Impacts of Harvest Residue Management on Soil Carbon Stocks in a Plantation Forest

Abstract

The impacts of plantation forest management on soil C stocks in New Zealand need to be better understood for the purposes of C accounting under the Kyoto Protocol. We investigated the impacts of three harvest residue management treatments on C and N stocks in a scoriaceous forest soil: whole‐tree harvesting plus forest floor removal, whole‐tree harvesting, and stem‐only harvesting. Volumetric samples were collected from the forest floor and three mineral soil depth increments (0–0.1, 0.1–0.2, and 0.2–0.3 m) 16 to 17 yr after harvesting. Both fine (<2‐mm) and coarse (>2‐mm) fractions of the mineral soil were analyzed. No significant impacts on stocks of C and N were observed in the mineral soil fine fraction. Averaged across treatments, the 0‐ to 0.3‐m mineral soil coarse fraction contained 5.2 ± 0.52 Mg C ha−1, representing 25% of total mineral soil C, with most derived from the fine‐earth‐coated scoria gravel. The inclusion of the coarse fraction stocks enabled the detection of significant treatment effects. Stem‐only harvesting (residue retention) had significantly larger C stocks in the 0‐ to 0.1‐m total mineral soil (fine + coarse fractions), forest floor, and total soil (forest floor + 0–0.3‐m total mineral soil) pools than whole‐tree harvesting (residue removal) plus forest floor removal. We recommend the retention of both harvest residues and forest floor materials for the maintenance of soil C stocks in plantation forests. Detection of residue management impacts on C stocks in soils with vesicular gravels may require analysis of the coarse fraction.