Increased microbial expression of organic nitrogen cycling genes in long-term warmed grassland soils

Global warming increases soil temperatures and promotes faster growth and turnover of soil microbial communities. As microbial cell walls contain a high proportion of organic nitrogen, a higher turnover rate of microbes should also be reflected in an accelerated organic nitrogen cycling in soil. We used a metatranscriptomics and metagenomics approach to demonstrate that the relative transcription level of genes encoding enzymes involved in the extracellular depolymerization of high-molecular-weight organic nitrogen was higher in medium-term (8 years) and long-term (>50 years) warmed soils than in ambient soils. This was mainly driven by increased levels of transcripts coding for enzymes involved in the degradation of microbial cell walls and proteins. Additionally, higher transcription levels for chitin, nucleic acid, and peptidoglycan degrading enzymes were found in long-term warmed soils. We conclude that an acceleration in microbial turnover under warming is coupled to higher investments in N acquisition enzymes, particularly those involved in the breakdown and recycling of microbial residues, in comparison with ambient conditions.