## 1 Introduction  
Agriculture is commonly regarded as the major source of contamination in the soil-water environment worldwide. Excessive use of manure and fertilizers has a negative impact on the groundwater quality and drinking water supplies, which can be detrimental to human health [1; 2; 3; 4]. One of the most problematic issues connected with the nitrogen cycle in agricultural areas (Figure 1) is nitrogen leaching to groundwater and surface water as a result of the abusive use of fertilizers [5; 6; 7]. The composition of nitrogen fertilizers is dominated by ammonium nitrate, which provides half of the nitrogen in the ammonium form and half in the nitrate form. These forms are easily available for crops but, in high concentrations, may cause hazardous effects to the aquatic system. Excessive concentrations of ammonium may cause a decrease of dissolved oxygen in natural waters and toxicity to aquatic organisms [8]. Ammonium ions can be adsorbed by clay particles, contrary to nitrates, which are easily transported in the soil-water environment and, subsequently, may lead to eutrophication [9; 10; 11].  
Leaching of nitrogen compounds depends on several factors, i.e. fertilization level, type of fertilizer, time and method of application, properties of soils (pH, texture, organic matter content), types of plants, agronomic practices, level of animal production, weather conditions, and watershed land use [12; 13; 14; 15; 16]. Due to the leaching of nitrates from the soil profile and the resulting possibility of spreading contaminants in the soil-water environment, it is reasonable to undertake extensive research aiming to recognize the basic processes, especially sorption, responsible for the migration of these contaminants [3; 7; 9; 17; 18; 19].  
Among the processes responsible for the migration of contaminants in the soil-water environment, the most significant are advection, diffusion, dispersion, biodegradation, and sorption. The role of particular processes associated with the migration of contaminants depends mainly on the type, the physical and chemical properties of pollution, and the properties of the medium in which the migration takes place [17; 20; 21]. Extremely important in solving problems related to risk assessment and groundwater protection in agricultural areas is recognition of the sorption processes occurring in natural soils [10; 12]. For ammonium ions, the key reactive processes controlling subsurface migration include sorption as a result of cation exchange and biological degradation [22]. Adsorption of positively-charged ammonium ions occurs on the surface of negatively-charged clay particles. [PERSON] and [PERSON] [23], [PERSON] et al. [24], [PERSON] [25], [PERSON] [26], [PERSON] et al. [27], [PERSON] [28], [PERSON] [29], and [PERSON] et al. [30] reported that montmorillonite has a higher adsorption capacity compared to kaolinite, fine and coarse sand, and for this reason natural clays are one of the most essential sorbents due to their availability and easy application in removal of nitrogen forms from the environment.  
Whereas many studies have been conducted in order to characterize the phenomena of sorption processes of various contaminants onto different sorbents [31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41], only very few reports have been presented on the evaluation of the adsorption of contaminants originating from agricultural sources. This gap in knowledge can result in reduced possibilities in the assessment of risks posed by ammonium contamination arising from many sources [22].  
In reference to the issues mentioned above, the main aims of this study were: (1) to assess the sorption capacities of agricultural soils from Central Poland; (2) to determine the kinetics of sorption of ammonium into soils with various contents of the clay fraction (0%-21%); and (3) to estimate the ammonium sorption characteristics using two- and three-parameter models.  
Figure 1: Simplified scheme of the nitrogen cycle in agricultural areas.