Long-term N-addition alters the community structure of functionally important N-cycling soil microorganisms across global grasslands

Authors: FREY, B - Swiss Federal Research Institute Wsl; MOSER, B - Swiss Federal Research Institute Wsl; TYTGAT, B - Ghent University; ZIMMERMANN, S - Swiss Federal Research Institute Wsl; ALBERTI, J - Consejo Nacional De Investigaciones Científicas Y Técnicas(CONICET); BIEDERMAN, L - Iowa State University; BORER, E - University Of Minnesota; BROADBENT, A - University Of Manchester; CALDEIRA, M - University Of Lisbon; DAVIES, KENDI - University Of Colorado; EISENHAUER, N - Leipzig University; ESKELINEN, A - University Of Oulu; Fay, Philip; HAGEDORN, F - Swiss Federal Research Institute Wsl; HAUTIER, Y - Utrecht University; MACDOUGALL, A - University Of Guelph; MCCULLEY, R - University Of Kentucky; MOORE, J - Monash University; NEPEL, M - University Of Vienna; POWER, S - Western Sydney University; SEABLOOM, E - University Of Minnesota; VASQZEZ, E - University Of Bayreuth; VIRTANEN, R - University Of Oulu; YAHDJIAN, L - Consejo Nacional De Investigaciones Científicas Y Técnicas(CONICET); RISCH, A - Swiss Federal Research Institute Wsl

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2022
Publication Date: 1/5/2023
Citation: Frey, B., Moser, B., Tytgat, B., Zimmermann, S., Alberti, J., Biederman, L.A., Borer, E.T., Broadbent, A., Caldeira, M.C., Davies, K.F., Eisenhauer, N., Eskelinen, A., Fay, P.A., Hagedorn, F., Hautier, Y., MacDougall, A.S., McCulley, R.L., Moore, J.L., Nepel, M., Power, S.A., Seabloom, E.W., Vásquez, E., Virtanen, R., Yahdjian, L., Risch, A.C. 2023. Long-term N-addition alters the community structure of functionally important N-cycling soil microorganisms across global grasslands. Soil Biology and Biochemistry. 176. Article 108887. https://doi.org/10.1016/j.soilbio.2022.108887.
DOI: https://doi.org/10.1016/j.soilbio.2022.108887

Interpretive Summary: Increasing environmental levels of reactive mineral elements such as nitrogen (N) from agro-industrial processes pose significant challenges to the biodiversity of grasslands on a global scale. Leguminous plants species are particularly sensitive levels of N in the soil because they harbor specialized bacteria that provide plant-available N by absorbing atmospheric N2 at considerable energetic cost to the plant. Fertilization with N decreases the abundance of legume species but effects on the diversity of soil microbes that process N remain unclear and are crucial for understanding effects of global change on grassland nutrient cycling. We applied genomic approaches to measure the abundance and diversity of soil microbes critical to nitrogen fixation and nitrogen cycling at thirty grasslands where N fertilizer is applied in a global coordinated experiment. This approach revealed that N fertilization increased the abundance and diversity of ammonia-oxidizing bacteria, in part by increasing soil pH. This change may increase environmental issues related to N in air and water. This study provides new insight into three functionally important N-cycling microbial groups and on how they respond to N-addition in grassland ecosystems spanning large biogeographic gradients.

Technical Abstract: Anthropogenic nitrogen (N) input is known to alter the soil microbiome, but how N enrichment influences the abundance, alpha-diversity and community structure of N-cycling functional microbial communities in grasslands remains poorly understood. Here, we collected soils from plant communities subjected to up to 9 years of annual N-addition (10 g N m-2 per year using urea as a N-source) and from unfertilized plots (control) in 30 grasslands worldwide spanning a large range of climatic and soil conditions. We focused on three key microbial groups responsible for two essential processes of the global N cycle: N2 fixation (soil diazotrophs) and nitrification (AOA: ammonia-oxidizing archaea and AOB: ammonia-oxidizing bacteria). Biological N2-fixation is critical for N supply and primary productivity of grasslands; nitrification dominates soil N-cycling and plays a key role in N losses and environmental pollution in grassland ecosystems. We targeted soil diazotrophs, AOA and AOB using Illumina MiSeq sequencing and measured the abundance (gene copy numbers) using quantitative PCR. N-addition shifted the structure of the diazotrophic communities, although their alpha-diversity and abundance were not affected. AOA and AOB responded differently to N-addition. The abundance and alpha-diversity of AOB increased, and their community structure shifted with N-addition. In contrast, AOA were not affected by N-addition. AOA abundance outnumbered AOB in control plots under conditions of low N availabilty, whereas N-addition favoured copiotrophic AOB. Overall, our results show that long-term N-addition may have important ecological implications for key microbial groups involved in two critical soil N-cycling processes. Increased AOB abundance and community shifts following N-addition may change soil N-cycling, as larger population sizes may promote higher rates of ammonia oxidation and subsequently increase N loss via gaseous and soil N-leaching. These findings bring us a step closer to predicting the responses and feedbacks of microbial-mediated N-cycling processes to long-term anthropogenic N-addition in grasslands.