Deep soil carbon loss offsets rapid aboveground carbon accumulation after reforestation

Carbon (C) sequestration following land restoration is an efficient measure of climate change mitigation. However, most studies including the international "4 per 1000 '' Initiative focus on topsoil C only. Here, we studied the response of deep soil C to different land restoration approaches. Samples were taken from rural sites in the Atlantic Forest Biome (Brazil), using a paired site design with triplicates of different 16-to-19-year-old restoration approaches of former arable land: reforestation, natural regeneration, and agroforestry sites. Adjacent arable land and a > 25-year-old secondary forest served as references. We determined aboveground and belowground C pools, including deep soil until 3 m depth. We did not find statistical differences among the three restoration methods for the C sequestration in the whole ecosystem. However, the C stock in the aboveground living biomass in the reforestation site was 85 +/- 15 Mg ha (-1), significantly exceeding the respective aboveground C in natural regeneration (40 +/- 13 Mg ha(-1)) and agroforestry system (25 +/- 5 Mg ha(-1)), respectively. This order, however, was reversed in soil, where reforestation caused a loss of soil organic carbon (SOC) by 27 Mg ha(- 1) below 40 cm depth, whereas SOC stocks in the agroforestry system and natural regeneration site increased by 41 and 25 Mg C/ha, respectively, compared to arable land. We thus conclude that deep soil carbon losses can negate aboveground carbon accrual after land restoration, which emphasizes the importance of including this pool in C sequestration studies.