Composted sewage sludge enhances mineral-associated organic carbon sequestration in tropical conservation agriculture systems

The agricultural use of composted sewage sludge (CSS) offers an integrated solution by enhancing crop nutrition while simultaneously promoting carbon (C) sequestration and stabilization in soils, a mechanism that is highly relevant for climate change mitigation. This study investigated the effects of CSS application rates on the dynamics of physical soil organic matter fractions in the Cerrado region. A field experiment was conducted in a Cerrado Oxisol managed under no-till, using a randomized complete block design with four replications. The five treatments consisted of residual CSS application rates (0.0, 22.5, and 37.5 Mg ha⁻¹), conventional mineral fertilization (MF), and a Native Forest (NF) area. Physical fractionation of soil organic matter was performed in soil layers down to 0.4 m depth to quantify particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). MAOC was the dominant fraction, accounting for 73–77% of total soil C. The application of 37.5 Mg ha⁻¹ of CSS resulted in a 63% increase in POC in the 0.1–0.2 m layer and, more notably, increased MAOC by 19% in subsurface layers (0.1–0.4 m). At this application rate, MAOC content approached those observed in the NF area. These results indicate that the continued use of CSS contributes to the saturation of mineral binding sites in subsurface layers, thereby promoting the medium-term accumulation of stable and resilient soil carbon.