Micro-Architectural Constraints on Urease Activity: How Aggregate Structure Modulates Microbial Trade-offs

This study investigates the relationship between aggregate micro-environments and microbial metabolic costs to quantify how soil physical structure constrains microbial physiology. Specifically, the objective was to determine whether aggregate size and structure impose distinct energetic trade-offs on nitrogen-cycling bacteria. Disturbed silty clay soil samples classified as Plinthic Acrisols and characterized by low pH (3.99 ± 0.02) and soil organic carbon of 2.59 ± 0.01 g kg^-1were taken under natural grassland in Yiyang city, Hunan Province, China. Natural aggregates sieved at 2-5 mm (large macroaggregates, LM) and 0.5-1 mm (hereafter small macroaggregates, SM) were inoculated with exogenous urease-producing bacteria (UPB) and incubated at 20 and 30 degrees Celsius (three replicates, each) for 7 days. The rate of SOC mineralization, urease activity, total extractable extracellular polysaccharides, and UPB populations were determined, and the temperature sensitivity (Q₁₀) of SOC mineralization was calculated. Afterward, LM and SM structures were characterized by pore-size distribution determined by mercury intrusion porosimetry (MIP), and by structural stability assessed using laser diffraction.

Results revealed that LM offered superior protection for UPB colonization compared to SM. Specifically, at 20 degrees Celsius, UPB population abundance in LM was 235.56 × 10³ CFU/g, whereas in SM was significantly lower at 196.66 × 10³ CFU/g. This higher biomass in LM supported a substantial C mineralization rate of 21.28 mg·kg^-1·d^-1. Notably, LM exhibited a lower Q₁₀ (0.96) compared to SM (1.19). MIP analyses refined this understanding, revealing that LM possesses a higher volume of habitable mesopores compared to SM. While this specific pore range facilitates extensive bacterial colonization, the accumulation of extracellular polysaccharides within the tortuous and constricted pore throats likely creates a bio-physical barrier that restricts diffusion. This physical architecture explains the paradox of high biological density yet low temperature sensitivity. In conclusion, large macroaggregates function as low-cost metabolic niches, where tortuous pore structures maximize bacterial survival but constrain metabolic flexibility through physical diffusion limits.