Microbial community responses in forest mineral soil to compaction, organic matter removal, and vegetation control

We tested three disturbance hypotheses in young conifer plantations: H-1: soil compaction and removal of surface organic matter produces sustained changes in microbial community size, activity, and structure in mineral soil; H-2: microbial community characteristics in mineral soil are linked to the recovery of plant diversity; and H-3: community responses are strongly modified by regional climate. Microbial biomass, respiration, carbon utilization, and phospholipid fatty acids were compared at two subtropical installations and one Mediterranean-type climate installation of the North American Long-Term Soil Productivity study. Treatments included combinations of compaction (none vs. severe), organic matter removal (none vs. complete), and weed control (none vs. complete), plus an uncut reference stand. Weed control resulted in the only consistent decline or shift in microbial indices at the subtropical sites. At the Mediterranean-type climate site, overstory harvesting resulted in declines in microbial biomass, respiration, and fungal phospholipid fatty acids that far outweighed the effects of the soil disturbance treatments. Severe compaction had no effect on community size or activity at any site. Microbial communities were generally tolerant of postharvest soil disturbance, leading to a rejection of the experimental hypotheses, with the exception of a link between microorganisms and recovery of plant diversity (H-2) at the subtropical sites.