Can short-term reforestation provide soil carbon sequestration in the brazilian semiarid region?

In the Brazilian semiarid region, beekeepers have adopted production models based on regenerative agriculture, such as reforestation, as a strategy to maintain bee swarms during the dry season, increase carbon (C) stocks and improve soil health. The study aimed to evaluate the impact of a reforestation process (Mimosa caesalpiniaefolia, Leucaena leucocephala, and Moringa oleifera), on the soil C sequestration potential in smallholder’s areas in the Brazilian semiarid region. Three rural properties with different soil clay content were selected: (1: Oxisol, 15 dag kg^-1; 2: Ultisol, 10 dag kg^-1; 3: Oxisol, 6 dag kg^-1 ) to determine the total organic C (TOC) stocks and its pools (particulate organic carbon – POC; mineral-associated organic carbon – MAOC) and the C sequestration or emission rate (0-10 cm, 10-20 cm and 20-40 cm) in areas under reforestation (RF) with different adoption times (2, 3 and 5 years), pasture (PT) and native caatinga vegetation (NV). In the 0-10 and 10-20 cm depths, TOC stocks were higher (p<0.05) in the areas under NV and RF, considering properties 1 and 2. In property 3, with lower clay content, TOC stocks, in the same layers, were also higher in NV, followed by the areas under PT and RF. Considering the accumulation of TOC in the three layers (0-40 cm), it was observed highest stocks in the area under NV in properties 1 (9.7 Mg ha^-1) and 3 (8.9 Mg ha^-1) and in the area under RF (8.97 Mg ha^-1), in property 2. There was a net carbon emission (C-CO₂) in the areas under PT in properties 1, 2 and 3 (3.1, 0.4 and 0.4 Mg ha^-1year^-1, respectively) and in the areas under RF in properties 1 (2.2 Mg ha^-1year^-1) and 3 (0.7 Mg ha^-1year^-1). On the other hand, in property 2, the RF sequestered C-CO₂ (0.4 Mg ha^-1year^-1). The proportion of the MAOC pool (compared to POC) in the TOC stocks, was dominant, ranging from 92% to 98%, 94% to 98% and 87 to 94% in the area under NV, RF and PT, respectively. Areas under reforestation will require longer adoption time to increase soil C stocks. The predominantly sandy soils, with an advanced history of degradation, combined with the still insufficient contribution of residues from the tree species, made it difficult to rebuild organic matter. Greater species diversification, especially with greater biomass production, may be the main change for increasing soil C stocks and enhancing beekeeping activity.