Introduction

In highly productive agricultural ecosystems, including arable fields and intensive grasslands, nitrogen inputs often largely exceed nitrogen consumption by crops. In addition to likely direct and indirect effects on edafon, this creates suitable conditions for nitrogen losses. Losses of nitrogen in its various forms from both arable and grassland soils can be considerable and represent the most serious environmental problem of intensive agriculture in many regions. A large proportion of nitrogen losses from soils is in the form of nitrogen gases, namely nitrous oxide (N2O) and molecular nitrogen (N2). Over recent decades, nitrogen cycling in terrestrial ecosystems has thus emerged as an important ecological issue resulting in an extensive research focused on many different aspects and levels of the topic. However, the nitrogen cycle is rather complex and there are still many uncertainties concerning, for example, role of individual factors controlling the occurrence and rates of the key nitrogen transformation processes, denitrification and nitrification, both producing nitrous oxide.

N2O-forming processes and N2O emissions have been considered in a number of recent reviews (e.g. Bouwman, 1990; Granli and Bockman, 1994; Mosier, 1994; Oenema et al., 1997; Oenema et al., 2005). However, different soil N processes can proceed at the same time, and formation of N2O in soil tends to have a complicated pattern of response to regulating factors resulting in a well-known extreme spatial heterogeneity and temporal variation of N2O fluxes (e.g. Parsons et al., 1991; Murray et al., 1995). Particularly N2O is therefore thought to be often produced in soil microsites ("hot spots"; Parkin, 1987) unevenly distributed in the soil (note: for the purpose of this study, also sites and events characterized by high rates of production and emission of N2O are called hot spots despite their physical size). Strong temporal pattern of N2O emissions (e.g. Parsons et al., 1991; Kammann et al., 1998) is related to both changes in environmental conditions (e.g. temperature and rainfall) and management practices (e.g. fertilization, irrigation, ploughing, grazing). Despite the fact that soils are the main sources of N2O, net N2O consumption has been found both in natural and agricultural systems too suggesting some soils as a sink for N2O (Chapuis-Lardy et al., 2007).

Arable (cultivated) soils prevail in the Czech Republic, covering more than 72% of agricultural land. They are common in the whole country; arable farming represents a typical land-use both in lowlands and highlands and occurs also in other marginal parts of the country. However, in less favourite agricultural areas, permanent and temporary grasslands represent a common type of agricultural ecosystems. Permanent grasslands cover about 22% of agricultural land and are used both for hay production (meadows, ca 70% of total area of grasslands) and for grazing (pastures, ca 30%). Although the majority of agricultural production originates in arable agriculture, grasslands make a substantial contribution to the national economy too, through their use in production meat, milk and other animal products. Currently there is a growing tendency to use grasslands for cattle and sheep grazing and the same tendency can be seen in surrounding areas in abroad in Central Europe area.

The purpose of the present paper is to briefly review the soil and environmental conditions supporting N2O formation, to identify and list the major emission hot spots and to emphasize the importance of spatial and time hot spots for the overall balance of N2O fluxes. In particular, it is focused on the cattle overwintering areas, representing specific and often significant emission hot spots. In addition to an overview of literature data about the topic, summary of data on N2O production and emission from the soils of cattle overwintering areas that are located in Central Europe is presented.

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