Soil organic carbon sequestration potential of reforesting riparian areas in an agricultural watershed in the state of São Paulo, Brazil

Fighting climate change has never been so urgent as today. As geoscientific research advances, more realistic and appalling future climate scenarios are unraveled. Decreasing greenhouse gases (GHG) emissions is not sufficient to avoid a bad outcome; hence, mitigation actions are needed to reduce climate-related risks in the future. The depletion of the soil organic carbon (SOC) pool due to land use change and soil degradation have substantially contributed to the increase in the atmospheric CO₂ concentration. Likewise, the sequestration of C by the soil is crucial to reverse this issue. These are especially important processes in the tropics where the replacement of native vegetation by agriculture still occurs at a high rate. Brazil is one of the biggest agricultural producers in the world. In 2018, agriculture and land use change represented 70% of the total Brazilian GHG emissions. Fortunately, Brazil also has opportune mitigation options. Since 1965, the Brazilian Forest Code requires landowners to conserve native vegetation by means of Riparian Preservation Areas, among other categories. Riparian forests provides several ecosystem services like water protection, biodiversity conservation and carbon sequestration. Frequently, the discussion over the carbon sequestration potential of riparian forests focus on the aboveground carbon, nevertheless, SOC stocks are more stable and protected from natural and anthropogenic hazards. We consider that the mandatory reforestation of riparian zones is a significant mitigation strategy in Brazil, owing to the potential of SOC sequestration by Brazilian biomes and the extensive area to be reforested. The objective of this study was to assess the SOC stocks of the main land uses of an agricultural watershed located in the state of São Paulo, Southeastern Brazil, and estimate the change in the SOC stocks that would occur with the reforestation of the riparian areas of this watershed. In order to achieve this goal, we compared the SOC stocks of riparian forests with the two main agricultural uses of the region, namely pasture and sugarcane. The mean SOC stock at 30 cm for riparian forests was of 44 Mg.ha^-1, for pastures was of 26 Mg.ha^-1 and for sugarcane was of 27 Mg.ha^-1. Although the riparian forests of the region are often poorly preserved, they contained considerably more SOC at 30 cm than the agricultural uses. Based on the estimates of the SOC stocks of the main land uses and the extent of the riparian zones of the sampled sites, we could foresee an accretion of 20% of organic carbon in the 30 cm soil layer of those areas. We hope that this study highlight the importance of the riparian forests and the ecosystem services they provide, and the relevance of the Brazilian Forest Code in the mitigation of climate change.