Compaction and organic matter retention in mixed-conifer forests of California: 20-year effects on soil physical and chemical health

Abstract

Soil resilience following harvest disturbance was studied at nine Long-Term Soil Productivity sites across the Sierra Nevada mountain range of California. We tested whether soil compaction, surface organic matter (OM) retention, and plant diversity have lasting effects on soil C, N, bulk density, or porosity. Study treatments following clearcut harvesting included soil compaction (none, moderate, severe) in factorial combination with three levels of OM retention (harvest slash retained, [OM2]; no slash retained, [OM1]; bare soil, [OM0]) and two plant diversity treatments (trees only; trees and shrubs). The sites encompassed a wide range in soil OM (5 to 12%) and clay content (5 to 36%) and represented common soil types in Sierran forests. At year 10, soil bulk densities were 10–29% greater for severely compacted compared to non-compacted plots across sites, corresponding to an average loss of 9–11% total porosity. Partial recovery was found by year 20, as bulk densities were 2–17% higher for compacted compared to non-compacted plots. In contrast to these physical responses, compaction had no adverse effect on soil C and N content at either year 10 or 20. Slash retention (OM2 versus OM1) and plant diversity treatments also had few effects on soil properties with the exception of a small increase in bulk density in the absence of shrubs, presumably due to reduced root and surface litter input. A more pronounced response was found following complete removal of surface organics (OM0), which produced a study-wide decline (12–17%) in soil C, N, and bulk density by year 20. All treatment responses were fairly consistent across sites, regardless of site differences in clay or soil OM content. Our results suggest substantial tolerance of soil C and N to harvest disturbance in Sierra Nevada forests despite a sustained change in soil physical properties and highlight the importance of the O horizon to long-term soil health.