How Do Leonardite and Elemental Sulfur Alter Nitrogen Immobilization in Vermicompost Enriched with Urea?

Bioavailable organic nitrogen is a valuable component of organic fertilizers, as microorganisms and plants can either uptake it directly or convert it into mineral nitrogen forms. Although vermicompost is an excellent source of readily available nutrient for plants, it contains low amounts of nitrogen. Therefore, this study investigated whether urea is capable of improving the organic nitrogen content of vermicompost in the presence of sulfur and leonardite as amendments. The vermicompost was amended with urea (0 and 1% w/w; designated U0 and U1), sulfur (0, 1, and 2% w/w; S0, S1, S2), and leonardite (0, 2, and 4% w/w; L0, L1, L2). The nitrogen assimilation as well as the changes in the properties of the nitrogen-enriched product were monitored at different time points (1st, 15th, 30th, and 60th days). The results showed that leonardite (L1 or L2) enhanced total nitrogen in U0 and U1 by improving nitrogen retention. In contrast, sulfur increased total nitrogen only in U0 and had no effect in U1. The application of urea resulted in a significant increase in organic nitrogen concentration relative to the U0 control. The highest concentration of organic nitrogen was recorded in the U1 treatment on day 30; however, sulfur addition (S1 or S2) significantly reduced it. The heatmap from hierarchical clustering analysis (HCA) showed that all measured parameters and treatments were grouped into four clusters at all time points. Sulfur and urea played a major role in the clustering of treatments.