Initial soil carbon losses may offset decades of biomass carbon accumulation in Mediterranean afforestation

Abstract

Afforestation of degraded areas was suggested as CO2 sink, contributing to climate change mitigation. Yet, few studies have assessed this sink by combining measurements on carbon (C) in the biomass and the soil, despite it being crucial to properly estimate the mitigation potential. Here, we assessed the combined C stocks of afforestation plots of different ages on former cropland in a Cambisol landscape in Extremadura, Spain. The plots were afforested with two native tree species (Quercus ilex L. and Quercus suber L. in a density ratio of 3:1), planted at several occasions between 1998 and 2011. Stocks of afforested areas in 2022 were compared to non-afforested negative controls on arable land, to a closeby olive grove and a forest with signs of degradation. Tree biomass was estimated from allometric equations, soil organic carbon (SOC) stocks were measured to 30 cm depth, based on equivalent soil mass. The biomass C accumulation rate in afforested plots increased with tree density and elevation (p < 0.05; range: 25 to 75 g C m−2 yr−1). SOC stocks, in contrast, were not significantly different in afforested and non-afforested plots at any depth and in tendency even lower in afforested plots younger than 20 years. Consequently, total (biomass plus soil) C stocks in afforested plots were not significantly higher than in non-afforested ones. Nevertheless, SOC stocks and contents between the tree rows were significantly lower compared to soil next to the trees in the olive grove (about 1200 vs. 2200 g C m−2 in the top 30 cm) and in tendency in the afforested plots (about 1200 vs. 1500 g C m−2 in the top 30 cm; p < 0.1). The fact that the degraded forest (about 6800 g C m−2) and the olive grove (about 5300 g C m−2) did have significantly higher total C stocks than the afforested and non-afforested sites (about 2300 and 1800 g C m−2) could indicate that afforestation could soon become a C sink. However, our study clearly shows that afforestation is not automatically a C sink. Timing of different C pools‘ losses and gains affect net ecosystem carbon sequestration. While improved soil management in afforestation may reduce SOC losses, afforestation with Mediterranean Quercus trees under current management practices may require decades before being a C sink. This finding should temper expectations that afforestation with such tree species is a rapid solution to combat climate change.