Mineralization characteristic of the organic nitrogen in effluents

Irrigation with reclaimed wastewater (effluent) is one way to dispose this waste particularly in water scarce arid and semi-arid areas. Secondary effluent used for irrigation contains considerable amounts of N, both in inorganic and organic forms. Whereas attention was given to transformations of inorganic N originated from effluent, little is known regarding the fate of the organic N in effluent and soils. An obstacle in dealing separately with these two forms is difficulty to separate them. In this study, a method based on ion-exchangers was developed to adsorb the mineral N and other ions from different effluents. These effluents were then incubated with soil (incubation). Two methods, i.e. shaking a slurry of soil and effluent and soil incubation at field capacity, were used to evaluate the mineralization rate (formation of ammonium and nitrate) of organic N originated from secondary effluents. The mixture (1:1) of the ion-exchangers, IRN-77 (H) and IRN-78 (OH) was found effectively in removing mineral N and other ions from effluents of varying qualities, without significantly affecting their content of organic N. The percentage of organic N in the investigated effluents ranged between about 10% to 70% of total N. The treated effluents were further used to study N transformations of the organic fraction. In experiments in which the effluents were shaken in a slurry of a clay soil (modification of the fast test used to evaluate potential nitrification), mineralization of the organic N began after about 2-4 days reaching a maximum after about 9 to 12 days. This indicated that under favorable conditions the organic N fraction could be mineralized in about one week. In soil incubation experiments, the mineralization of the organic N from the effluent was completed in about for 3-4 weeks in a sandy loam and a loess soil, while the original organic N continued to mineralize. For Grumusol, the mineralization of the organic N from the effluent was completed in less than two weeks, and again the original organic N continued to mineralize. Application of the different effluents to short soil columns indicated that while most of the ammonium was adsorbed to the soil a significant fraction of the organic N in the effluent was leached out. The relatively high mobility and mineralization of effluent organic N implies that this fraction affects significantly the short (days) and middle (weeks) range transformations of N in effluent irrigated soils. This may affect N availability to plants and N losses via leaching or volatilization (e.g. denitrification/nitrification products, or ammonia).