Nutrient addition increases grassland sensitivity to droughts

Authors: BHARATH, S - University Of Minnesota; BORER, E - University Of Minnesota; BIEDERMAN, L - Iowa State University; Blumenthal, Dana; Fay, Philip; GHERADI, L - Arizona State University; KNOPS, J - Xian Jiao University; LEAKEY, A - University Of Illinois; YAHDJIAN, L - University Of Buenos Aires; SEABLOOM, E - University Of Minnesota

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2019
Publication Date: 5/1/2020
Citation: Bharath, S., Borer, E., Biederman, L., Blumenthal, D.M., Fay, P.A., Gheradi, L., Knops, J., Leakey, A., Yahdjian, L., Seabloom, E. 2020. Nutrient addition increases grassland sensitivity to droughts. Ecology. 101(5):e02981. https://doi.org/10.1002/ecy.2981.
DOI: https://doi.org/10.1002/ecy.2981

Interpretive Summary: Grassland productivity may be increasingly affected by both drought and nutrient enrichment. Here, we tested how experimental nutrient fertilization influenced resistance to and recovery from severe regional droughts in 13 sites across North America. We found that fertilization increased sensitivity to drought, particularly in more arid sites, and also the speed of recovery from drought. Consequently, fertilization and nitrogen deposition may have more negative effects in dry regions, where they can destabilize plant productivity, than in wet regions, where they may increase post-drought productivity. These results can be used by land managers and policy makers to understand how fertility management and nutrient pollution may interact with changing precipitation regimes.

Technical Abstract: Grasslands worldwide are expected to experience an increase in extreme events such as drought, along with simultaneous increases in mineral nutrient inputs, as a result of human activities. These changes are likely to interact, because elevated nutrient inputs may alter plant diversity and increase the sensitivity to droughts. Here, we examine the effects of experimental nutrient fertilization and resulting diversity loss on the resistance to and recovery from severe regional droughts. We do this at 13 sites spanning gradients of aridity in North America – 5 sites in the California Annual Grasslands and 8 in the Great Plains. We measure resistance and recovery as the change in annual aboveground biomass (ANPP) relative to change in growing season precipitation during and after a drought. Resistance and recovery were similar across our sites spanning a 400 mm range of mean growing season precipitation. Although Great Plains grasslands were equally sensitive to drought across a wide range of aridity, chronic nutrient fertilization reduced drought resistance in arid sites. Fertilization also increased drought recovery for both arid and mesic sites in the Great Plains. Within the California annual grasslands, neither nutrient addition nor the regional aridity gradient had consistent effects on ANPP sensitivity to drought. Across both regions, we found that species richness in naturally occurring grasslands was not associated with either resistance to or recovery from the drought, in contrast to findings in experimentally assembled grassland communities. Taken together these results suggest that human-induced eutrophication will likely increase drought recovery, but at sites experiencing increasing aridity, eutrophication may destabilize grassland primary production.