Effects of Reclaimed Water Irrigation on Heavy Metal Contents and Enzyme Activities in Garden Soil

Objective  The study investigated the impact of reclaimed water irrigation treatments on the heavy metal contents and enzyme activities in soil, which could provide scientific evidence and practical support for promoting the recycling and utilization of raw sewage.

Methods  This study selected Schima superba, Castanopsis hystrix, and Castanopsis fissa as experimental plants and irrigated them with tap water, raw sewage, and reclaimed water. The chemical properties of the soil (pH, organic matter, total nitrogen, total phosphorus, alkali-hydrolyzed nitrogen, ammonium nitrogen), the contents of heavy metals (Pb, Cr, Cd), and soil enzyme activities (catalase, S-CAT; urease, S-UE; sucrose, S-SC) and neutral phosphatase (S-NP) were analyzed and compared, along with their interrelationships.

Results  Compared with tap water irrigation, irrigation with raw sewage and reclaimed water significantly increased the soil's pH, organic matter, total nitrogen, total phosphorus, alkali-hydrolyzed nitrogen, ammonium nitrogen, Cr, and Cd contents, while Pb content showed no significant difference. The activities of S-CAT and S-SC were significantly higher under raw sewage and reclaimed water irrigation than with tap water. While the S-NP activity was the highest under tap water irrigation. Redundancy analysis revealed that pH was the main factor affecting Cr and Cd contents. Pearson correlation and principal component analysis indicated a significant positive correlation between Pb content and S-NP activity, while Cr content showed a highly significant positive correlation with S-CAT and S-SC activities and a highly significant negative correlation with S-UE and S-NP activities. Cd content showed a highly significant positive correlation with S-SC activity and a significant negative correlation with S-NP activity.

Conclusion Irrigation with raw sewage and reclaimed water alters the properties of garden soil, increases soil nutrient contents, enhances S-CAT and S-SC activities, and reduces S-NP activity. However, it also leads to the accumulation of some heavy metals, although their concentrations remain below the limits set by national standards.