New Accounting Method Puts Algae at the Center of Nitrogen Trading Markets

Nitrogen pollution is one of the most stubborn environmental problems of our time, choking rivers, fueling algal blooms, and suffocating coastal ecosystems. For decades, regulators have struggled with a deceptively simple question: is a ton of nitrogen from a sewage treatment plant the same as a ton of nitrogen washing off a eroding riverbank? A new study published in Environmental Management argues that the answer is a resounding no—and that getting this wrong could quietly undermine the environmental markets built to protect our waterways. Researchers led by Jing Lu of Griffith University, together with colleagues from the University of Queensland, Queensland University of Technology, and the consulting firm Alluvium, have developed an exploratory accounting method that, for the first time, builds the different biological punch of nitrogen from different sources directly into the arithmetic of nutrient trading.

Nutrient trading schemes work like carbon markets for water pollution. A point source—typically a sewage treatment plant (STP) or an aquaculture farm—facing expensive upgrades to meet tighter effluent limits can instead pay a third party to reduce nitrogen loads elsewhere in the same watershed. Those offsets usually come from diffuse sources: eroding streambanks, fertilizer runoff, or urban stormwater. The logic is economically elegant. Where diminishing returns make every additional kilogram of removal at a treatment plant prohibitively costly, watershed mitigation actions such as riverbank stabilization, riparian revegetation, and wetland restoration can often deliver reductions more cheaply—while also stacking co-benefits like biodiversity gains, carbon sequestration, flood mitigation, and habitat connectivity. These actions fall under the umbrella of nature-based solutions, formally defined by the United Nations Environment Assembly as actions to protect, conserve, restore, and sustainably manage ecosystems that address societal challenges while providing human well-being and ecosystem services.

But the market’s core promise—that the buyer’s discharge is fully neutralized by the seller’s reduction—rests on an assumption that scientists have long known to be shaky. Most existing schemes simply count total nitrogen (TN), treating every kilogram as environmentally equivalent no matter where it comes from. The new research, building on a series of bioassay studies by the same team, shows why that assumption fails. In standardized three-day laboratory experiments, the researchers exposed a nitrogen-starved freshwater alga to effluents from tertiary-treated sewage plants and aquaculture ponds, and to laboratory-simulated erosion runoff—so-called soil slurries prepared from soils collected across five eastern Australian watersheds, including the Lockyer, Brisbane, Logan, and Bowen River catchments in Queensland and the Hawkesbury–Nepean in New South Wales. The critical discovery: the best predictor of algal photosynthetic response was not total nitrogen but total dissolved nitrogen (TDN), the fraction that algae can take up immediately.

The proportions of dissolved nitrogen vary wildly between sources. In STP effluents and aquaculture pond samples, roughly 94 percent of total nitrogen was present as dissolved nitrogen. In soil-derived runoff, the dissolved fraction ranged from as little as 2 percent up to 20 percent, depending on the soil and site. That means a ton of nitrogen in treated sewage hits algae very differently than a ton locked in eroding soil particles. To quantify the difference, the team fitted Michaelis–Menten models—the same saturating kinetics used to describe enzyme reactions—to the algal response curves for each source. The model yields a half-saturation constant, K, the concentration at which algae achieve half their maximum photosynthetic response. Because the maximum response appeared consistent across sources, the ratio of half-saturation constants between buyer and seller provides a biologically grounded equivalence measure.

The resulting numbers are striking. When a sewage treatment plant is the credit buyer and soil erosion mitigation is the seller, the equivalency ratio is approximately 0.26 to 0.27, meaning each unit of STP dissolved nitrogen generates roughly four times the algal impact of a unit of dissolved nitrogen from eroded soil. For aquaculture ponds, the ratio is higher, around 0.68 to 0.86. Sensitivity testing showed these values barely change across plausible maximum algal response values, giving the approach a measure of robustness. Translated into market terms: not all nitrogen credits are created equal, and ignoring that fact either over- or under-compensates for real ecological damage.

To turn these findings into a practical accounting framework, the researchers decomposed the traditional trading ratio—the multiplier dictating how much reduction a seller must deliver per unit of buyer discharge—into four separable components. The equivalency ratio captures the source-specific biological impact. The delivery ratio accounts for transport and processing of nitrogen within waterways between seller and buyer locations. The uncertainty factor provides a safety margin for imperfect load estimates and mitigation performance. And a co-benefit factor, currently a placeholder set at one, would discount required reductions when mitigation actions deliver additional environmental value. In their illustrative formula, the required seller reduction equals the buyer’s nitrogen load multiplied by all four factors. The beauty of this decomposition is transparency: rather than a single opaque ratio negotiated behind closed doors, each assumption becomes an explicit, testable, and updatable parameter.

The team demonstrated the method with a case study built around the Oxley sewage treatment plant, which discharges into the mid-Brisbane River estuary. Offsetting 10 tonnes of nitrogen per year under the conventional baseline—total nitrogen with an equivalency ratio of one, a delivery ratio of one, a co-benefit factor of one, and a conservative uncertainty factor of 1.5—requires about 15 tonnes per year of nitrogen reduction from riverbank mitigation. But switching the accounting currency to dissolved nitrogen changes the picture dramatically. Because eroded soil delivers so little of its nitrogen in dissolved form, the TDN-based scenario with full equivalency would demand between 70.5 and 705 tonnes per year depending on the soil’s dissolved fraction—up to 47 times the baseline requirement. Applying the empirically derived equivalency ratio of 0.3 brings that back down to 21 to 211 tonnes per year, or 1.4 to 14 times the baseline. The lesson is clear: where a mitigation project is sited matters enormously, and schemes that prioritize erosion sites with higher dissolved nitrogen proportions can achieve the same ecological protection with far less work.

The authors are candid about the limitations. The delivery, co-benefit, and uncertainty factors are placeholders reflecting current knowledge gaps rather than rigorously calibrated values, though the uncertainty range of 1.5 to 4 mirrors trading ratios used in United States programs. The equivalency ratio derives from a single biological indicator—algal photosynthetic response—and does not capture other impacts such as oxygen demand, biodiversity loss, or hypoxia; related work by the team has shown that organic carbon from different nutrient sources can differentially drive estuarine oxygen consumption. The bioassay data also come exclusively from Australian watersheds and treatment systems, and the method assumes negligible in-stream processing between buyer and seller, a simplification reasonable for event-driven flood exports but potentially wrong where denitrification or long residence times prevail. Temporal mismatches—continuous sewage discharge versus episodic erosion pulses—and lag times before mitigation takes effect remain unsolved, likely requiring seasonal crediting rules, dynamic ratios, or credit discounts until monitoring confirms performance.

Even so, the framework offers something nutrient markets have sorely lacked: a scientifically defensible way to equate environmental impacts across fundamentally different pollution sources, while explicitly inviting refinement as evidence accumulates. By disaggregating trading ratios into transparent components, the method promotes adaptive management and reduces the risk of arbitrary ratio-setting that has historically eroded regulator and investor confidence. The researchers position nitrogen trading not as a license to pollute but as an engine for funding watershed restoration—an investment stream that has been chronically underfunded because diffuse loads are so hard to quantify. If markets are to deliver real ecological outcomes rather than paper compliance, they must rest on accounting that respects the biology of the receiving waters. This study provides a template for how that might work, and a challenge for the field: measure the nitrogen that actually matters, not just the nitrogen that is easy to count.

Subject of Research: An environmental accounting method for nitrogen trading between point and diffuse pollution sources based on algal response to dissolved nitrogen

Article Title: An Exploratory Accounting Approach for Balancing Environmental Impacts of Point and Diffuse Sources in Nitrogen Trading

Article References: Lu, J., O’Brien, K. R., Egger, F., Weber, T., Olley, J. M., Adams, M. P., & Burford, M. A. (2026). An Exploratory Accounting Approach for Balancing Environmental Impacts of Point and Diffuse Sources in Nitrogen Trading. Environmental Management, 76(10), Article 325. https://doi.org/10.1007/s00267-026-02626-7

Image Credits: AI Generated

DOI: 10.1007/s00267-026-02626-7

Keywords: nitrogen trading, nutrient trading, point source pollution, diffuse source pollution, total dissolved nitrogen, water quality, algal blooms, watershed mitigation, soil erosion, nature-based solutions, environmental equivalency, environmental markets