Nature’s warning, science’s answer: ZJU team charts road map to sustainable emissions control

As spring transitions into summer, the weather often takes a wild turn: one moment the sun blazes overhead and temperatures spike above 30°C, the next moment skies darken and torrential rain sends the mercury tumbling. These extreme weather events aren’t just seasonal quirks; they’re part of a broader signal from nature. The global climate is undergoing profound change.

Carbon and nitrogen, two essential elements in Earth’s biogeochemical cycles, are closely bound up with major climate and environmental concerns, from greenhouse gas emissions to PM2.5 air pollution. Now, a research team at Zhejiang University is offering new insight into how these elements interact and how we might manage them more effectively.

Led by Professor GU Baojing from the College of Environment & Resource Sciences, the team has developed a coupled carbon-nitrogen model based on the Coupled Human And Natural System (CHANS) framework. By tracking the movement of carbon and nitrogen across soil, water, air, and key industrial sectors, the researchers have painted a detailed picture of how these elements circulate through both ecosystems and human activities in China. Their findings point to a promising solution: an integrated management approach that could dramatically cut emissions while delivering climate, health, and ecological benefits, all at a lower cost.

Published in Science on June 6, 2025, the study is titled “Integrated carbon and nitrogen management for cost-effective environmental policies in China”. The paper’s lead author is XU Xin, a PhD student at Zhejiang University, and its corresponding author is Professor GU Baojing.

In living systems, carbon and nitrogen are like twin brothers. Carbon forms the structural backbone of organic molecules, while nitrogen powers life’s engine, fueling protein synthesis and driving genetic diversity.

At the macro-ecosystem level, the interplay between carbon and nitrogen affects soil health, vegetation growth, and biodiversity. From individual organisms to entire ecosystems, carbon and nitrogen maintain a delicate equilibrium, one that helps sustain the planet’s ecological balance.

However, since the dawn of industrialization, extensive development patterns have disrupted this equilibrium. Intensive human activities, such as the burning of fossil fuels and excessive fertilizer use in agriculture, have led to excessive emissions of carbon and nitrogen. This has triggered a series of environmental issues, including air pollution, acid rain, eutrophication, and the greenhouse effect, which are increasingly pushing the planet beyond its environmental limits.

Spatiotemporal characteristics of China's carbon and nitrogen emissions and their relative shares

“Our model data show that between 1980 and 2020, nitrogen losses in China (through atmospheric and water emissions) increased by 2.3 times, while carbon emissions surged by a staggering 6.5 times,” said XU Xin. “High carbon emissions are mainly concentrated in the Yangtze River Delta, the Pearl River Delta, and the Beijing-Tianjin-Hebei region. Nitrogen losses are most severe in areas such as the middle and lower Yangtze River plain, the North China Plain, and the Sichuan Basin. During this period, the carbon-to-nitrogen emission ratio surged from 35:1 to 119:1, an alarming signal that ecosystems are under intense strain.”

“This imbalance is like a seesaw tilted to one side,” explained Professor GU Baojing. “When carbon and nitrogen inputs are out of sync, it can hamper plants’ ability to absorb excess greenhouse gases. If carbon emissions are curbed without simultaneously addressing nitrogen pollution, we run the risk of weakening forest carbon sinks and layering one environmental problem on top of another. That’s why coordinated carbon-nitrogen management is essential.”

With the implementation of China’s “dual carbon” strategy (peaking emissions before 2030 and achieving carbon neutrality by 2060) and policies aimed at curbing pollution, the research team has shifted its focus to coordinated carbon-nitrogen governance.

“Our goal is to build a coupled carbon-nitrogen model that could systematically capture the interactions in integrated management and evaluate both the economic trade-offs and policy implications,” said GU Baojing.

To this end, the team explored the mechanisms of carbon and nitrogen cycling across various human-nature systems. “We analyzed over 6,000 carbon and nitrogen flow pathways from 1980 to 2020 in China. We divided the research area into 16 key subsystems, including cropland, livestock, aquaculture, forests, grasslands, energy, industry, transportation, human, solid waste, and wastewater,” XU Xin explained. “On the strength of these findings, we developed the CHANS-CN model, which builds upon our team’s 2015 CHANS nitrogen cycle model and represents a significant leap from a single-element model to a coupled carbon-nitrogen model.”

“For a long time, dynamic carbon-nitrogen management across interfaces (such as soil, water, or air) and sectors (such as industry, agriculture, or forestry) has not been fully explored,” GU Baojing noted. To assess the potential and feasibility of integrated carbon-nitrogen management, the team developed a suite of China-specific management strategies based on extensive field experiments and big data analysis, considering system mechanisms, emission reduction potential, health and environmental impacts, and policy viability.

The team tested four scenarios: integrated carbon-nitrogen management, carbon-only reduction, nitrogen-only reduction, and business-as-usual. “Simulation results show that the integrated carbon-nitrogen approach has the most optimal overall effect,” XU Xin said. “By 2060, this strategy could reduce carbon emissions by 91% and nitrogen emissions by 74%, bringing the carbon-to-nitrogen emission ratio back within a safe and sustainable range.”

To better understand the full value of integrated management, the research team introduced a “carbon-nitrogen impact equivalent” index to monetize the climate, ecological, and health impacts of emissions. Compared with managing carbon or nitrogen alone, integrated carbon-nitrogen strategies cut the unit abatement cost by 37%. In financial terms, an investment of $424 billion is projected to generate $1.8 trillion in societal benefits. Among the most cost-effective measures are optimizing energy systems, applying fertilizers more precisely in agriculture, and boosting energy efficiency in transportation.  

With the advantages of integrated carbon-nitrogen governance now clearly demonstrated, the research team proposed a series of recommendations to align production processes with China’s natural and socioeconomic conditions, in the hope of taking orderly control of carbon and nitrogen emissions and achieving a green transformation.

“From the perspective of policies, we should prioritize cost-effective measures. For instance, optimizing power transmission and storage infrastructure in the energy system, accelerating the adoption of clean energy, improving fuel efficiency in transportation, and fine-tuning feed formulae in livestock farming,” GU Baojing explained.  

Furthermore, region-specific strategies should be formulated. Areas like the Yangtze River Delta, Northeast China, or Northwest China each have unique natural and economic conditions. Customized action plans and clear emission reduction priorities are vitally important. A holistic perspective is needed to navigate the complex interplay among different subsystems and implement effective cross-sector solutions.

The research team led by Prof. GU Baojing (second from the left)

“Improving environmental and resource challenge is a long-term, systematic project. It starts with field trials and model simulations to form scientific conclusions, followed by technological development and policy updates, which ultimately lead to tangible results,” said GU Baojing. “The most visible outcome of this transformation is blue skies, cleaner water, and a more stable climate. That’s what our team has been striving for.”

Looking ahead, the team plans to deepen its focus on maximizing the synergies among various ecosystem services and incorporating broader external factors, such as climate change, into regional carbon-nitrogen models, thus making more flexible and layered environmental governance policies.

Adapted and translated from the article by ZHOU Tianyu, ZHA Meng  
Translator: FANG Fumin
Photo: SHI Mengsu, ZHE Ying, and the research team 
Editor: TIAN Minjie