Soil Pore Structure and Nitrogen Availability as Drivers of Microbial Carbon Use Efficiency

Soil pore systems create diverse microhabitats that influence how microorganisms process carbon (C), yet the mechanisms driving their functioning remain poorly understood. We investigated how pore size and nitrogen (N) availability interact to affect microbial C use efficiency by selectively delivering ¹⁴C-labeled glucose with or without N, to large (30-150 µm) or small (3-10 µm) pores in three soils of loamy sand, sandy loam, and loam textures. We traced the fate of added glucose C through CO₂ emissions and microbial biomass analysis over a 45-day incubation to calculate C use efficiency. Without N addition, the small pores supported much higher C use efficiency (0.64 vs. 0.54) than large pores. N addition, however, eliminated these pore size differences and produced contrasting responses: C use efficiency decreased in small pores and remained at the same levels in large pores. Identical pore size classes differed in C processing depending on soil texture and pore connectivity. While fine-textured soils generally followed the paradigm of higher efficiency in smaller pores, the coarse-textured soil exhibited distinct processing patterns. These findings show that soil texture, pore connectivity, and resource availability interact to create soil-specific processing, dependent on pore structure, that require distinct conceptual models.