Soil Nitrogen Mineralization in a Clearcutting Chronosequence in a Northern California Conifer Forest

Abstract

AbstractSoil ammonification, nitrification, and N mobility were studied for 1 yr in three Sierra Nevada (USA) mixed‐conifer stands to examine the long‐term influence of timber harvesting on soil N dynamics. Clearcutting had a persistent effect on soil N mineralization, detectable by in situ incubation but not by conventional low‐tension lysimetry. Mineralization rates were greater in 5‐ and 17‐yr‐old regenerated clearcuts than in an adjacent 100‐yr‐old uncut forest. Annual rates of N mineralization averaged 12 kg ha−1 for the uncut forest, and 49 and 31 kg ha−1 for the 5‐ and 17‐yr‐old clearcuts, respectively. Mineralization continued throughout the summer, even when soil matric potentials were <‐1500 kPa. Soil solution NO3 concentrations were much higher in the younger clearcut than in either the older clearcut or the uncut forest. Sustained effects of clearcutting seem due largely to higher substrate availability and to moisture and temperature conditions favoring microbial activity. Chemical inhibition of nitrification may become important as the forest floor develops.