Soil microbial communities vary in composition and functional strategy across soil aggregate size class regardless of tillage

Abstract

The physicochemical environment within aggregates controls the distribution of carbon and microbial communities in soils. Agricultural management, such as tillage, can disrupt aggregates and the microscale habitat provided to microorganisms, thus altering microbial community dynamics. Categorizing microbial communities into life history strategies with shared functional traits—as has been done to understand plant community structure for decades—can illuminate how the soil physicochemical environment constrains the membership and activity of microbial communities. We conducted an aggregate scale survey of microbial community composition and function through the lens of the yield–acquisition–stress (Y–A–S) tolerator life history framework. Soils collected from a 7-year tillage experiment were separated into 4 aggregate size classes and enzyme activity, multiple-substrate-induced respiration, and carbon use efficiency were measured to reveal trade-offs in microbial resource allocation. Microbial community structure was interrogated with bacterial and fungal marker gene sequencing, and metagenomic features such as community weighted genome size and traits conferring stress tolerance were predicted using PICRUSt2. Consistent with our hypothesis, aggregates of different size classes harbored distinct microbial communities manifesting distinct life history strategies. Large macroaggregate communities >2 mm were classified as acquisition strategists based on increased enzyme activity relative to other aggregate size classes. Small and medium microaggregate (0.25–2 mm) communities did not show a strong tendency toward any particular life history strategy. Genes conferring stress tolerance were significantly enriched in microaggregates <0.25 mm (indicative of stress tolerators); however, these communities also had the highest carbon use efficiency (indicative of yield strategists). We found tradeoffs in resource allocation between communities classified as yield and acquisition strategists consistent with the Y–A–S framework. Tillage did not alter life history strategies within aggregates, suggesting that the aggregate physicochemistry plays a larger role than agricultural management in shaping microbial life history at the scale studied.