Ponding Water Quality of Rice Paddies Fertilized with Anaerobically Digested Liquid Pig Manure as Affected by Fly Ash and Zeolite

Anaerobically digested liquid pig manure (LPM) is enriched with nutrients and thus can be used as an alternative nutrient source and substitute for chemical fertilizer (CF) in rice (Oryza sativa L.) farming. However, there are concerns regarding the contamination of the surrounding water due to the discharge of ponding water containing dissolved organic carbon (DOC), nitrogen (N), and phosphorus (P) from rice paddies fertilized using LPM. This study investigated the effects of the co-application of fly ash (FA) and zeolite (Z) amendments (FAZ amendments) on the concentration of DOC, N, and P in the ponding water of rice paddies fertilized with LPM at two different rates (standard (LPM_S) and double (LPM_D) at 11 and 22 g N m⁻², respectively). Rice was cultivated using four nutrient treatments, including no input, CF (11 g N m⁻²), LPM_S, and LPM_D, with or without FAZ amendments. When FAZ was not amended, LPM_S and LPM_D application increased the DOC concentration by 32% and 41%, respectively, compared to CF treatments (11 g N m⁻²), reflecting a high DOC concentration in LPM. The total N and P concentrations in the ponding water were lower in LPM_S treatment (by 5 and 8%, respectively) but higher (by 94% and 47%, respectively) in LPM_D treatment compared to CF treatments in the absence of FAZ, indicating a high potential for water pollution with a double LPM application rate. With a given nutrient treatment, FAZ amendments decreased DOC by 15–39%, supporting the immobilization of DOC by Z. FAZ consistently decreased the NH₄⁺ concentration by 6–51% across the nutrient treatments, likely via the sorption of NH₄⁺ onto the negatively charged sites of Z, but its effect on total N concentration was not consistent. Unexpectedly, total P concentration increased (by 77–167%) following the FAZ amendment. FAZ amendments tended to increase rice biomass and grain yield for LPM treatments, but these rice growth parameters were poor compared to CF regardless of FAZ amendment. Our results show that the application of LPM as a complete replacement for CF may hamper rice yield while increasing the likelihood of water pollution with DOC and P, although the co-application of FAZ may help to reduce rice yield loss and decrease DOC and NH₄⁺ concentrations.