XANES Synchrotron analysis uncovers the effects of extensive grazing on the composition of mineral-associated organic carbon

Pasture degradation, often associated with extensive grazing, reduces soil quality and negatively impacts livestock production. Intensive grazing involves techniques that increase forage production for livestock through soil and animal management, offering a solution to restore degraded pasture (DP) productivity and improve soil carbon stocks. By optimizing forage yields and consumption, these practices enhance soil organic carbon (SOC) inputs. Understanding the mechanisms of SOC stabilization and transformation in pastures is essential for developing sustainable agricultural practices that increase both soil carbon stocks and pasture productivity. This is particularly relevant as the degraded pasture (DP) system has lost soil carbon stocks, whereas the intensified system has shown an increase in soil carbon. We evaluated the chemical composition of mineral-associated organic matter (MAOM) to monitor shifts in chemical composition caused by pasture management and SOC stabilization mechanisms. Physical fractionation was used to separate soil organic carbon pools into particulate organic matter (POM) and MAOM in a long-term field experiment initiated in 1996 in São Carlos, São Paulo, Brazil, by Embrapa Pecuária Sudeste. We employed X-ray absorption near-edge structure (XANES) spectroscopy to assess MAOM composition at the Carbon K-edge across two pasture systems: a degraded pasture (DP) and a rainfed pasture with a moderate stocking rate (RMS, 200 kg N ha^-1 year). A native forest (FO) was included as a reference. Samples were analyzed at two soil depths: 0–5 cm and 20–30 cm. XANES spectra were measured at the SIRIUS - Brazilian Synchrotron Light Laboratory (LNLS). The C K-edge XANES spectra were deconvoluted enabling the distinction of the main peaks at 284.5 eV (quinone C), 285.4 eV (aromatic C), 287.0 eV (phenolic C), 287.8 eV (aliphatic C), 288.8 eV (carboxylic C), and 289.9 eV (O-alkyl C) in all samples. The DP system showed reduced MAOM carbon stocks (91 Mg C ha^-1), indicating that extensive grazing and lack of soil management negatively affect SOC stability. In contrast, the RMS system enhanced MAOM stocks to 148 Mg C ha^-1, compared to FO (120 Mg C ha^-1). The reduced carbon stocks in DP were accompanied by increased SOC oxidation, as evidenced by a higher relative proportion of carboxylic acid (32–42%) compared to other systems (33–39%). The RMS system, with higher carbon stocks, showed accumulation of aliphatic compounds compared to DP, indicating the role of intensive grazing and soil management in enhancing carbon stocks.