Changes in Microbial Nitrogen Across a 100‐Year Chronosequence of Upland Hardwood Forests

Soil microorganisms mediate many of the major processes involved in soil N cycling. Also, they are strong competitors with plants for available soil N. Thus, changes in microbial N because of forest harvesting may have significant impacts on N availability and overall forest N cycling. A chronosequence of upland hardwood forest stands in southern Indiana, USA, ranging in age from 1 to 100 yr since last harvest, was chosen to evaluate changes in microbial N with stand development. Microbial N was measured in the A and B soil horizons during different seasons from 1997 to 1999. Peak levels of microbial N were highest in the youngest forest stand (1 to 3 yr old), but seasonal variability was greater than differences by stand age. Microbial N concentration (mg kg−1) varied significantly by season and soil horizon within stands ranging in age from 1 to 30 yr since harvest, but not in the mature, 80‐ to 100‐yr‐old stand. There were few significant differences in microbial N content (kg ha−1). Harvesting did not appear to have long‐term effects on microbial N, but spatial variability in microbial N appeared to be greater in the younger forest stands.