Mineralization of organic N originating in treated effluent used for irrigation

Reclaimed wastewater and, particularly, secondary effluent used for irrigation, may contain considerable amounts of mineral and organic N. The knowledge regarding N-transformations of effluent-originated organic N in soil is not well established. A method based on ion-exchangers (IE) was developed to remove the mineral N and other ionic species from the effluents, enabling a better follow-up of the reactions of effluent-originated organic N. Modifications of two incubation methods were used to evaluate net mineralization rates and the contributions of ammonification and nitrification of the effluent-originated organic N. A mixture of the ion-exchangers, IRN-77 (H⁺) and IRN-78 (OH^–), was found effective in removing mineral N and other ions from effluents without significantly affecting the content of organic N. In suspension-based experiments performed with a microbially active calcareous clay soil, the nitrification started after about a 1 to 4 d lag (higher lag associated with higher BOD), and the total mineral N reached plateau values after about 9 to 14 d. The time estimated for completion of ammonification of the organic N in the well-mixed and aerated suspensions was 3 to 6 d. Soil incubations were performed after adding the IE-treated effluents to small soil columns. Ammonification of both soil and effluent-originated organic N occurred concomitantly with the nitrification, making the evaluation of rates more complicated. Tracing the time differences in total mineral N between the soils irrigated with the IE-treated effluent and the blank (no added N) enabled the estimation of first order rate constants for the net mineralization of the effluent-originated organic N in: a sandy loam (0.3 wk^–1), a loess (0.4 wk^–1), and in the calcareous clay (1.1 wk^–1). About two thirds of the organic N added to the soils in the columns during the pre-incubation stage were not retained in the soils, whereas ammonium was practically not leached out. The relatively fast movement of the effluent-originated organic N in soil and its mineralization characteristics indicate that this fraction significantly affects the short (days) and middle (weeks) range transformations of N in effluent-irrigated soils.