Rewrite Carbonate weathering enhances nitrogen assimilatory uptake in rivers globally as a headline for a science magazine post, using no more than 8 words

Study area description

The PRB consists of seven sub-basins, respectively, NBP, Yujiang River Basin (YJ), Hongliu River Basin (HL), XJ, BJ, DJ and PRD (Fig. 1). The Geologic composition of the PRB is shown in Fig. 1 (calculated from ref. ⁴⁷), where the carbonate rocks account for 44% of the total area in the western part (NBP, YJ, HL and XJ), and only represent 8% in the eastern region (BJ, DJ and PRD). In the eastern region of the PRB (BJ, DJ and PRD), the proportion of silicate sedimentary rock (22%) is much higher than that of carbonate rock, especially in the DJ (silicate of 32% and carbonate of 1%) and the PRD (silicate of 16% and carbonate of 1%). On the basis of the bedrock difference, the west four sub-basins are grouped as the carbonate-dominated region and the east three as the non-carbonate-dominated region for analysis.

Field sampling and laboratory analysis

Field samplings were conducted from 10 August to 12 September 2020 (wet season) and 19 December 2022 to 5 March 2023 (dry season). More than 220 river water sites across the entire PRB were sampled, together with 57 groundwater samples (47 in the wet season and 9 in the dry season). We also sampled the discharged water from 25 wastewater treatment plants in the PRD. All water samples were filtered using a membrane with 0.45 μm pore size for dissolved chemical analysis. Environmental variables, such as water temperature, pH, dissolved oxygen, oxidation and reduction potential and so on, were measured in situ using a multiparameter meter (HANNA HI-98194) with accuracy of ±0.15 °C, ±0.02 pH units, ±0.1 mg l⁻¹ and ±1.0 mV. The total alkalinity (TA) was titrated (Hach Digital Titrator, accuracy ±1%) after water collection. Nutrients including \({{\rm{NO}}}_{3}^{-}\), \({{\rm{NH}}}_{4}^{+}\), \({{\rm{NO}}}_{2}^{-}\), \({{\rm{PO}}}_{4}^{3-}\) and dissolved silicon (Si) were analysed with the Seal AA500 AutoAnalyzer (accuracy ±1%), and TDN was measured using the Jena Multi N/C 3100 analyser (accuracy ±3%). DON was calculated as the difference between TDN and DIN. In addition, ¹⁵\({\rm{N}}\text{-}{\rm{NO}}_{3}^{-}\) and ¹⁸\({\rm{O}}\text{-}{\rm{NO}}_{3}^{-}\) were measured with the Thermo Scientific 253 plus isotope ratio mass spectrometer (accuracy ±0.2%).

We also conducted field sampling at 21 sites in Malang, Indonesia from 6 September to 9 September 2024. The river water samples were collected and filtered using a 0.45 μm membrane filter for dissolved chemical analysis. DIC and TDN were measured using the Jena Multi N/C 3100 analyser (accuracy ±3%), and nutrients including \({{\rm{NO}}}_{3}^{-}\), \({{\rm{NH}}}_{4}^{+}\), \({{\rm{NO}}}_{2}^{-}\) and \({{\rm{PO}}}_{4}^{3-}\) were analysed with the Seal AA500 AutoAnalyzer (accuracy ±1%).

Incubation experiment

The 24-h in situ incubations were conducted to identify the mechanism and validate the basin-level implication. During the wet season, incubation experiments were conducted at the low-DIC site from 21 to 22 September 2023, and at the high-DIC site from 24 to 25 September 2023. During the dry season, experiments were conducted at the low-DIC site from 12 to 13 January 2024, and at the high-DIC site from 15 to 16 January 2024. The experiment started at 6:30 am and ended at 6:30 am the following day. In the field, the original river water was transferred into 625 ml transparent glass bottles. Then, 0.25 ml of 1.207 g l⁻¹ K¹⁵NO₃ was added to the bottle, serving as the tracers. The total amended ¹⁵\({{\rm{N}}{\rm{O}}}_{3}^{-}\) was less than 10% of the ambient \({{\rm{NO}}}_{3}^{-}\) pool to alleviate its impact on the natural N cycle¹¹. Triplicate incubation samples were then placed back on the surface of the river and submerged in the water to main the incubation bottles at the in situ temperature and surface solar radiation. Measurements were taken every 3 h (a total of 27 bottles). Environmental variables were measured using a multiparameter meter (HANNA HI-98194) and TA was titrated with the Hach Digital Titrator. Incubated water samples were sequentially filtered using the 0.7-μm Waterman GF/F membrane and the membrane with 0.45 μm pore size. The Waterman GF/F membrane was carefully folded and the filtrate was analysed for nutrients. For ¹⁵N analysis, PON was analysed with the Thermo Scientific 253 plus isotope ratio mass spectrometer (accuracy ±0.2%).

We also conducted the in situ incubation experiment with and without adding \({{\rm{PO}}}_{4}^{3-}\) on 9 August 2024 at the low-DIC site, to determine whether the DON zonation is led by the P limitation. Two incubation groups (blank and experiment) were set up and 0.25 ml of 1.207 g l⁻¹ K¹⁵NO₃ was added to the 625-ml bottle serving as the tracer. Then, 1 ml of 6.25 mmol l⁻¹ NaH₂PO₄ was added to the experiment group with an additional 10 μmol l⁻¹ of \({{\rm{PO}}}_{4}^{3-}\). After the addition, the ratio of DIN and dissolved inorganic phosphorus (DIP) in the experiment group reached around 11:1, far lower than the Redfield ratio (N:P of 16:1)⁴⁰, resulting in an environment enriched in P and eliminating any potential P limiting factors. Triplicate incubation samples for both groups were then placed on the surface of the river and submerged in the water to maintain the incubation bottles at the in situ temperature and surface solar radiation. The ancillary water chemistry, such as pH and dissolved oxygen, were measured every 3 h (a total of 27 bottles). The remaining procedures were the same as those described previously.

Data acquisition and analysis

The DIC is calculated using the CO2SYS programme⁴⁹ based on field-measured TA, pH, temperature and other relevant parameters. The monthly solar radiation dataset for the PRB analysis is acquired from ref. ⁴⁸, and the hourly solar radiation for incubation sites is acquired from ref. ⁵⁰. Furthermore, PON concentrations and carbonate proportion datasets from 40 rivers (1 river from the original dataset is excluded owing to lack of carbonate rocks information) across the UK from the LOCATE project⁴³ were collected, and more than 150,000 rows of metadata for global river DON concentration are collected from previous studies^{51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69}. The TON (located in Europe and the USA) and corresponding DIC data are collected from the GLORICH Project⁴². In addition, more than 360,000 rows of the global DIC dataset^70,71 are collected for analysis. The 20-yr mean global solar radiation and temperature⁴⁸ were calculated to determine the latitudinal variation. In this study, the organic forms of N, DON, PON and TON were analysed in parallel to multi-explore and verify the impact of lithologic formation on organic N distributions. On the basis of the proportion of carbonate, the top half of rivers (n = 20) are grouped as higher-carbonate regions in the UK, while the lower half are grouped as low carbonate regions. For all the statistical analyses, P < 0.05 is regarded as statistically significant and P < 0.01 is regarded as highly significant.

Growth rate calculation

In biogeochemistry models, the phytoplankton uptake rate of nutrients, or phytoplankton growth rate, is calculated as a function of temperature, solar radiation and nutrient limitation (including N and P)^36,41. We further incorporate the DIC as a limitation factor for the terrestrial river ecosystems on the basis of the Michaelis–Menten kinetic as

where μ is the phytoplankton growth rate in d⁻¹, μ_max is the light and temperature dependent maximum growth rate (d⁻¹), L_N, L_P and L_C are the limitation factor of DIN, DIP and DIC, respectively.

where μ₀ represents the phytoplankton growth rate at 0 °C (d⁻¹), T is water temperature in ref. ⁴¹, while air temperature (°C) is used here for estimation due to data scarcity, α is the initial slope of the photosynthesis-irradiance curve (m² W⁻¹ d⁻¹), I is the solar radiation that is available for photosynthesis in W m⁻² and is calculated as the downward short wave solar radiation × 0.43 (ref. ⁴¹). For the limitation factors

where \({L}_{{\mathrm{NO}}_{3}^{-}}\) and \({L}_{{\mathrm{NH}}_{4}^{+}}\) represent the \({{\rm{NO}}}_{3}^{-}\) and \({{\rm{NH}}}_{4}^{+}\) limitation factors, and the inhibition effect of the presence of \({{\rm{NH}}}_{4}^{+}\) is considered in \({L}_{{\mathrm{NO}}_{3}^{-}}\) (ref. ⁴¹). \({\mathrm{NO}}_{3}^{-}\), \({\mathrm{NH}}_{4}^{+}\), DIP and DIC are the concentrations of \({{\rm{NO}}}_{3}^{-}\), \({{\rm{NH}}}_{4}^{+}\), DIP and DIC in rivers (μmol l⁻¹), respectively. \({k}_{{\mathrm{NO}}_{3}^{-}}\), \({k}_{{\mathrm{NH}}_{4}^{+}}\), \({k}_{\mathrm{DIP}}\) and k_DIC are half-saturation concentrations for phytoplankton uptake of \({{\rm{NO}}}_{3}^{-}\), \({{\rm{NH}}}_{4}^{+}\), DIP and DIC (μmol l⁻¹), respectively. μ_max is used to represent the global latitudinal phytoplankton growth rate potential line, and μ_maxL_C is calculated as the DIC-controlled growth rate potential. The median DIC for each 5° latitude bin is used for the estimation.