Deep soil carbon accrual after land restoration in the Atlantic Forest, Brazil

Abstract

Soils play a crucial role in the global carbon cycle, not only by storing most of terrestrial carbon but also by actively participating in carbon sequestration and emission processes. Reliable estimates of soil organic carbon stocks changes after land use change are essential for developing effective climate change mitigation strategies. However, traditional estimates often focus on topsoil, overlooking the substantial contribution of deeper soil layers, particularly in the tropics. This thesis addresses this gap by providing global estimates of deep soil carbon stocks and examining the impact of various land restoration approaches on these stocks within the Atlantic Forest of Brazil, in addition to assessing deep soil carbon stability and temperature sensitivity. <br /> I compiled a comprehensive database of over 12,000 soil profiles, ranging in depth from 200 to 500 cm, to update global estimates of deep soil carbon stocks in relation to climate, soil type, and land use. Additionally, I studied the response of deep soil carbon to different land restoration approaches, including reforestation, natural regeneration, and agroforestry systems, using a paired site design in triplicates for each method. Samples were taken from rural sites in the Atlantic Forest, with adjacent arable land and a secondary forest serving as references. I also evaluated soil carbon stability and temperature sensitivity at various depths along the soil profile to understand the factors influencing temperature sensitivity in restored lands. <br /> The soil organic carbon stock for the 0-200 cm depth interval was 19% larger than previously thought, adding an extra 336 Pg of carbon stored in soils compared to previous global estimates. Soils in tropical climate have the highest stock in the 0-300 cm layer, with an average of 314 Mg ha_-1. Forests significantly contribute to this deep carbon pool, with 69% of the soil organic carbon stock in the 0-300 cm layer located below 40 cm. The land restoration case study in the Atlantic Forest showed no significant differences in total ecosystem carbon sequestration among the three restoration methods, although reforestation sites promoted higher plant aboveground carbon stocks than natural regeneration and agroforestry systems. There was an inverse relationship between aboveground carbon and deep soil carbon sequestration, and reforestation sites with fast aboveground growth caused a loss of 27 Mg ha_-1 of soil organic carbon in the 40-300 cm layer. Moreover, reforestation sites had the lowest deep soil carbon stability, whereas agroforestry system presented the highest. Further, deep soil temperature sensitivity in restored lands was mostly influenced by soil fertility, particularly phosphorus content in the subsoil and nitrogen content in the deep soil. <br /> The results showed that agroforestry systems and natural regeneration, which sequestered more carbon in deep soil in addition to increasing deep soil carbon stability, can enhance climate change mitigation benefits more effectively than reforestation, which is the most commonly used restoration approach in the Atlantic Forest. Transitioning from reforestation to these alternative approaches, where feasible, can bolster carbon stability and maximize the climate mitigation potential of land restoration efforts in the Atlantic Forest. In conclusion, this thesis demonstrated the significant contribution of deep soil carbon to total soil carbon dynamics, emphasizing the need to include deep soil layers in assessments of carbon sequestration following land restoration.