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Negative effects of nitrogen override positive effects of phosphorus on grassland legumes worldwide

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in low-nutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents.

 

Pedro M. Tognetti, Suzanne M. Prober, Selene Báez, Enrique J. Chaneton, Jennifer Firn, Anita C. Risch, Martin Schuetz, Anna K. Simonsen, Laura Yahdjian, Elizabeth T. Borer, Eric W. Seabloom, Carlos Alberto Arnillas, Jonathan D. Bakker, Cynthia S. Brown, Marc W. Cadotte, Maria C. Caldeira, Pedro Daleo, John M. Dwyer, Philip A. Fay, Laureano A. Gherardi, Nicole Hagenah, Yann Hautier, Kimberly J. Komatsu, Rebecca L. McCulley, Jodi N. Price, Rachel J. Standish, Carly J. Stevens, Peter D. Wragg, and Mahesh Sankaran

PNAS July 13, 2021 118 (28) e2023718118

Significance

Predicting the effects of anthropogenic nutrient enrichment on plant communities is critical for managing implications for biodiversity and ecosystem services. Plant functional types that fix atmospheric nitrogen (e.g., legumes) may be at particular risk of nutrient-driven global decline, yet global-scale evidence is lacking. Using an experiment in 45 grasslands across six continents, we showed that legume cover, richness, and biomass declined substantially with nitrogen additions. Although legumes benefited from phosphorus, potassium, and other nutrients, these nutrients did not ameliorate nitrogen-induced legume decline. Given global trends in anthropogenic nutrient enrichment, our results indicate the potential for global decline in grassland legumes, with likely consequences for biodiversity, food webs, soil health, and genetic improvement of protein-rich plant species for food production.

Abstract

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in low-nutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non–nitrogen-fixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species.

 

See https://www.pnas.org/content/118/28/e2023718118

 

Figure 1: Change in legume cover (A), richness (B), and biomass (C) for the third year (top row) and last (third to sixth) year after initiation of the experiment (bottom row). Changes were expressed as response ratios, the natural logarithm of the relative change from initial values (Methods); positive and negative values indicate increases and decreases, respectively. Bars represent means ± SEMs, and dots (•) indicate treatment means that were statistically different from the controls. No response ratio in control plots were statistically different from zero, indicating that controls remained the same on average over time. Note the different y-axis ranges. Cover and richness data were available for 45 sites and biomass data for 26 sites.

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