Microbial life-history strategies in soils under long-term fertilizations

Background: The rRNA operon (rrn) copy number is associated with protein synthesis and reproduction, reflecting microbial r- and K-life strategies and influencing soil ecosystem function. Although the positive relationship between microbial community-level rrn copy numbers and nutrient availability has been reported, the association between rrn copy number and soil stoichiometry or environmental stress remains largely unknown, particularly in the context of long-term nutrient inputs. Results: Using long-term (> 30 years) field experiments across three agro-ecosystems, we consistently found that N fertilization increased the microbial community-level rrn copy number. This increase was equivalently explained by soil CN stoichiometry (22%) and soil acidification (21%). Balanced soil CN stoichiometry favored the growth of N-dependent copiotrophs such as Bacilli and Flavobacteriia containing high rrn copy numbers (an average of 2.5), and enhanced their nutrient competition ability. Moreover, N fertilization-induced soil acidification, as an environmental stressor, increased the abundance of pH-negative responders such as Clostridia and Ktedonobacteria which also contained high rrn copy numbers (2.8), and threatened rare taxa with low rrn copy numbers. Conclusions: Consequently, our finding challenges the concept of microbial life-strategy regulation solely by nutrient availability, highlighting the novelty of significant contributions of soil stoichiometric balance and environmental stress to microbial strategies in agro-ecosystems under long-term nutrient inputs.