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Nitrogen is the most yield-increasing nutrient of plants and the importance of its role in agricultural production is beyond question. Nevertheless, there are non-production consequences of nitrogen use in agriculture, among which the increase in nitrate content in crop plants as well as in ground and surface waters is the most important one. Mineral nitrogen leached from the topsoil layers to the 60-90 cm layer of the soil profile is considered to be unavailable for the main root mass of crop plants. Therefore, research into nitrogen migration to deeper layers of the soil profile is extremely important, both in terms of nitrogen loss estimation and potential groundwater pollution. Research is also being carried out concerning the possibility of phytoremediation of nitrates from the soil, for example Marecik et al. (2013) showed high nitrogen removal rates for the treatment of industrial wastewater with a high concentration of N-NO3 using sweet flag (Acorus calamus), common reed (Phragmites communis) and broadleaf cattail (Typha latifolia). It should be noted that much attention is now being paid to nitrogen losses and the resultant problems of water pollution in the context of the document known as the Nitrate Directive and the Helsinki Convention (Directive, 91/676/ CEE; Convention, 1992; Directive, 2000/60/EC). The main © 2019 Institute of Agrophysics, Polish Academy of Sciences A. WATROS et al. 482 assumptions of the Nitrate Directive are focused on preventing the causes of eutrophication as a serious threat to water quality (Directive, 91/676/CEE; Randal and Goss, 2008). Hence, the quantities of soil mineral nitrogen, which have been recorded for years, are an important indicator of the effects of nitrogen on water quality (Regulation, 2002; Fotyma et al., 2010; Lipiński, 2010). Apart from the environmental aspect, monitoring nutrient changes in various soil layers over the years and making an attempt to correlate these changes with the influence of various factors is significant both from a scientific and practical point of view. Such information is the basis for constructing simple empirical models based on, for example, multiple regression equations, that allow for the prediction of nutrient content in the studied soil and thus help farmers to plan the rational use of fertilizers in order to ensure an optimal nutrient supply for crop growth and to increase soil productivity using only a limited set of soil properties (Tkaczyk et al., 2017; 2018a; 2018b). Knowledge concerning nutrient content, especially NPK balance, and their relationship with other soil physicochemical properties are important for agrophysical modelling (Walczak et al., 1997; Lamorski et al., 2013). All of these models need input data to run, therefore representative data, especially at a large scale, are sought by modellers using and developing crop growth and yield models. Detailed information put into agrophysical models may constitute a new quality in research on how crop production will change in the conditions of a changing climate (Pirttioja et al., 2015; Fronzek et al., 2018) and how different cultivation and fertilization methods will impact this production (Ruiz-Ramos et al., 2018; Rodríguez et al., 2018). The balance of NPK also depends on the root system of the plant species and the level of mineral and organic fertilization (Antonkiewicz et al., 2018; Watros et al., 2018). The content of mineral nitrogen in the soil is also impacted by the biological activity of the soil, which in turn may be influenced by soil moisture (Wnuk et al., 2017) or N fertilization (Walkiewicz et al., 2018), and by atmospheric conditions. For this reason, research concerning the spatio-temporal variability of meteorological series from different climate zones (Baranowski et al., 2015; Hoffmann et al., 2017; Krzyszczak et al., 2017a; 2017b; 2019) and their prediction (Murat et al., 2018) is extremely important. It allows for the assessment of the likely impact of climate change not only on agricultural production, but also on the content of macroelements (NPK) in the soil. About 1/5 of the total area of soil used for agriculture is occupied by grasslands (GUS, 2018). The location, area and shape of these grasslands are physiographically determined, which prevents arbitrary changes. This results in high grassland area stability (Wasilewski, 2010), also the location of grasslands among crop fields, water bodies and watercourses as well as the large variations in habitat conditions indicate their exceptional role in the natural environment (Wasilewski, 2009). Despite a decrease in the area of both arable land and grasslands in the 21st century, the yields of plants obtained per surface unit are increasing, which is related to the intensity of plant production, and thus with the more pronounced impact of agricultural land on the natural environment. Therefore, it is necessary to search for solutions optimizing the management of macroand microelements, including nitrogen, that will take into account the diversity of Polish soils and land use. The results of previous research mainly concerned the scale of nitrogen outflow, as well as the impact of various factors on the migration of this nutrient into the soil profile, mainly on arable land. Grasslands, including those located on organic soils, have not been the subject of wider analyses. Thus far, the amount of mineral nitrogen in the soil layer 60-90 cm from grassland soils with regard to agricultural fodder crops has not been examined. Also, the potential impact of this form of nitrogen on water quality in the hydrographic regions of Poland has not been studied previously. Therefore, taking into account the environmental effects, as well as production and economic considerations, this study attempted to evaluate the mineral nitrogen content in the 60-90 cm soil layer in grassland soils with regard to selected agricultural fodder crops in Poland’s hydrographic regions. MATERIALS AND METHODS The evaluation of the soil mineral nitrogen content was performed based on the results of environmental investigations conducted by the accredited Regional Chemical and Agricultural Stations in agricultural farms across Poland over the period 2010-2012. The sampling sites were divided between Poland’s hydrographic regions (Table 1). The vector layer of Poland’s division into hydrographic areas (secondorder catchments) used in this study was developed based on the Institute of Meteorology and Water Management (IMGW) hydrological atlas of 1986 “Hydrographic division of Poland – Part II. A map series of the hydrographic division at a scale of 1:2”. The locations of the sites were characterized by geographical coordinates using a GPS tracker, which allowed for sampling to be carried out in the same places in successive years of the study. The location of the sampling sites, divided into soil type and land use, is presented in Fig. 1. Soil samples were taken in spring and autumn from three soil profile layers, 0-30, 30-60, and 60-90 cm, from fields with a total area of not more than 4 ha. Each total sample (with a weight of about 100 g) consisted of 15-20 primary samples collected from an area of not more than 100 m. In this study only soil samples from the 60-90 cm layer were subjected to the analysis. Sites where the same land use was continued throughout the entire study period were selected NITROGEN CONTENT IN POLAND HYDROGRAPHIC AREAS 483 from the grassland area. For comparison, sites where maize and mixed cereals were grown over the period 2010-2012 were also selected (if the same crop was grown during successive years, the average value from the respective years was taken into consideration). The number of soil samples collected for each soil type and land use is listed in Table 2. Additionally, information about the mineral nitrogen fertilization rate for each soil sampling point (land use) was acquired. The fields were classified according to the doses of nitrogen fertilization: 0 (without fertilization), 1-40, 41-80, 81-120 and >120 kg N ha. The soil samples were subjected to analyses of Nmin in accordance with the PN-R-04028:1997 norm. The collected soil samples were used to determine the mineral nitrogen content in the 60-90 cm layer which were kept at a temperature of -18°C. The soil samples with a natural moisture content (after defrosting) were subjected to extraction with a 1% potassium sulphate solution at a ratio of 1:10. For the extracts obtained, both nitrate and ammonium nitrogen content were determined spectrophotometrically using a Skalar San Plus System auto-analyser. Dry matter content was also determined in the examined samples. Mineral Fig. 1. Location of soil sampling points from a) grasslands on mineral soil, b) grasslands on organic soil, c) maize and d) mixed cereals. Ta b l e 1. Number of samples collected from the 60-90 cm soil layer in grasslands and soils under maize and mixed cereals in the selected hydrographic regions of Poland Hydrographic region Grassland on mineral soils Grassland on organic soils Maize Mixed cereals Poland (in total) 859 167 826 951 Vistula basin 553 95 431 685 Odra basin 223 36 370 227 Basin of coastal rivers 83 36 25 39 A. WATROS et al. 484 nitrogen content, in the form of total nitrate and ammonium nitrogen, was expressed in mg kg of dry weight of the soil sample. Based on the average mineral nitrogen content calculated for individual sites, its spatial distribution was shown in Poland’s hydrographic areas and analysed in relation to specific land uses. Interpolation was performed using ArcView software which applied the IDW (inverse distance weighting) method, which is based on the assumption that the value of the trait being investigated at a given point is dependent on the value of this trait in the closest base points). The data from 12 adjacent base points were included. Interpolation was performed with a strength of 3 and without barriers. The ge

Mineral nitrogen content in hydrographic areas of Poland depending on land use