Current management and harvesting practices can compact forest soils and degrade soil health. However, effects of soil compaction on microbial processes and composition are poorly understood. We measured microbial community responses to compaction in a sandy loam and a clay loam soil under laboratory and field conditions. Treatments of no, moderate (approximately 20% increase in bulk density), and severe (approximately 40% increase) compaction were manually applied to intact soil cores. A 67-d laboratory experiment, punctuated by four sampling dates, was used to evaluate microbial indices (biomass, respiration, total and culturable bacteria and fungi, N mineralization, surface CO2 efflux, C use (Biolog), and phospholipid fatty acids PLFA) and their relationship to soil physical properties (bulk density, pore-size distribution, water-holding capacity WHC, gas diffusion). Macropores (>30 micrometer diam.) were reduced 50 to 90% in compacted samples. In contrast, habitable-sized pores for microorganisms (0.2-30 micrometer diam.) increased at least 40% in both soils with compaction. Despite these changes, microbial measures were either unaffected by compaction or showed inconsistent increases (e.g., fungal hyphae, C use, total PLFA) across sampling periods and soil types. Surface CO2 efflux was reduced 34 to 51% in severe compaction samples. Minimal changes in microbial respiration indicate that reduced efflux was due to restricted gas diffusion. Microbial indifference to compaction also was verified at two mixed-conifer plantations in northern California. Soil strength values, ranging from 75 to 3800 kPa (no to severe compaction), were unrelated to either microbial respiration or biomass. The results show broad tolerance of microbial communities from contrasting soil textures to compaction, and indicate a poor link between physical and biological indices of soil health.
