Abstract
Microplastic contamination can affect soil properties, microbial communities, and nutrient cycling. This study evaluated whether expanded polystyrene (EPS) microplastic exposure during vermicomposting affected nitrite-related absorbance in vermicast produced by Eisenia fetida. Fifty independent experimental units were divided into a control treatment without EPS (n = 25) and an EPS treatment (n = 25). After a 12-day exposure period, vermicast samples were extracted with calcium chloride and analyzed using a modified Griess-type colorimetric reaction. Griess-reactive absorbance was measured at 540.1 nm with a visible-light spectrophotometer. Mean absorbance was 1.25 absorbance units (AU) in the EPS treatment and 0.766 AU in the control, a 63% difference between group means. A Mann-Whitney U test indicated a significant difference between treatments (U = 75, z = -4.6, p < 0.01). Under these experimental conditions, EPS exposure was associated with greater Griess-reactive absorbance in vermicast. Because no nitrite calibration curve was generated, the results cannot be interpreted as absolute nitrite concentrations, and the mechanism underlying the observed difference remains unresolved.