Conversion of grassy cerrado into riparian forest and its impact on soil organic matter dynamics in an Oxisol from southeast Brazil

The purpose of this study was to evaluate possible changes in soil organic matter (SOM) dynamics after establishing riparian forests on soils previously under Brazilian savannah (“cerrado”). We selected a site with a homogeneous Typic Acric Red–Yellow Latosol (Anionic Acrustox). Part of this site was maintained under native vegetation (grassy cerrado C4-dominated), and part was planted with riparian species (C3) in 1992. Litter and soil samples were collected and analysed (total organic carbon, total nitrogen, δ13C isotopic analysis, and SOM density fractionation). Due to the predominance of grasses, carbon input was mainly below ground in cerrado. In such a soil, the decomposition process was more efficient, and much C and N were transferred to the heavy fraction. When forest was planted, there was a change from belowground to aboveground litter input (largely superficial), leading to higher C and N stocks in the light and lower stocks in the heavy fraction (resulting in lower stocks for bulk soil). The introduction of the C3 vegetation decreased the soil δ13C signature. It has occurred particularly in the topsoil (0–5 cm) due to the deposition of C3 litter on the soil surface. At the same time, the presence of cerrado-remaining C below 5 cm maintained higher δ13C values in this layer. During the 8 years after forest plantation, the input mode influenced both the δ13C distribution with depth, and the C replacement: between 0 and 2.5 cm, nearly 50% of cerrado-derived C was replaced by forest-derived C, while below 5 cm, replacement was around 20%. The relatively rapid C dynamics in this Oxisol (27% replacement in the top 20 cm after 8 years of forest plantation) shows that, under tropical conditions, significant changes may occur in a short period of time.