Stronger fertilization effects on above ground versus belowground plant properties across nine U.S. grasslands

Authors: KELLER, ADRIENNE - University Of Minnesota; WALTER, CHRISTOPHER - West Virginia University; Blumenthal, Dana; BORER, ELIZABETH - University Of Minnesota; COLLINS, SCOTT - University Of New Mexico; DELANCEY, LANG - University Of Minnesota; Fay, Philip; HOFMOCKEL, KIRSTEN - Iowa State University; KNOPS, JOHANNES - Xian Jiao University; LEAKEY, ANDREW - University Of Illinois

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/2022
Publication Date: 10/8/2022
Citation: Keller, A.B., Walter, C.A., Blumenthal, D.M., Borer, E.T., Collins, S.T., DeLancey, L.C., Fay, P.A., Hofmockel, K.S., Knops, J.M., Leakey, A.D. 2022. Stronger fertilization effects on above ground versus belowground plant properties across nine U.S. grasslands. Ecology. 104. Article e3891. https://doi.org/10.1002/ecy.3891.
DOI: https://doi.org/10.1002/ecy.3891

Interpretive Summary: Understanding how root productivity responds to increased nutrient inputs in grasslands is key to understanding effects on carbon cycles. We measured how a decade of nitrogen (N) and phosphorus (P) fertilization influenced aboveground and belowground biomass and productivity at nine grassland sites spanning the continental United States. Although N and P fertilization effects were strong aboveground, belowground effects were weaker and ranged from positive, negative, or neutral across sites. Nitrogen addition increased root biomass at sites with low N deposition but decreased it at sites with high N deposition. These results suggest that the effects of nutrient supply on belowground plant properties are context dependent and that increases in aboveground biomass with grassland fertilization may not translate into increased productivity or biomass belowground.

Technical Abstract: There is widespread evidence that nutrient supply limits plant productivity. Therefore, increased nutrient inputs due to anthropogenic activity are expected to enhance primary productivity across terrestrial ecosystems. Given that roots are major contributors to soil carbon (C) storage, understanding belowground net primary productivity (BNPP) and biomass responses to changes in nutrient availability is essential to predicting carbon-climate feedbacks in the context of interacting global environmental changes. To address this gap, we measured how a decade of nitrogen (N) and phosphorus (P) fertilization influenced aboveground and belowground biomass and productivity at nine grassland sites spanning the continental United States, which are part of a globally distributed nutrient addition experiment (The Nutrient Network). We measured aboveground biomass, and root production, biomass, and turnover alongside a suite of climate and soil factors and plant diversity to assess how fertilization, site conditions, and their interactions influenced above- and belowground plant biomass and productivity. Fertilization effects were strong aboveground, with both N and P addition stimulating aboveground biomass. In contrast, belowground responses to fertilization were weaker and ranged from positive, negative, or neutral across sites. Root properties were highly variable across sites, ranging up to 6-fold. Atmospheric N deposition was positively related to root turnover rates, and this relationship was amplified with N addition. Nitrogen addition increased root biomass at sites with low N deposition but decreased it at sites with high N deposition. However, much of the large variation in root properties among sites was unexplained by climate, edaphic factors, or N deposition. Overall, these results suggest that the effects of nutrient supply on belowground plant properties are context dependent, particularly with regard to background N supply rates, and therefore site factors must be considered when predicting how grassland ecosystems will respond to increased nutrient loading from anthropogenic activity.