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

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.