Batch versus column experiments of soils amended with ammonium charged zeolitites to quantify inorganic nitrogen leaching and retention

To assess the leaching behavior from soil of inorganic nitrogen species like nitrate, nitrite and ammonium, the most employed method is the equilibrium batch for its simplicity and cost effectiveness. This approach was compared with the more time consuming but more realistic soil leaching column chromatography technique. Both the methods were tested during the ZEOLIFE project, aimed to explore the capability of natural zeolites (chabasite) to be used as agricultural fertilizer once charged with ammonium from pig slurry. The studied soil is a Humic Thionic Fluvisols located near Codigoro (Ferrara, IT) in a saline paleomarsh environment reclaimed one century ago, intensively cultivated from late ‘70. The soil amended with 7 kg/ha of ammonium charged chabasite was tested using batch tests with Milly-Q water and a liquid/solid ratio of 1:5 and 1:10, stirred for 24 hours to reach equilibrium conditions and then analyzed for ammonium nitrate and nitrite. The soil was repacked in 15 cm length HDPE columns with inner diameter of 2 cm, equipped with 50 μm Nitex mesh to prevent soil washing. The columns were washed with Milly-Q water for at least 30 pore volumes at variable rates, from 0.5 to 2.5 ml/min to leach all the water soluble reactive nitrogen. Soil water samples of 2 ml were collected at discrete intervals to reconstruct the elution curve of ammonium nitrate and nitrite. The results of the batch tests showed low ammonium concentration in the leachate demonstrating the good retention of this cation within the chabasite framework, while higher concentrations of nitrate were recorded due to their presence in the pig slurry. Nitrite were always detected at low concentration indicating bacterial mediated reactions, like nitrification and denitrification. The results of the column experiments displayed elevated nitrate concentrations in the very first water samples and a rapid decrease towards values below detection limits after the first two pore volumes eluted. Ammonium and nitrite were detected at much lower concentrations than nitrate. The comparison of the reactive inorganic nitrogen leached by batch and eluted by column showed higher values in batches, possibly induced by the elevated contact area between soil particle and water phase. Beside, the column experiments highlighted that most of the nitrogen was exported in the form of nitrate at the beginning of the experiment. Thus is concluded that, despite their more demanding requirements, soil columns give essential information respect to simple equilibrium batch experiments.