or retained within microscopic pores. Such fractions are often described as bound or residual forms and are generally less immediately accessible. Other fractions remain weakly adsorbed or dissolved in soil water and can be taken up by roots, encountered by microbes, or transported toward groundwater. The study also investigated the occurrence forms of the antibiotics, tracing how amendments shifted residues from more mobile pools into more strongly bound or residual pools. This approach provides a more realistic assessment of remediation performance than simply asking how much antibiotic remains.
At an early stage, the amendments produced distinct effects. Seven days after application, a 2 percent biochar treatment produced the strongest reduction in OTC bioavailability, lowering it by 50.34 percent. Biochar is a carbon-rich material created by heating biomass under oxygen-limited conditions. Its porous structure, high internal surface area, aromatic carbon domains, and oxygen-containing functional groups can provide numerous sites for contaminant retention. Oxytetracycline can interact with biochar through π–π electron donor–acceptor interactions, hydrogen bonding, electrostatic attraction, pore filling, and cation bridging. However, when the biochar dose was increased beyond the effective level, OTC bioavailability rose rather than continued to fall. The researchers suggest that high amendment doses may introduce dissolved organic matter or alter soil chemistry in ways that compete with sorption, increase antibiotic solubilization, or make previously retained molecules more mobile.
Sulfamethoxazole followed a different pattern. Its bioavailability consistently declined as the dosages of both biochar and pig manure increased. This contrast reflects the different molecular structure and ionization behavior of SMX. Pig manure contributes organic carbon, humic-like substances, minerals, nutrients, and microbial communities. These components can create new sorption domains and promote the formation of less mobile antibiotic–organic matter associations. At the same time, manure-derived dissolved organic matter can sometimes increase contaminant transport by keeping antibiotics in solution or by competing for mineral surfaces. The net result depends on the balance between these processes. In the reported experiments, increasing manure and biochar doses produced an overall decline in SMX bioavailability, indicating that retention and transformation into less accessible forms outweighed potential mobilizing effects.
The role of salinity became particularly clear when the researchers increased salt levels from a non-saline control to 7 grams per kilogram. Under these conditions, 2 percent biochar inhibited the salinity-related increase in OTC bioavailability by 39.40 percent. For SMX, the same treatment constrained the increase in bioavailability to only 3.83 percent. These results suggest that biochar can partly buffer the chemical changes caused by salt accumulation, potentially by providing additional sorption surfaces and modifying the distribution of ions and organic molecules in the soil solution. Pig manure also showed compound-specific behavior. At a 2 percent dose, it initially increased and then decreased OTC bioavailability, revealing a transient mobilization phase followed by stronger retention or transformation. For SMX, the manure treatment directly reduced bioavailability by 32.06 percent. The findings emphasize that amendment performance cannot be generalized across all antibiotics or all salinity levels.
The most striking results emerged after 60 days of incubation. By that time, 2 percent biochar had reduced OTC bioavailability by 83.03 percent and SMX bioavailability by 65.21 percent. The corresponding reductions under 2 percent pig manure were 50.47 percent and 60.42 percent. These changes indicate that time was an important component of remediation. Antibiotics may gradually diffuse into biochar pores, become incorporated into increasingly stable organic associations, bind to mineral surfaces, or undergo chemical and biological transformation. This process is often called aging or sequestration. Aging can reduce the speed at which contaminants desorb into soil water, although it does not necessarily destroy the molecules. A strongly bound antibiotic may be less immediately bioavailable but could become mobile again if soil pH changes, organic matter decomposes, salts are leached, or the amendment itself ages under field conditions.
The study has implications well beyond a laboratory comparison between two amendments. Pig manure is frequently used as a fertilizer, especially in agricultural systems where livestock production and crop cultivation are closely linked. Yet manure can also introduce antibiotic residues and antibiotic-resistance genes into soil. Biochar, by contrast, is often promoted as a circular-economy material that can improve soil structure, retain nutrients, and immobilize pollutants. The new findings show that its application must be optimized rather than treated as a universally safe solution. An unsuitable biochar dose, feedstock, production temperature, particle size, or surface chemistry could produce weaker retention or even increase short-term contaminant availability. The authors’ results support a targeted strategy in which amendment rates are selected according to antibiotic type, soil salinity, pH, organic-matter content, and the intended duration of remediation.
The research also highlights why antibiotic contamination in salt-affected farmland deserves urgent attention. Salinization is expanding in many arid and semi-arid regions because of evaporation, insufficient drainage, irrigation practices, seawater intrusion, and climate-driven water stress. At the same time, antibiotics continue to enter agricultural environments through livestock manure, treated wastewater, sludge, and irrigation water. When these pressures overlap, soil chemistry may amplify the mobility and ecological accessibility of pharmaceutical residues. Bioavailable antibiotics can influence microbial communities, disturb nutrient cycling, affect soil organisms, enter crops, and contribute to the selection and dissemination of antibiotic-resistance genes. The study does not claim that biochar or pig manure alone eliminates these risks. Instead, it offers evidence that properly managed amendments can shift antibiotics toward more strongly bound and residual forms, reducing their immediate environmental activity while also underscoring the need for long-term monitoring, dose control, and field-scale validation.
Subject of Research: Effects of biochar and pig manure on the occurrence forms and bioavailability of oxytetracycline and sulfamethoxazole in saline–alkali soil.
Article Title: The effect of soil amendments on the occurrence forms and bioavailability of antibiotics in saline–alkali soil
Article References: Chen, R., Yao, R., Chen, Y. et al. “The effect of soil amendments on the occurrence forms and bioavailability of antibiotics in saline–alkali soil.” Environmental Geochemistry and Health, volume 48, article 555 (2026).
Image Credits: AI Generated
DOI: 10.1007/s10653-026-03454-8
Keywords: Saline–alkali soil; antibiotics; oxytetracycline; sulfamethoxazole; biochar; pig manure; occurrence form; bioavailability; soil amendments; antibiotic contamination
