We’re excited to share that our 2024 New Phytologist Tansley Review, Empirical evidence and theoretical understanding of ecosystem carbon and nitrogen cycle interactions, has been recognised as one of the top 10 most viewed papers published in New Phytologist in 2024.

Led by Beni Stocker and developed by LEMONTREE’s Nitrogen Working Group, this paper brought together years of discussion, collaboration and synthesis to improve how we understand and model plant responses to changing environmental conditions, with particular reference to the coupling between plant carbon (C) and nitrogen (N) cycles.

At the time of publication, we described the review as a “labour of love”. Seeing it resonate so strongly with the wider plant and Earth system science community makes that effort seem even more worthwhile.

Why This Review Mattered

We tackled a problem at the heart of predicting future climate change – how the terrestrial C and N cycles interact – and presented results that are at variance with the assumptions made in many current global vegetation and land surface models. The review’s “subtext” is that the time has come, or is overdue, for a root-and-branch revision of how C-N interactions are represented in models.

There is a long backstory. Early “C-only” models predicted very strong increases in plant growth under rising CO₂, assuming C supply was the main limiting factor. But in reality, plant growth requires N as well as C. A key paper by Bruce Hungate and others, published in Science in 2003, analysed future projections made by six carbon-only models. That paper concluded that these projections of increased C uptake were unrealistic, because the projected growth increases called for the uptake of far more N than was possible…. assuming that rates of N supply to ecosystems could not change.

That paper kicked off a global rush to make ecosystem models (hopefully!) more realistic by explicitly including C-N cycle interactions. The problem was that no one actually knew how to do it. So the resulting models involved a lot of guesswork, with different groups making different guesses.

With hindsight, we can now point to two additional, equally fundamental, problems. If these C-only models were so flawed, how could they already predict the observed magnitude of the land carbon sink? And is it really sensible to “bet against nature”, by assuming that plants could not find ways to access additional supplies of N?

Our 2024 review examined in some detail how today’s models represent C-N interactions, and the consequences for ecosystem model predictions. More importantly, we made use of meta-analyses of experimental results and field observations to test whether some of the assumptions underlying modern, N-enabled models were correct.

Plants Are Flexible

One of our clearest findings was that models make very different assumptions about how C and N interact. Some assume that plant N uptake increases in direct proportion with soil N supply. Others allow for saturation. Some models keep root and shoot allocation fixed, while others allow plants to shift investment depending on N availability. Many (but not all) tightly link photosynthetic capacity to leaf N. These differences matter, because they shape predictions of how much C land ecosystems can continue to absorb in a still-changing global environment.

It turned out that when C-only and C-N models are compared, on average there is little difference in their predictions of land C uptake (the land sink). However, the C-N models show much larger variations from one another than the C-only models. The average C-N model does a good job of predicting the current C sink; but some models make it too small, and others too large. So one major effect of the push for C-N cycle coupling has been a large increase in uncertainty.

A key resource for our analysis of the real world was the MESI data base, which brings together data on many experimental studies where leaf and plant measurements have been made on plants subjected to N addition or CO2 enhancement. We were particularly interested to find out whether experimentally increasing N supply would lead to higher leaf N content and higher photosynthetic capacity.

The answer is that adding N generally increases leaf N – but not photosynthetic capacity. However, not all studies in the data base reported photosynthetic capacity. A larger number reported light-saturated photosynthesis (Asat, which depends on photosynthetic capacity), with puzzling results: sometimes adding N increased Asat, and sometimes it didn’t. (You can find this information already in the Supplementary of the Liang et al. meta-analyis paper – but without comment.) We tried to find a pattern behind these differences amomng studies, but we failed. It remains a puzzle.

Meanwhile, Beni developed a C-N model based on a very simple principle: that the allocation of C to leaves versus roots should match N demand with N supply. This model also formally assumes that leaf-level photosynthetic capacity is determined only by climate and light (an instance of eco-evolutionary optimality (EEO) theory, which informs much of the research by LEMONTREE). He showed that the model could quantitatively reproduce a long list of plant responses documented in the MESI data base. For example, EEO theory predicts that adding N should preferentially increase above-ground biomass while increasing CO2 should favour below ground-biomass. Both predictions are true. Most models don’t reproduce them, but Beni’s does.

Ning Dong carried out an analysis of field observations of how key plant traits respond (spatially) to environmental variables. Here too were some surprising findings, including a total lack of any effect of atmospheric N deposition on photosynthetic capacity. Others were less surprising – such as the decline in photosynthetic capacity (when assessed at a standard temperature) with increasing growth temperature. This is expected in the light of EEO theory because at higher temperatures, less Rubisco is required to achieved a given catalytic effect – although most models also neglect this (widely observed) response.

We concluded that models often underestimate plant flexibility, particularly in how plants adjust N demand, N allocation and resource-use strategies in response to rising CO₂ and climate change.

Building on the Review

One of the most rewarding outcomes of the Tansley Review has been seeing its ideas tested and extended through new LEMONTREE research. Several recent studies by the team have pushed forward the review’s central questions: how plants balance carbon gain with nutrient availability, and how those processes shape the land carbon sink.

Work led by Alissar Cheaib, published in Ecology Letters, showed that soil N supply has the strongest influence on leaf N in environments where plants exert the greatest N demand, particularly in colder and drier climates – supporting the idea that leaf N is regulated by plant demand and optimal resource use, rather than soil N supply alone.

Jan Lankhorst’s work showed that greater nutrient availability increases photosynthetic capacity and reduces the C cost of acquiring N, but does not change the fundamental balance between C gain and water loss. Plants became more productive but still maintain the same optimal carbon–water trade-off predicted by EEO theory.

Evan Perkowski found that sugar maple trees can trade N for water. When N is more available, trees invest less efficiently in N use and instead conserve water, maintaining stable photosynthesis across nutrient gradients. This study offered strong evidence for the “least-cost” EEO theory in action. Evan’s work also demonstrated that leaf-level photosynthetic responses to elevated CO₂ are often driven more by N demand than supply.

Work by Maoya Bassiouni, following up an idea first proposed by Ning (Dong et al. 2022 New Phytologist), showed that declining leaf N in European forests can be explained as a predictable consequence of acclimation to CO2 and does not (as has been widely asserted) indicate that plants are suffering from a shortage of N.

Trevor Cambron examined how nutrient acquisition shapes the future of the global land C sink. This Nature Climate Change review highlighted that terrestrial ecosystems currently absorb around one-third of human C emissions, but the persistence of this sink depends heavily on how plants acquire both N and phosphorus (P) under elevated CO₂.

All these studies reinforce the central message of our Tansley review: nutrients are not simply constraints but rather form part of a dynamic optimization strategy plants use to balance carbon gain, water use and growth.

Where the Working Group Goes Next

The recognition of this review is encouraging not simply because of visibility, but because it shows how important these questions remain for the scientific community.

The original LEMONTREE Working Group has expanded its purview to become a  “CNP synthesis” group. A major focus is on assessing how P is represented in terrestrial biosphere models, and whether those assumptions are creating similar uncertainties to those we identified for N.

Synthesis papers matter. They connect experiments, theory, and modelling across disciplines, and may reveal that the biggest advances in modelling come not from adding complexity in the form of new processes, but from revisiting the fundamentals on which models were built.

Blog by Colin Prentice.

References

Bassiouni M., Smith N.G., Reu J., Peñuelas J., & Keenan T.F. (2025). Observed declines in leaf nitrogen explained by photosynthetic acclimation to CO₂. Proceedings of the National Academy of Sciences, 122 (33), https://doi.org/10.1073/pnas.2501958122

Cambron, T.W., Fisher, J.B., Hungate, B., Stocker, B.D., Keenan, T., Prentice, I.C. & Terrer, C., (2025). Plant nutrient acquisition under elevated CO₂ and implications for the land carbon sink. Nature Climate Change. https://doi.org/10.1038/s41558-025-02386-y

Cheaib, A., Waring, E.F., McNellis, R., Perkowski, E.A., Martina, J.P., Seabloom, E.W., Borer, E.T., Wilfahrt, P.A., Dong, N., Prentice, I.C. & Wright, I.K. (2025). Soil nitrogen supply exerts largest influence on leaf nitrogen in environments with the greatest leaf nitrogen demand. Ecology Letters, 28 (1), https://doi.org/10.1111/ele.70015

Dong, N., Wright, I.J., Chen, J.M., Luo, X., Wang, H., Keenan, T.F., Smith, N.G. & Prentice, I.C. (2022). Rising CO2 and warming reduce global canopy demand for nitrogen. New Phytologist, 235(5), https://doi.org/10.1111/nph.18076

Lankhorst, J.A., de Boer, H.J., Behling, D.C., Drake, P.L., Perkowski, E.A. & Rebel, K.T. (2025). Nutrient availability increases photosynthetic capacity without altering the cost of resource use for photosynthesis. AoB Plants. https://doi.org/10.1093/aobpla/plaf061

Perkowski, E.A., Frey, D.W., Goodale, C.L. & Smith, N.G. (2025a). Increasing nitrogen availability increases water use efficiency and decreases nitrogen use efficiency in Acer Saccharum. Tree Physiology, tpaf119, https://doi.org/10.1093/treephys/tpaf119

Perkowski EA, Ezekannagha E, Smith NG. (2025b). Nitrogen demand, availability, and acquisition strategy control plant responses to elevated CO2. Journal of Experimental Botany. 76 (10), https://doi.org/10.1093/jxb/eraf118

Stocker, B.D., Dong, N., Perkowski, E.A., Schneider, P.D., Xu, H., de Boer, H., Rebel, K.T., Smith, N.G., Van Sundert, K., Wang, H., Jones, S.E., Prentice, I.C., & Harrison, S.P. 2024. Tansley Review. Empirical evidence and theoretical understanding of ecosystem carbon and nitrogen cycle interactions. New Phytologist. https://doi.org/10.1111/nph.20178