Extension Note 59 — Influences of Clearcut Harvesting and Fertilization on Structure and Function of the Soil Food Web in ICH Forests of Southern Interior British Columbia

This Extension Note describes some effects of clearcut harvesting and broadcast NPS fertilization on nitrogen (N) mineralization and components of the soil food web at three sites in the ICHmw subzone. We found that the forest floor in clearcuts had lower fungal hyphal lengths, total microbial biomass, microbial biomass N, and potential N mineralization than the forest floor in adjacent forests. Population densities of fungivorous, ominvorous, and predacious nematodes were reduced, and the ratio of bacterivorous/fungivorous nematodes was increased in the clearcuts. Nitrogen mineralization was correlated with bacterivorous and fungivorous nematodes in forest soils but not clearcut soils, suggesting that grazing by microbivorous nematodes may contribute to regulation of N mineralization in forests, and reduced grazing in clearcuts may account for the reduced N mineralization. The effects of fertilization were more pronounced. We found that fertilization resulted in higher N mineralization and nitrate leaching, but had no effect on the amount of N sequestered in the microbial biomass, suggesting high potential for N losses to leaching and denitrification before seedlings are large enough to fully utilize the mineralizable N. Fertilization also resulted in an increased ratio of bacterial/fungal biomass and increased populations of bacterivorous nematodes, which together suggest that greater N mineralization was the result of increased bacterial turnover via nematode grazing. These results suggest that clearcut harvesting has the potential to reduce N availability in the short-term, but recovery may be hastened through forest floor and plant retention. In addition, broadcast fertilization at the time of planting can result in high N losses, but these losses may be minimized by delaying application until sites are fully occupied, and/or by applying fertilizer locally near individual seedlings.