Abstract
Rising levels of atmospheric carbon dioxide (CO2) and nitrogen (N) deposition affect plant communities in numerous ways1,2,3,4,5,6,7,8,9,10,11. Nitrogen deposition causes local biodiversity loss globally12,13,14, but whether, and if so how, rising CO2 concentrations amplify or dampen those losses remains unclear and is almost entirely unstudied. We addressed this knowledge gap with an open-air experiment in which 108 grassland plots were grown for 24 years under different CO2 and N regimes. We initially found that adding N reduced plant species richness less at elevated than at ambient CO2. Over time, however, this interaction reversed, and elevated CO2 amplified losses in diversity from enriched N, tripling reductions in species richness from N addition over the last eight years of the study. These interactions resulted from temporal changes in the drivers of diversity, especially light availability, that were in turn driven by CO2 and N inputs and associated changes in plant biomass. This mechanism is likely to be similar in many grasslands, because additions of the plant resources CO2 and N are likely to increase the abundance of the dominant species. If rising CO2 generally exacerbates the widespread negative impacts of N deposition on plant diversity, this bodes poorly for the conservation of grassland biodiversity worldwide.
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Data availability
All the data and the reproducible research document are publicly available at https://doi.org/10.6073/pasta/bc1e3e405c7d49e3880ca626e54425fa.
Code availability
All the code used in this paper is available at https://www.google.com/url?q=https://doi.org/10.6073/pasta/bc1e3e405c7d49e3880ca626e54425fa&source=gmail-imap&ust=1728237675000000&usg=AOvVaw29979cAyd_X4xoXFN019cw.
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Acknowledgements
We thank K. Worm, D. Bahauddin and the many BioCON undergraduate interns (see the reproducible research document at https://doi.org/10.6073/pasta/bc1e3e405c7d49e3880ca626e54425fa) for assistance with experimental maintenance, data collection and data curation. We acknowledge US National Science Foundation long-term ecological research grants DEB-0620652, DEB-1234162 and DEB-1831944; long-term research in environmental biology grants DEB-1242531 and DEB-1753859; ecosystem sciences grant DEB-1120064; biocomplexity grant DEB-0322057; biological integration institutes grant NSF-DBI-2021898 to P.B.R., S.E.H. and F.I.; and US Department of Energy programs for ecosystem research grant DE-FG02-96ER62291 to P.B.R.
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P.B.R. led the design and implementation of the experiment, conceived the questions addressed, led the data collection and analysis, wrote the first draft of the paper and led the subsequent editing, writing and revisions. N.M. contributed to the analysis and visualization. S.E.H. assisted in experiment implementation. P.B.R, N.M., S.E.H., F.I. and E.E.B. contributed to data interpretation, editing, review and writing.
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Extended data figures and tables
Extended Data Fig. 1 Simplified illustration of change in species richness under CO2 and N to aid in interpretation of Fig. 1.
a,b, Realized species richness (SR) in the sampled area of the plot as % of planted richness in the entire plot at (a) ambient CO2 and (b) elevated CO2. c, % difference in SR due to +N (compared to ambient N [aN]) at two CO2 levels. Percent difference in SR under +N in c was calculated as [SR in +N- SR in aN]*100/SR in aN] for each level of CO2. d, eCO2 mediation of richness loss due to +N; described by the change in +N effect on SR under eCO2 (compared to ambient CO2[aCO2]) and calculated as the difference between +N effect on SR in elevated and ambient CO2 levels (i.e, difference between dashed and solid lines in Fig. 1b). The straight purple and orange arrows indicate change due to N addition under ambient and elevated CO2 conditions, respectively, and straight green arrows indicate impact of CO2 on SR loss due to +N. The long-curved lines illustrate where a specific contrast in values in one type of comparison is located in another type of comparison. All values in Figs. 1–4 averaged over 9- and 16-species plots. For visualization purposes here and in other figures, loess polynomial fits are shown when the patterns are non-linear43. The dashed line at 0 represents no change in response to the treatments and the shaded region represents 95% confidence interval.
Extended Data Fig. 2 Realized species richness measured at neighborhood scale (0.1–0.5 m2) and estimated at near-whole plot scale (3.24 m2) for ambient treatments.
a, For plots originally planted with 9 species. b, For plots originally planted with 16 species. c, Neighbourhood richness as a proportion of whole plot richness. The shaded regions around the trend lines represent the 95% confidence intervals.
Extended Data Fig. 3 Change in species diversity and light availability expressed as a function of CO2 treatment.
Same data as in Fig. 1 but examined from the perspective of CO2 effects. a, % difference in species richness due to eCO2 (compared to ambient CO2) at two N levels. Percent difference in SR under eCO2 in A was calculated as [SR in eCO2- SR in aCO2]*100/SR in aCO2] for each level of N and averaged over 9- and 16-species plots. b, Effect of CO2 on species evenness, calculated as its difference between eCO2 and ambient CO2 at each N level. c, Effect of CO2 on percent light availability, calculated as its difference between eCO2 and ambient CO2 at each N level. The dashed line at 0 represents no change in response to the treatments and the shaded region represents 95% confidence interval. The results are supported by linear mixed models presented in Table 1, Extended Data Figs. 1–3.
Extended Data Fig. 4 Effect of N on percent species richness (richness) loss at contrasting CO2 levels along different environmental gradients.
a–h, Associations between effect of added N on species richness percent loss at ambient or elevated CO2 (top and bottom row, respectively) and effect of added N on: a,e, light; b,f, log Soil N (mid-summer soil solution N concentration); c,g, moisture (soil volumetric water content averaged across the growing season); and d,h, soil pH. The effect of N for the environmental variables were calculated as a difference between control and treatment at each CO2 level whereas effect of N on SR was calculated as a proportional difference as in Fig. 1. Solid black lines represent significant (p < 0.05) associations in a multiple regression model, dashed black lines represent marginally significant associations (0.05 < p < 0.1) and dashed grey lines represent insignificant associations (p > 0.1). The shaded regions around the trend lines represent the 95% confidence intervals. The results are supported by linear models in Extended Data Table 4 based on F statistic.
Extended Data Fig. 5 Species-specific losses over time due to global change treatments.
Temporal trends in species lost in sampled neighborhoods under different global change manipulations. The proportion lost was calculated as the number of plots in which a planted species had 0 cover divided by the total number of plots the species was planted. For 9 species plots, not all species were planted in every plot. The treatments are aCO2aN: ambient CO2 and ambient N; aCO2 + N: ambient CO2 and enriched N; eCO2aN: elevated CO2 and ambient N; and eCO2 + N: elevated CO2 and enriched N. Because we sample 12.5% of the plot each year, species missing in any sampled plot in any one year might have been present elsewhere in the plot and able to recolonize the sampled cover area in a subsequent year. Species can also recolonize a plot from seed from nearby plots. Hence, proportion of plots from which a species was lost in not necessarily unidirectionally increasing. Each species by treatment combination has n = 24 data points.
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Reich, P.B., Mohanbabu, N., Isbell, F. et al. High CO2 dampens then amplifies N-induced diversity loss over 24 years. Nature 635, 370–375 (2024). https://doi.org/10.1038/s41586-024-08066-9
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DOI: https://doi.org/10.1038/s41586-024-08066-9
