Quantity Versus Quality: Links Between Soil Organic Matter and Bacterial Community Composition Along a Geoclimatic Gradient

Soil organic matter (SOM) quantity drives soil bacterial community composition from the regional to global scale. Qualitative characteristics of SOM are known to affect soil bacterial communities in manipulation experiments. However, it remains unresolved how strongly SOM characteristics affect soil bacterial community composition at the macroscale. Here, we investigated how quantity versus qualitative characteristics of SOM shape community composition along a biogeochemical gradient of grassland soils. We assessed relative abundance patterns of soil bacteria and characterised SOM based on scalable methods. Soils with higher SOM content (along a continuum between 0.6% and 18.7% SOC) and acidic pH (along a continuum between pH 4.1-6.7) hosted fewer narrowly distributed taxa (i.e., taxa occurring in few sites) and therefore had lower bacterial alpha diversity. We could explain a larger fraction of bacterial community composition (up to 59.6% of 16S rRNA reads) in these soils. Consequently, we understand community composition in low-SOM soils less than in high-SOM soils, because the drivers of narrowly distributed taxa remain poorly understood. Qualitative SOM characteristics did not strongly affect biogeographical patterns of widely distributed soil bacterial taxa. This suggests that broad aspects of SOM quality do not dominate soil bacterial community composition at the investigated macroscale.