Combined Effects of Sewage Sludge and Phosphogypsum on Soil Nematode Communities and Soil Food-Web Functioning under Hemp (Cannabis sativa L.) Cultivation

The increasing agricultural use of sewage sludge and phosphogypsum within circular economy strategies highlights the need to evaluate their effects on soil biological communities and ecosystem functioning. This study evaluates the impact of fertilization with sewage sludge and varying doses of phosphogypsum (100, 500, and 1000 kg ha−1) on the structure and functional diversity of soil nematode communities in hemp (Cannabis sativa L.). Nematode communities were assessed two and six months after fertilizer application, corresponding to the June and October soil sampling dates, to determine their density, genus diversity, dominance, and trophic structure. The results indicate that fertilization significantly increased overall nematode density six months after application, while descriptive analyses suggested lower genus richness in fertilized treatments than in the unfertilized control. The trophic structure was highly responsive to the amendments; unfertilized soil exhibited a higher proportion of hyphal-feeding nematodes (dominated by Aphelenchoides spp.), whereas fertilized treatments were dominated by bacterial feeders, primarily Acrobeloides spp. Consequently, the nematode Channel Index (CI) values reflected a shift toward bacterial decomposition in fertilized soils, suggesting accelerated element cycling. Among the tested phosphogypsum rates, the intermediate dose (500 kg ha−1) was associated with favorable biological responses, although no consistent dose-dependent response to phosphogypsum application was observed. These results demonstrate that soil nematodes are highly sensitive bioindicators of soil transformations induced by waste-derived fertilizers, providing valuable insights for optimizing sustainable fertilization strategies.