Dissipation kinetics of sulfadimethoxine in manure-amended savanna soils under field conditions in the dry season

Veterinary sulfonamides introduced via manure application pose significant environmental risks in tropical agroecosystems, yet their fate under authentic savanna field conditions remains poorly documented. This study investigated the dissipation kinetics of sulfadimethoxine (SDM) in a sandy clay loam savanna soil amended with 10% (w/w) cattle manure under open-field dry-season conditions (July-October 2024, central Tanzania; soil surface temperature 28–42 °C). Initial SDM concentrations ranged from 10 to 100 mg kg − 1 dry weight. Dissipation followed pseudo-first-order kinetics (R² > 0.993) with half-lives of 1.0–4.0 days, among the shortest reported for sulfonamides under field conditions. Dissipation rate constants decreased with increasing initial concentration ( k = 0.17–0.46 day − 1 ), a pattern consistent with potential concentration-dependent inhibition of degrading microorganisms at higher doses. Increasing soil moisture from 10% to 25% (w/w) and manure amendment from 0% to 25% shortened half-lives by up to threefold, an effect likely attributable to enhanced microbial degradation facilitated by greater water availability and organic inputs, although surface photodegradation may also have contributed under high irradiance. Under typical dry-season conditions, > 99% of applied SDM dissipated before the onset of rains, substantially reducing the potential for leaching to groundwater. These findings highlight the strong influence of temperature, moisture availability, and organic amendments on antibiotic attenuation in seasonally dry tropical environments and provide evidence-based guidance for timing manure application to minimize environmental release while preserving its agronomic benefits.