Accelerated soil carbon turnover under tree plantations limits soil carbon storage.

Four million hectares of global primary forests have been lost annually since 1990, mostly in tropical and subtropical areas. This loss has coincided with an annual 2% increase in the total area of tree plantations, which now cover over a quarter of a billion hectares of land1. Soils of native old-growth forests have a large carbon (C) storage capacity and can act as net sinks for atmospheric CO22. Soil organic carbon (SOC) comprises three-fourths of all terrestrial C and is a long-term buffer against atmospheric CO2 increases3,4,5. This important role of native forests in terrestrial C storage raises concerns about long-term consequences of their replacement by plantations, though plantations have been advocated as a measure to sequestrate C from the atmosphere and to mitigate future climate change6. Global-scale meta-analyses7,8 suggested that replacement of native forests by tree plantations has generally reduced SOC stocks by 13–19%, the effect being most serious in the first two decades, although in the tropics, conversion of native forests to plantations has had no significant effect on SOC9. Meta-analyses necessarily include studies spanning diverse sites and species to capture statistical patterns that single studies cannot, but with an acknowledged risk that the mechanisms underlying those patterns can sometimes be obscured9. An additional problem with the soil C data available for meta-analysis is that most measurements are restricted to the first two decades of forest regrowth8; data covering all stages of plantation development are scarce. This hinders our ability to reliably predict the long-term dynamics of soil C during reforestation and to understand their controlling mechanisms. Mechanistic understanding of the processes regulating soil C storage requires direct measurements of belowground C pools and fluxes. An ecosystem’s SOC stock declines if SOC losses exceed inputs. Inputs are those from vegetation (litter, dead roots) and organic C exuded from roots. Losses occur as mainly respiratory CO2 produced by heterotrophic microbes that decompose SOC, autotrophic respiration, and smaller C fluxes as methane and volatile organics. C inputs generally decrease following conversion of natural forests to plantations: on average, aboveground litterfall and fine-root biomass are respectively 34% and 66% smaller in plantations than in natural forests8. Thus, decreased plant C input could be an important driver of SOC reduction during reforestation. However, the extent to which that reduction is caused by accelerated decomposition of SOC is still unknown. In situ measurements of the CO2 flux from SOC decomposition (Rm) are difficult since the CO2 produced by that process is usually mixed with fluxes from other pathways, particularly litter decomposition (Rl) and autotrophic respiration by roots and symbionts (Rr). The sum of these three fluxes constitutes the total soil-surface CO2 flux (Rs)10,11. Most studies of soil CO2 production report only Rs, but there is an increasing need to separate Rs into its components to determine the distinct constraints on different pathways of SOC loss12. In this study, we used a chronosequence of Chinese fir plantations established by successively clearing mixed-species native forests over 88 years (Supplementary Table S1)13. As an important timber species, Chinese fir covers over 12 Mha, almost 11% of the global planted forest area1. We established experimental plots that allowed Rs, Rh and Rm to be measured and Rl derived, and from which SOC residence times and functional aspects of soil microbial activity were estimated. We were therefore able to directly test long-term effects of replacing native forests by plantations on processes controlling the size and dynamics of the SOC pool.