Sewage sludge-based fertilizer enhances the soil organic carbon stocks under conservation agriculture in the Brazilian Cerrado

Organic fertilizers made from sewage sludge (SS) are sustainable alternatives that can improve soil health and increase crop productivity. In addition, they can contribute to increasing soil organic carbon (SOC) stocks, contributing to climate change mitigation. However, there is still little research on the use of organic fertilizer ‘composted sewage sludge’ (CSS) associated with crop rotation in tropical regions. It is hypothesized that consecutive fertilization with CSS, along with no-till practices, may be an effective strategy for increasing SOC sequestration in areas under conservation agriculture, with the potential to mitigate greenhouse gas emissions and improve overall soil quality. This study was aimed to evaluate the effect of CSS application rates on the SOC stocks in the layers 0.05; 0.1; 0.2; 0.3, and 0.4 m deep, in an Oxisol under conservation agriculture (i.e., no-tillage grown with soybean, maize, beans, wheat, oats, and marandu grass) in the Brazilian Cerrado. A field experiment was conducted in Selvíria, MS, Brazil. A randomized complete block design with four replicates and five treatments [native forest (NF), conventional mineral fertilization (CMF), and residual effect of accumulated rates of CSS (0.0, 22.5, and 37.5 Mg ha^-1, wet basis) over six crop years] was used. To evaluate the SOC stocks, the total C content in the soil and the bulk density were obtained. The SOC stock was higher in the NF along the soil profile. The application of CSS significantly influenced the subsurface layers (0.2–0.4 m), with the rate of 37.5 Mg ha^-1 CSS promoting an increase of 15.3% to 16.2% in SOC stocks compared to the rate of 0.0 Mg ha^-1. The lowest SOC stocks were observed at the rate of 0.0 Mg ha^-1 and treatment with CMF. Based on the NF results, it was possible to note the influence of the agroecosystem in the surface layer (0.0–0.1 m), with losses of total C in the treatments that received CMF and CSS. It was observed that the organic fertilizer CSS associated with the cultivation of cover crops and under a no-till system increased SOC stocks up to 0.4 m deep. Results suggest that the recycling of SS via CSS is a promising strategy for increasing SOC sequestration in areas under conservation agriculture in tropical regions, mitigating greenhouse gas emissions. Our results are aligned with the concepts of circular economy and the ‘Sustainable Development Goals’ (SDGs) of the UN’s ‘2030 Agenda’, with an emphasis on ‘SDG2’, ‘SDG6’, and ‘SDG13’.