Drivers of the microbial metabolic quotient across global grasslands

Authors: RISCH, A - Swiss Federal Research Institute Wsl; ZIMMERMANN, S - Swiss Federal Research Institute Wsl; SCHUTZ, M - Swiss Federal Research Institute Wsl; BORER, E - University Of Minnesota; BROADBENT, A - University Of Manchester; CALDEIRA, M - Universidade Nova De Lisboa; DAVIES, K - University Of Colorado; EISENHAUER, N - Leipzig University; ESKELINEN, A - Helmholtz Centre For Environmental Research; Fay, Philip; HAGEDORN, F - Swiss Federal Research Institute Wsl; KNOPS, J - Jiaotong University; LEMBRECHTS, J - University Of Antwerp; MACDOUGALL, A - University Of Guelph; MCCULLEY, R - University Of Kentucky; MELBOURNE, B - University Of Colorado; MOORE, J - Monash University; POWER, S - Western Sydney University; SEABLOOM, E - University Of Minnesota; SILVIERA, M - University Of Florida; VIRTANEN, R - Leipzig University; YAHDJIAN, L - National Council Of Science And Technology (CONACYT); OCHOA-HUESO, R - University Of Cadiz

Submitted to: Global Ecology and Biogeography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2023
Publication Date: 4/4/2023
Citation: Risch, A.C., Zimmermann, S., Schütz, M., Borer, E.T., Broadbent, A.A., Caldeira, M.C., Davies, K.F., Eisenhauer, N., Eskelinen, A., Fay, P.A., Hagedorn, F., Knops, J.M., Lembrechts, J., MacDougall, A.S., McCulley, R., Melbourne, B.A., Moore, J.L., Power, S.A., Seabloom, E.W., Silviera, M.L., Virtanen, R., Yahdjian, L., Ochoa-Hueso, R. 2023. Drivers of the microbial metabolic quotient across global grasslands. Global Ecology and Biogeography. 32(6):904-918. https://doi.org/10.1111/geb.13664.
DOI: https://doi.org/10.1111/geb.13664

Interpretive Summary: The potential for grasslands to mitigate climate change by absorbing atmospheric CO2 depends on the balance between CO2 uptake by plants and CO2 loss from the decomposition of organic carbon by microbes in the soil. Up to 30% of the world’s soil C is stored in grassland soils, but these soils are now being disturbed by human activities, including nutrient enrichment and herbivory but their effects on microbial activity remains unclear. The Microbial Metabolic Quotient, MMQ, is a key parameter for understanding microbial activity and its effects on soil C sequestration. Here, we experimentally tested the individual and interactive effects of multiple nutrient addition (NPK + micronutrients) and herbivore exclusion on the MMQ at 23 sites on 5 continents. MMQ was higher in locations with greatest temperature of the wettest quarter and with lower water holding capacity, soil organic C concentration, and pH. MMQ did not consistently respond to nutrient addition or herbivore exclusion, instead interacting with soil pH or soil organic carbon differences between sites. These findings highlight the importance of local edaphoclimatic conditions in understanding how fertilization or herbivory may affect the balance between carbon uptake and loss in grasslands.

Technical Abstract: The proportion of microbial CO2 respired (MR) per unit of soil microbial biomass C (MBC), i.e., the microbial metabolic quotient (MMQ), is a key soil parameter for understanding the microbial regulation of the carbon (C) cycle, including soil C sequestration. Up to 30% of the world’s soil C is stored in grassland soils, but these soils are now being disturbed by human activities, including nutrient enrichment and altered herbivory. Here, we experimentally tested the individual and interactive effects of multiple nutrient addition (NPK + micronutrients) and herbivore exclusion on MR, MBC, and their ratio, MMQ, across 23 sites on 5 continents. Given that sites encompassed a wide range of edaphoclimatic conditions, we also tested the context dependency of the response of MMQ to anthropogenic disturbance. MMQ was higher in locations with greatest temperature of the wettest quarter, lower water holding capacity, soil organic C concentration and pH, but it did not show a consistent response to nutrient addition or herbivore exclusion across sites. We attributed this lack of consistent response to the moderating influence of climatic and soil variables. For example, nutrient addition without herbivore exclusion increased MMQ only at sites with lower soil pH, whilst herbivore exclusion regardless of fertilization led to greater MMQ only at sites with lower soil organic carbon. Thus, our results emphasize the importance of the local edaphoclimatic conditions in understanding the response of the C cycle to anthropogenic stressors and contribute to explain why single site studies often produce conflicting results.