Nitrate pollution has become one of the most widespread water quality challenges in intensively farmed regions around the world, threatening drinking water safety, aquatic ecosystems, and downstream lakes. A new study published in Nitrogen Cycling reveals how human activities in rural urban transition zones are reshaping the nitrogen cycle, allowing nitrate to move through rivers and groundwater and ultimately reach large freshwater lakes.
The research focuses on the rural urban ecotone of the Yangtze River Delta in eastern China, an area where agriculture, aquaculture, and residential development coexist. Using an integrated multi tracer approach, the research team traced nitrate sources and transformation processes across surface water and groundwater systems, offering one of the most detailed pictures to date of how nitrogen pollution travels through complex landscapes.
"Nitrate pollution does not come from a single source, and it does not stay in one place," said corresponding author Yanhua Wang. "Our goal was to understand not only where nitrate originates, but also how it moves, transforms, and accumulates as water flows from farmland and villages toward major lakes."
The team combined water chemistry measurements, dual stable isotope analysis, Bayesian mixing models, and a county scale nitrogen cascade model. This approach allowed them to distinguish nitrate derived from manure, chemical fertilizers, aquaculture effluent, soil leaching, and atmospheric deposition. Field samples were collected across multiple seasons from rivers, canals, and groundwater wells in two contrasting river networks upstream of Taihu Lake, one of China's largest freshwater lakes.
The results show that nitrate contamination was widespread in both surface water and groundwater, often exceeding regional water quality thresholds. In traditional agricultural areas, manure was identified as the dominant source of nitrate, contributing nearly 70 percent of surface water nitrate and about 60 percent of groundwater nitrate across seasons. In more industrialized and aquaculture intensive zones, aquaculture effluent emerged as the main nitrate source during dry and wet seasons, highlighting a pollution pathway that is often underestimated.
"Our findings show that manure and aquaculture are major drivers of nitrate pollution, yet they are frequently overlooked compared to chemical fertilizers," said Wang. "This has important implications for how we design pollution control strategies."
The study also revealed clear differences in nitrogen transformation processes. Nitrification dominated in agricultural surface waters, especially during dry periods, while denitrification was more pronounced in groundwater, where low oxygen conditions allow microbes to convert nitrate into other nitrogen forms. These underground processes can reduce nitrate concentrations locally, but they may also produce greenhouse gases such as nitrous oxide.
By integrating isotopic evidence with a nitrogen cascade model, the researchers linked observed water pollution to long term changes in land use and agricultural practices. Rapid expansion of aquaculture and commercial crop production has significantly increased nitrogen losses to aquatic systems in some counties, while reductions in cropland area and livestock restructuring have altered nitrogen pathways in others.
"Surface water and groundwater are not separate systems," Wang explained. "They act together as major transport pathways that deliver nitrogen from land to lakes. Effective water protection must consider both."
The study underscores the need for coordinated watershed management strategies that address manure application, pond aquaculture, and fragmented cropland management. According to the authors, reducing nitrate pollution in large lakes like Taihu will require integrated solutions that balance agricultural productivity with environmental protection, especially in rapidly urbanizing regions.
This multi tracer framework provides a powerful tool for policymakers and scientists seeking to untangle complex pollution sources and design targeted strategies to protect water resources in agricultural landscapes worldwide.
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Journal Reference: Zhao Z, Zhao J, Chen S, Ning L, Cai Z, et al. 2025. Tracing the nitrate source and process in rural-urban ecotone: integrated multi-tracer approach. Nitrogen Cycling 1: e011
https://www.maxapress.com/article/doi/10.48130/nc-0025-0011
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Nitrogen Cycling is a multidisciplinary platform for communicating advances in fundamental and applied research on the nitrogen cycle. It is dedicated to serving as an innovative, efficient, and professional platform for researchers in the field of nitrogen cycling worldwide to deliver findings from this rapidly expanding field of science.
