Freshwater Macrophytes Restore Soil Functionality Through Species-Specific Effects on Soil Organic Carbon and Enzymatic Activity

Freshwater macrophytes harvested from eutrophic water bodies may be repurposed as soil amendments, but their species-specific effects on soil chemical and biological quality remain insufficiently understood. This study evaluated the effects of dried biomass from Egeria densa, Myriophyllum aquaticum, Pistia stratiotes, and Salvinia auriculata, incorporated at 20 g kg−1 soil, on soil chemical attributes, organic carbon, and enzymatic activity after 30, 60, and 90 days of incubation. The experiment was conducted in a completely randomized 5 × 3 factorial design, comprising five amendment treatments (four macrophyte species and an unamended control), three incubation periods (30, 60, and 90 days), and five replications per treatment combination, under controlled conditions at 25 ± 1 °C and 60–70% of soil water-holding capacity. Macrophyte treatment, incubation period, and their interaction significantly affected most soil attributes, whereas the treatment × incubation-period interaction was not significant for soil organic carbon (SOC; p = 0.180). Myriophyllum aquaticum maintained the highest SOC values (55.0–56.4 g kg−1), approximately 22–26% above the unamended control. Salvinia auriculata showed a delayed biological response, with β-glucosidase activity increasing from 17.7 to 114.4 μg PNP g−1 soil h−1 between 30 and 90 days. Pistia stratiotes maintained base saturation at approximately 82% throughout incubation and was more strongly associated with chemical fertility indicators. The first two RDA axes explained 77.2% of the total variation. Redundancy analysis confirmed that macrophyte identity and incubation time explained substantial variation in soil quality indicators. These findings indicate that freshwater macrophytes can be selected according to agronomic objectives, with M. aquaticum and S. auriculata showing greater potential to improve soil biological quality and P. stratiotes contributing more strongly to chemical fertility.