Long‑term impacts of a composted sewage sludge–based Class B organic fertilizer on soil micronutrients and crop production in the Tropics

Organic fertilizers, such as Class B materials ( e.g. , composted sewage sludge, CSS), may contain high levels of plant micronutrients, but their use in crop production needs to be monitored to avoid environmental problems. In this study, we sought to understand the Cu, Mn, and Zn dynamics in tropical soils and their residual effects on maize and wheat productivity, after three CSS applications. A randomized complete block experimental design with six treatments and four replicates was implemented. CSS rates (0.0, 5.0, 7.5, 10.0, and 12.5 Mg ha⁻¹) were compared with conventional mineral fertilizer (CMF). Over a six-year period encompassing three CSS applications, we monitored maize and wheat yields, available and total Cu, Mn, and Zn content in the soil, as well as the distribution of these nutrients among the different soil fractions (mobile, exchangeable, bound to Mn oxides, OM, amorphous Fe oxides, crystalline Fe oxides, and residual). Soil micronutrient availability increased with CSS applications. Total Cu and Mn content increased up to intermediate rates but showed slight reductions at the highest rate (12.5 Mg ha⁻¹). CSS application at 12.5 Mg ha⁻¹ rate increased linearly Cu and Zn availability in the soil by up to 67% and 600%, respectively. Micronutrients from Class B organic fertilizers, exemplified here by CSS, enhanced maize and wheat yields, demonstrating a positive association between crop production and Cu and Zn availability. Successive applications of CSS represent an agronomically effective and environmentally sustainable strategy for supplying micronutrients in crop production, contributing to circular economy objectives and the maintenance of global food security; however, its long-term environmental sustainability depends on continuous, strategic monitoring. • Composted sewage sludge (CSS) application increased soil Cu, Mn, and Zn availability. • Micronutrients accumulated predominantly in the residual soil fraction. • CSS maintained or improved maize and wheat yields over six years. • Soil micronutrient availability positively correlated with crop productivity. • Long-term CSS use supports circular economy in tropical agroecosystems.