In the complex and interwoven biogeochemical cycles that regulate Earth’s ecosystems, the assimilation of nitrogen into terrestrial plants stands as a fundamental process with far-reaching implications for global carbon budgets. Recent research has unveiled a startling dimension of this process: the carbon cost of nitrogen assimilation, a previously underappreciated aspect, is not only substantial but also sensitive to climate warming. This new insight emerges from a sophisticated modeling framework developed to quantify the global carbon demand plants incur while assimilating various forms of nitrogen under current and future climate scenarios.
Nitrogen is an essential nutrient for plant growth, serving as a building block for amino acids, proteins, and nucleic acids. Nevertheless, assimilating nitrogen, particularly in its various soil forms such as nitrate, ammonium, and extractable organic nitrogen, requires significant energy expenditure that consumes carbon fixed via photosynthesis. This interplay introduces a carbon cost that has been notably absent or poorly constrained in Earth system models. Researchers now report that this cost is not marginal; it rivals and even surpasses carbon emissions from well-known anthropogenic disturbances like deforestation and degradation fires.
The study presents a meticulous quantification showing that under present-day climate conditions, terrestrial plants globally expend approximately 208 teragrams of carbon annually assimilating nitrogen. When subjected to a warming scenario of approximately 2°C—a threshold relevant to international climate targets—this carbon cost escalates by nearly 20%, reaching 249 teragrams per year. Such an increase is significant, underscoring how climate warming amplifies carbon demands inherent to nutrient acquisition processes.
Spatial distribution of this increased carbon cost is particularly revealing. Higher latitudes demonstrate a disproportionate rise, with the carbon cost related to nitrogen assimilation swelling by roughly 47% in these regions. This latitude-dependent pattern emerges partly because warming accelerates soil microbial activity, thereby enhancing the availability of inorganic nitrogen forms like nitrate and ammonium. Consequently, plants intensify uptake and assimilation of these nitrogen forms, which are more carbon-demanding relative to organic nitrogen, compounding the overall metabolic costs.
The research deploys a novel mechanistic model capable of distinguishing among nitrogen forms and calculating the associated carbon costs of each assimilation pathway. This approach marks a significant advancement beyond previous models that either aggregated nitrogen into a singular category or neglected the detailed energetic expenditures altogether. By incorporating distinct assimilation costs for nitrate, ammonium, and extractable organic nitrogen, the model offers a nuanced perspective of plant nitrogen metabolism under current and projected climates.
Crucially, the model’s output reveals that the carbon costs of nitrogen assimilation not only match but surpass carbon emissions from deforestation and degradation fires, two of the largest terrestrial carbon fluxes tied to human activity. Additionally, this carbon expenditure aligns closely with the carbon fixed by forests in response to atmospheric nitrogen deposition, a key driver of terrestrial carbon sequestration. These comparisons underscore the importance of including nitrogen assimilation costs in global carbon cycle assessments, as ignoring them risks underestimating terrestrial ecosystem carbon demands and fluxes.
The warming-induced increase in carbon costs stems from several interrelated ecological and physiological mechanisms. Rising temperatures enhance microbial mineralization rates, boosting soil nitrogen availability, particularly of inorganic forms. However, assimilation of these forms is energetically more expensive than organic nitrogen, requiring additional biochemical reductions and protein synthesis in plants. This complex feedback loop effectively raises the metabolic carbon investment necessary for nitrogen acquisition as climates warm.
From a broader perspective, the findings herald significant implications for modeling terrestrial carbon cycle feedbacks in Earth system models. Current predictive frameworks often assume static or minimal carbon costs associated with nutrient uptake. This research clearly indicates that carbon demands for nitrogen assimilation are dynamic and sensitive to climate changes, requiring explicit representation in models to improve forecasts of ecosystem carbon storage, productivity, and resilience under future climate scenarios.
Importantly, these insights also inform carbon management strategies and climate mitigation policies. Understanding that nitrogen assimilation’s carbon cost increases with warming challenges assumptions about ecosystem carbon sinks’ capacity to mitigate atmospheric CO2 rise. It suggests that warming could dampen the net carbon uptake by plants because a larger share of fixed carbon is diverted towards nutrient acquisition rather than biomass accumulation, altering the carbon balance in forested and non-forested landscapes alike.
Beyond carbon cycling, the study contributes to our understanding of plant physiological ecology by quantifying the metabolic investment plants allocate towards managing their nitrogen economy amidst changing environmental conditions. It highlights the critical role of nutrient availability and energy allocation trade-offs in determining plant growth and ecosystem productivity, especially in the face of anthropogenic climate forcing.
Additionally, the latitude-dependent amplification of carbon costs reveals potential vulnerabilities in high-latitude ecosystems, such as boreal forests and tundra, which serve as key carbon reservoirs. The disproportionate carbon demand for nitrogen assimilation in these zones could modulate their ability to serve as stable carbon sinks, thereby influencing global climate trajectories.
The researchers advocate for integrating these carbon costs into global carbon budgets and climate mitigation models. Doing so will yield more accurate projections of carbon fluxes and improve understanding of terrestrial ecosystem responses under complex, interacting climate and biogeochemical drivers. Such refined models are essential for developing strategies that balance carbon sequestration efforts with nutritional limitations and metabolic constraints inherent in plant physiology.
This study opens new avenues for experimental validation and model refinement, encouraging the scientific community to explore diverse ecosystems, varying nitrogen forms, and broader temperature ranges. Such efforts are crucial to capture the full scope of nitrogen-carbon interactions and their climate sensitivity, thereby enhancing the reliability of global carbon cycle predictions.
As the planet continues warming, the intersection between carbon assimilation and nitrogen acquisition will likely become a defining feature of terrestrial biogeochemistry. This work sets a foundational framework for deciphering this linkage, spotlighting a critical but overlooked element in the carbon cycle narrative—and compelling scientists and policymakers alike to reconsider how nutrient costs shape the carbon balance of the biosphere under a changing climate.
Ultimately, this research underscores an urgent need to rethink terrestrial carbon budgets by incorporating the metabolic carbon costs of nitrogen assimilation and their sensitivity to warming. This conceptual advancement provides a more holistic understanding of how ecosystem nutrient dynamics influence global carbon cycling, offering new insights necessary to navigate the intertwined fates of climate, vegetation, and atmospheric chemistry in the Anthropocene.
—
Subject of Research: Plant carbon costs associated with nitrogen assimilation and their response to climate warming.
Article Title: Increased carbon cost for nitrogen assimilation in plants under a warming climate.
Article References:
Hu, CC., Tian, CG., Chen, CJ. et al. Increased carbon cost for nitrogen assimilation in plants under a warming climate.
Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01816-y
Image Credits: AI Generated
