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

Citation

Frey B, Moser B, Tytgat B, Zimmermann S, Alberti J, Biederman LA, Borer ET, Broadbent AAD, Caldeira MC, Davies KF, Eisenhauer N, Eskelinen A, Fay PA, Hagedorn F & Hautier Y (2023) Long-term N-addition alters the community structure of functionally important N-cycling soil microorganisms across global grasslands. Soil Biology and Biochemistry, 176, Art. No.: 108887. https://doi.org/10.1016/j.soilbio.2022.108887

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). 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 availability, whereas N-addition favoured copiotrophic AOB. Overall, N-addition showed a low impact on soil diazotrophs and AOA while effects for AOB communities were considerable. These results reveal that long-term N-addition has 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.

Keywords
Ammonia oxidizer; Biogeography; Diazotroph; Grassland; N-cycling microbial community; N-Fertilization; N2-fixong bacteria; nifH; Nutrient network (NutNet); Urea

Notes
Additional authors: Andrew S. MacDougall, Rebecca L. McCulley, Joslin L. Moore, Maximilian Nepel, Sally A. Power, Eric W. Seabloom, Eduardo Vázquez, Risto Virtanen, Laura Yahdjian, Anita C. Risch

Journal
Soil Biology and Biochemistry: Volume 176