A team led by Prof. LIU Xueyan from the Institute of Geochemistry of the Chinese Academy of Sciences has developed a new plant-soil nitrogen isotope process model, which quantifies the fractional contribution of three nitrogen forms (nitrate, ammonium, and dissolved organic nitrogen) to the total nitrogen in global terrestrial plants.

Using this model, combined with global data on terrestrial plant carbon-to-nitrogen (C/N) ratios and gross primary productivity (GPP), the researchers estimated that the gross carbon consumed for nitrogen assimilation by global terrestrial plants amounts to 208 ± 12 teragrams of carbon per year (Tg-C/yr). Notably, this figure surpasses the annual carbon emissions from forest fires and degradation (155 Tg-C/yr) and is comparable to the magnitude of forest carbon sequestration driven by atmospheric nitrogen deposition.

The study was published in Nature Geoscience on Oct. 6.

Plant uptake of soil nitrogen (N) and internal nitrogen assimilation both rely on carbon (C) fixed through photosynthesis. Biochemical studies have shown that the theoretical carbon costs for plant assimilation of nitrate, ammonium, and dissolved organic nitrogen average 5.81, 4.32, and 2.16 grams of carbon per gram of nitrogen (g-C/g-N), respectively.

Prior research has largely focused on the positive impacts of nitrogen on plant growth and carbon sequestration, while the metabolic carbon costs of plant nitrogen assimilation have not been thoroughly evaluated. This gap has long been a "blind spot" in terrestrial carbon cycle research. The new study highlights that this hidden carbon expenditure must be included in carbon balance accounting.

Climate warming is expected to boost soil nitrogen transformations, increasing the supply of bioavailable nitrogen—especially inorganic forms like nitrate and ammonium. Additionally, it will raise plants' nitrogen demand due to accelerated growth. These changes may lead to a higher proportion of inorganic nitrogen being assimilated by plants and a corresponding increase in associated carbon consumption, thereby driving up the total carbon cost of plant nitrogen assimilation.

To test this hypothesis, the researchers simulated the contributions of different nitrogen sources under a 2.0 ℃ warming scenario. By combining these simulations with corresponding data on plant C/N ratios and GPP, they projected that the total carbon consumed for nitrogen assimilation by global terrestrial plants under this warming scenario will reach 249 ± 15 Tg-C/yr. This represents an average increase of 47% (41 ± 19 Tg C) compared to the current level (208 ± 12 Tg-C/yr). Regionally, the increases are 9% in tropical areas, 62% in temperate regions, and 105% in boreal zones.

Climate warming enhances the availability of soil inorganic nitrogen and its contribution to plant total nitrogen assimilation. However, it also increases the metabolic carbon expenditure related to plant nitrogen assimilation, offsetting a portion of the carbon gained through photosynthesis. This effect is particularly significant at high latitudes.

This study reveals new mechanisms of coupling between vegetation carbon-nitrogen cycling processes and their climate responses and feedbacks, providing new evidence for refined accounting of the global carbon balance and for formulating future carbon-neutral and climate-response strategies.