Targeted sludge application for carbon sequestration: A novel approach deployed in semi-arid soils

Municipal wastewater sludge can contribute to nutrient recycling and carbon accumulation in soils. However, achieving these benefits requires both an appropriate application rate and suitable field selection. This study proposes an approach to guide field selection to optimize carbon accumulation. The approach integrates hydrological and topographical data with historical records of sludge applications and soil chemical properties to predict medium- to long-term organic carbon concentration in soils following sludge application. An opportunity map was developed using a model trained with 14 years of data from a wastewater company in Chile. Following a scenario analysis, two fields (each with eight paddocks) with contrasting soil conditions were selected to evaluate model performance and assess organic carbon distribution within the soil profile. The model achieved a root mean square error of 6.2 g kg⁻¹ for topsoil organic carbon, with an R² value of 0.43. The opportunity map revealed that only 20 % of the paddocks that had received sludge since the company started its operations were located in soils with potential for organic carbon accumulation. The case study indicated that fallow-corn rotations promoted organic carbon loss, regardless of the sludge application rate. Direct observations of the paddocks showed that organic carbon accumulated beyond the ploughing depth. Deep, fine-textured soils contained 156.7 ± 17.0 tons ha⁻¹ of organic carbon, whereas shallow, coarse-textured soils had 131.3 ± 14.9 tons ha⁻¹ after similar sludge application rates. The carbon balances for the topsoil, both predicted and observed, were negative for the studied fields. Since sludge application records included only topsoil data, it was not possible to calculate a net carbon balance. The proposed approach accurately reflected paddock conditions and could serve as a decision-making tool for selecting suitable soils for sludge disposal to maximise soil carbon accumulation.