From Waste to Wealth: Integrating Fecal Sludge-Based Co-Compost with Chemical Fertilizer to Enhance Nutrient Status and Carbon Storage in Paddy Soils

This study evaluated the effects of applying fecal sludge-based co-compost (CC) integrated with chemical fertilizers on soil nutrient status, organic carbon (OC) storage, and economic returns in paddy soils. Ten integrated nutrient management (INM) treatments were tested, i.e., BRRI recommended dose of fertilizer (RDF), CC 5.0 t ha−1, RDF + CC 2.0 t ha−1, RDF + CC 1.5 t ha−1, RDF + CC 1.0 t ha−1, RDF + CC 0.5 t ha−1, 75% RDF + CC 2.0 t ha−1, 75% RDF + CC 1.5 t ha−1, 75% RDF + CC 1.0 t ha−1, and 75% RDF + CC 0.5 t ha−1. Two rice varieties were cultivated over two consecutive seasons—winter rice (boro) and monsoon rice (aman)—in the experimental field. Soil samples (0–15 cm) were collected before and after the seasons and fractionated into labile particulate organic matter (>53 µm) and stable mineral-associated organic matter (<53 µm). Bulk soils and CC were analyzed for OC, nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and heavy metals, while the fractions were analyzed for OC and N. Across both seasons, 75% RDF combined with 2.0 t ha−1 or 1.5 t ha−1 of CC consistently showed the highest OC, total N, and soil C stock, with moderate P, K, and S levels. Sole RDF produced the lowest OC and N. Among fractions, stable OC was the highest in the 75% RDF + 2.0 t ha−1 CC treatment, statistically similar to 75% RDF + 1.5 t ha−1 CC, and the lowest under RDF alone. Economically, sole RDF yielded the highest profit, while full RDF + CC achieved competitive returns. Reduced RDF + CC treatments (75% RDF + 1.5 or 2.0 t ha−1 CC) offered slightly lower returns but improved soil sustainability indicators. Overall, applying 75% RDF + 1.5 t ha−1 CC provided the most cost-effective balance of nutrient enrichment, soil C stock, and profitability. This CC-based INM approach reduces chemical fertilizer dependency, enhances soil health, and promotes sustainable waste management, supporting environmentally resilient rice production.