Soil pH regulates organic carbon pool by changing microbial life-history strategy.

INTRODUCTION: The stability of the vast soil carbon pool, crucial for climate regulation, depends on microbial processes that govern carbon loss as CO2 or its stabilization in soil. Microbial life-history strategies, representing tradeoffs between resource acquisition (A-strategy) and growth yield (Y-strategy), are central to soil organic carbon (SOC) dynamics. However, how abiotic factors modulate these strategies and, in turn SOC fate remains unclear. OBJECTIVES: Using the black soil region of Northeast China, which harbors substantial yet vulnerable SOC reserves, this study aimed to identify the dominant abiotic driver shaping microbial life-history strategies and to elucidate how this driver influences SOC stabilization pathways. METHODS: We conducted a field survey combining metagenomic profiling of microbial attributes (diversity, functional potential, and inferred life-history strategy) with measurements of soil properties including extracellular enzyme activities and SOC fractions. This integrative approach traced the pathway from abiotic drivers to microbial traits and ultimately to carbon allocation. RESULTS: Soil pH emerged as the key environmental gradient, with a threshold at pH 6.43 marking a systemic shift in microbial ecology and carbon processing. Acidic soils (pH 4.60-6.43) favored A-strategists, characterized by large genomes, enriched carbohydrate-active enzymes, and high extracellular enzyme activity, enabling polymer degradation and humification but limiting mineral-associated organic carbon (MAOC) formation. In contrast, neutral soils (pH 6.43-8.87) supported Y-strategists with streamlined genomes and biosynthetic metabolism, promoting microbial necromass accumulation and MAOC stabilization. Distinct functional guilds underpinned the A- and Y-strategies and frequent horizontal gene transfer in acidic soils further reinforced the A-strategy dominance under low pH. CONCLUSION: Our findings reveal a mechanistic link between microbial life-history strategies and SOC stabilization, demonstrating that pH may shape the balance between A- and Y-strategists and their contrasting carbon pathways. This insight enhances predictive models of SOC dynamics and highlights pH management as a key lever for agroecosystems carbon retention.