Changes of nitrate concentrations in surface waters influenced by land use in the crystalline complex of the Czech Republic

Abstract

Changes in nitrate pollution in surface waters brought by the construction of tile drainage systems and by conversion of cultivated land to grassland (followed by decrease in fertilizer use) in two districts were examined. The goal was to find out how nitrate concentrations have responded to changes in agricultural management. There were no major changes in land use in the Pelhrimov district after 1990, apart from decrease in mineral fertilizer use. In the Cesky Krumlov district, permanent grassland area increased by 55% (from 228.9 km² to 353.3 km²) and by 241% (from 5.4 km² to 13 km²) in the catchments investigated between 1990 and 2000 followed by reduction in use of fertilizers. In these two districts, 31 small catchments where long-term, or repeated, nitrate concentration monitoring was carried out between 1986 and 2005, were analyzed. Trend, regression and factor analyses were conducted on changes in nitrate concentrations, and to assess the impact of several factors on nitrate concentrations in water before and after conversion to grassland.

Over the monitoring period 1986–2005, there was a significant decreasing linear trend in $\text{N}''–''{\text{NO}}_3^{-}$ concentration in surface waters in all catchments in the Cesky Krumlov district. In the Pelhrimov district 3 of 23 catchments showed a significant decreasing trend in nitrate concentrations and 2 a significant increasing trend despite a decrease in mineral nitrogen consumption. The most important environmental factor affecting nitrate pollution was the proportion of ploughed land in the catchment. The greater the share of ploughed land in the catchment, the greater the nitrate contamination of surface water. The proportion of drained land in the catchment after grassing has no markedly influence on nitrate nitrogen concentration.

Introduction

Agriculture in the Czech Republic (CR) underwent major changes after 1990, both with respect to food production and land use. Changes in land use were accompanied by a decrease in application of fertilizer. Manifestations of land use changes are different in different regions of the country (Table 1).

In general, emission of nitrates (and other nutrients) from agricultural land is considered as a major cause of their elevated concentrations in ground and surface water. Comparisons of nitrate levels observed with different land uses have revealed the relationship between land use and mean nitrogen concentrations in water flow. Concentrations increase from forest stands to meadows and pastures to cultivated land (Edwards et al., 1990, Reynolds and Edwards, 1995, Hudson et al., 1997, Lord et al., 1999, Kvítek, 2001, Ruiz et al., 2002, Kvítek and Doležal, 2003, Worrall et al., 2003, Kvítek et al., 2005). Compared to cultivated land, grassland is capable of absorbing and utilizing larger quantities of added nitrogen. The main reason that relatively small quantities of nitrogen are leached from meadows is that grassland takes up larger quantities of nitrogen than cultivated land, and it retains that ability for most of the year (Whitehead, 1995). The negative (upward) effect of ploughing on nitrate concentrations has been described by Neill, 1989, Thornton and Dise, 1998, Stoate et al., 2001.

Leaching of nitrates into groundwater and their presence in surface runoff depends on nitrogen excess, hydrological regime, land use, soil type, and climatic conditions (Shepherd et al., 2001, Oenema et al., 2005). There are two main sources of water nitrate pollution – point and non-point (background nutrient losses, nutrient losses from agricultural activities, atmospheric deposition of nutrients and nutrient losses from forestry). Kronvang et al. (2005) indicate that approximately 80% of nitrate contamination of water in the Svihov reservoir (Pelhrimov district) is due to non-point sources of pollution. Similar data have been reported by Neal et al. (2006) from catchments outside the CR.

The changes of land use after 1990 in Central and Eastern Europe are followed by the decrease of use of mineral fertilizer. Pekárová and Pekár (1996) reported that nutrient concentrations in surface waters in the Ondava river in Slovakia have decreased after a general and substantial reduction in the use of fertilisers. Furthermore, both Mander et al., 2000, Loigu and Vassiljev, 1997 found downward trends in small agricultural catchments in Estonia. However, neither Dzikiewicz (2000) in Polish rivers nor Tumas (2000) in Lithuanian rivers detected downward trends.

Before 1990 the socio-economic system in the countries of the former east European block gave priority to maximum production of field crops. Therefore a widespread construction of tile drainage occurred in the CR between 1968 and 1985 to create sufficient land area for production of field crops, especially cereals. By 1990, 1.087 million ha (about 25%) of agricultural soil was drained. Systematic draining, with field spacing of 7–20 m, was often followed by the ploughing of wet meadows and waterlogged soils (Kulhavý et al., 2007).

Installation of tile draining systems is a common agricultural management practice to help improve moisture and aeration conditions that shorten water retention periods in biologically active unsaturated soil zones. This change in moisture and air conditions in soil is manifested in greater organic matter mineralization, and thus a greater risk of non-point contamination of surface waters and groundwater with nutrients and pesticides (Gilliam et al., 1979, Skaggs and Gilliam, 1981, Southwick et al., 1995, Fenelon and Moore, 1998, Tomer et al., 2003, Doležal and Kvítek, 2004, Behrendt et al., 2005, Tiemeyer et al., 2006). The nitrate surplus is washed out from freshly drained soils in large amounts and often hangs on henceforward (Janeček, 1981).

Our goal was to find out how nitrate levels in surface waters have responded to the dramatic changes in agriculture management (land use change and decrease in use of mineral fertilizers) in Cesky Krumlov district compared to Pelhrimov district (decrease in use of mineral fertilizers, but no change in land use) with relation to the EU Nitrates Directive (91/676/EEC). This Nitrates Directive requires members states (including Czech Republic) to reduce nitrate pollution by introducing controls on agriculture in catchments where the nitrate concentration exceeds 50 mg l⁻¹ or is in danger of doing so. Nitrate levels in drinking water are regulated by the EU Drinking Water Directive (80/778/EEC), which established a maximum admissible concentration of 50 mg l⁻¹, based on World Health Organisation guidelines. We engaged several statistical methods (as trend tests, multiple regression analysis and factor analysis) to explore possible trends and mutual relationships between nitrate concentrations and selected catchment characteristics – factors; in particular ploughed land and tile drained land proportion.