Carbon neutral? No change in mineral soil carbon stock under oil palm plantations derived from forest or non-forest in Indonesia

Sustainability criteria for palm oil production guide new planting toward non-forest land cover on mineral soil, avoiding carbon debts caused by forest and peat conversion. Effects on soil carbon stock (soil C-stock) of land use change trajectories from forest and non-forest to oil palm on mineral soils include initial decline and subsequent recovery, however modeling efforts and life-cycle accounting are constrained by lack of comprehensive data sets; only few case studies underpin current debate. We analyzed soil C-stock (Mg ha(-1)), soil bulk density (BD, g cm(-3)) and soil organic carbon concentration (C-org, %) from 155 plots in 20 oil palm plantations across the major production areas of Indonesia, identifying trends during a production cycle on 6 plantations with sufficient spread in plot age. Plots were sampled in four management zones: weeded circle (WC), interrow (IR), frond stacks (FS), and harvest paths (HP); three depth intervals 0-5, 5-15 and 15-30 cm were sampled in each zone. Compared to the initial condition, increases in Core (16.2%) and reduction in BD (8.9%) in the FS zone, was compensated by decrease in C-org (21.4%) and increase in BD (6.6%) in the HP zone, with intermediate results elsewhere. For a weighted average of the four management zones and after correction for equal mineral soil basis, the net temporal trend in soil C-stock in the top 30 cm of soil across all data was not significantly different from zero in both forest- and non-forest-derived oil palm plantations. Individual plantations experienced net decline, net increase or U-shaped trajectories. The 2% difference in mean soil C-stock in forest and non-forest derived oil palm plantations was statistically significant (p < 0.05). Unless soil management changes strongly from current practice, it is appropriate for C footprint calculations to assume soil C-stock neutrality on mineral soils used for oil palm cultivation. (C) 2015 The Authors. Published by Elsevier B.V.