Land-use change affects soil organic matter dynamics without changing carbon stocks in Amazonian Acrisols

Land-use change (LUC) alters carbon (C) dynamics and affects the persistence of soil organic matter (SOM). This study evaluated the impact of LUC on C sequestrations and turnover in Acrisols down to 1 m depth in the southwestern Brazilian Amazon. The study was conducted at Embrapa headquarter in Rio Branco, Acre. Soil samples were collected from six layers under six land-uses: regenerating forest after a 2011 fire (BA) and its reference native forest (NFB); grass-based pasture with Cynodon nlemfuensis (PP) and mixed grass–legume pasture (C. nlemfuensis and Arachis pintoi) (IP), both under continuous grazing since 2018; grazing exclusion pasture (EP) since 2018; reference native forest for pasture systems (NFP). C contents were determined by elemental analysis, C stocks were calculated by the equivalent soil mass method, and δ¹³C was measured by isotope ratio mass spectrometry. Statistical analyses were performed for the integrated soil profile and individual layers using one-way ANOVA. Carbon stocks did not differ among land-use types in the integrated profile. However, significant differences occurred at 70–100 cm, with greater stocks in EP than PP, while IP and NFP were intermediate. No differences were observed between BA and NFB. Pasture soils showed less negative δ¹³C values than forest references, indicating incorporation of C₄-derived C. The contribution of “new” C (C₄-plant) ranged from 37-52% in the surface to 6-10% in the 70-100 cm layer. Despite unchanged C stocks, pasture introduction altered SOM cycling by mobilizing endogenous C₃-SOM.