Assessing Forest Soil Disturbance through Biogenic Gas Fluxes

Soil damage from wet‐weather timber harvests may persist after some physical and chemical properties have been restored through tillage and fertilization. This study's objective was to determine if the gaseous products of aerobic and anaerobic soil biological activity could reveal harvest‐damage effects, even after applying costly mitigation treatments. Fluxes of CO 2 , CH 4 , and N 2 O across the soil surface were measured in Ultisols of a Coastal Plain pine flat damaged during timber harvest, then mitigated by bedding and fertilization (100 kg ha ‐1 N, P, and K). Gas fluxes were measured with large static chambers (0.5 by 1.0 m sampling area) to compensate for high microsite variability and the presence of coarse debris on the forest floor. Carbon dioxide evolution was a robust and consistent indicator of residual damage, declining an average of 34% in planting beds on damaged vs. undamaged soils. For example, in a late summer reading, efflux of CO 2 ‐C from beds installed over former skid trails was 143 mg m ‐2 h ‐1 vs. 258 mg m ‐2 h ‐1 from undamaged beds and 231 mg m ‐2 h ‐1 from undisturbed forest floor. Methane and N 2 O fluxes were ephemeral and, thus, generally unreliable as indicators of harvest damage — though bedding produced scattered high peaks in both. Carbon dioxide was also the only gas flux that responded significantly to fertilization, with an average 26% increase up to 4 mo after fertilization. These results suggest that suppression of gross soil biological activity by harvest damage was not restored by intensive mitigation in the next rotation's establishment phase.