Soil carbon stocks in temperate grasslands differ strongly across sites but are insensitive to decade-long fertilization

Authors: KELLER, ADRIENNE - University Of Minnesota; BORER, ELIZABETH - University Of Minnesota; COLLINS, SCOTT - University Of New Mexico; DELANCEY, LANG - University Of Minnesota; Fay, Philip; HOFMOCKEL, KIRSTEN - Pacific Northwest National Laboratory; LEAKEY, ANDREW - University Of Illinois; MAYES, MELANIE - University Of Tennessee; SEABLOOM, ERIC - University Of Minnesota; WALTER, CHRISTOPHER - West Virginia University; WANG, YONG - University Of Tennessee; ZHAO, QIAN - Pacific Northwest National Laboratory; HOBBIE, SARAH - University Of Minnesota

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2021
Publication Date: 11/21/2021
Citation: Keller, A.B., Borer, E.T., Collins, S.L., DeLancey, L.C., Fay, P.A., Hofmockel, K.S., Leakey, A.D., Mayes, M.A., Seabloom, E.W., Walter, C.A., Wang, Y., Zhao, Q., Hobbie, S.E. 2021. Soil carbon stocks in temperate grasslands differ strongly across sites but are insensitive to decade-long fertilization. Global Change Biology. 28:1659-1677. https://doi.org/10.1111/gcb.15988.
DOI: https://doi.org/10.1111/gcb.15988

Interpretive Summary: Enhancing soil carbon (C) storage has the potential to offset human-caused increases in atmospheric CO2. Meanwhile, concurrent anthropogenic increases in inputs of nutrients including nitrogen (N) and phosphorus (P) are widely expected to stimulate primary productivity, ultimately promoting soil C sequestration, particularly in grasslands, which make up nearly a third of non-agricultural land world-wide and a third of terrestrial primary production. Together with university collaborators we examined how a decade of N and P fertilization influenced above and belowground plant biomass and productivity, total soil C, and three soil C fractions at nine sites spanning the continental United States. The study revealed that despite strong fertilization effects on aboveground biomass, effects on soil C pools ranged from strongly positive to strongly negative, and total soil C storage varied by more than an order of magnitude across sites. Overall, site factors such as water availability, plant productivity, and soil texture and mineralogy were key predictors of cross-site soil C storage. These findings suggest that prioritizing the protection of highly productive temperate grasslands could be an effective tool for climate change mitigation, as part of a suite of socio-ecological goals to be considered in land management decision-making.

Technical Abstract: Enhancing soil carbon (C) storage has the potential to offset human-caused increases in atmospheric CO2. However, rising CO2 has occurred concurrently with increasing supply rates of biologically limiting nutrients such as nitrogen (N) and phosphorus (P). It is unclear how these increased supplies of N and P will alter soil C sequestration, particularly in grasslands which make up nearly a third of non-agricultural land world-wide and a third of terrestrial primary production. Here, we leverage a globally distributed nutrient addition experiment (The Nutrient Network) to examine how a decade of N and P fertilization (alone and in combination) influenced soil C and N stocks at nine sites spanning the continental United States. We measured changes in total soil C and N stocks and in three soil C fractions (light and heavy particulate organic matter and mineral-associated organic matter fractions). Nutrient amendment had variable effects on soil C and N pools that ranged from strongly positive to strongly negative while soil C and N pool sizes varied by more than an order of magnitude across sites. Piecewise SEM clarified that small increases in plant C inputs with fertilization did not translate to greater soil C storage. Instead, peak season aboveground plant biomass (but not root biomass or production) was strongly positively related to soil C storage at seven of the nine sites, and across all nine sites soil C covaried with moisture and soil mineralogy. Overall, we show that site factors such as climate, productivity, soil texture and mineralogy are key predictors of cross-site soil C while nutrient amendment has weaker and more variable within-site effects on C sequestration. This suggests that prioritizing the protection of highly productive temperate grasslands could be an effective tool for climate change mitigation, as part of a suite of socio-ecological goals to be considered in land management decision-making.