This article presents the results of studies on lysimeters and drainage carried out in the karst zone of Lithuania. The studies included the estimation of the changes of water quality in a soil profile of moraine loam and sandy loam soils in respect of the land usage and fertilization intensity. Fluctuations of N-NO 3 concentrations contained in soil water mainly depend on N losses through the yield (r 2 ¼ 0.78). The highest N concentrations (up to 28.7 mg l À1 ) were observed in the fields where potatoes were grown; six times lower N concentrations occurred in the fields of barley with undercrop and the lowest N concentrations (up to 5.1 mg l À1 ) were determined in fields of perennial grass. This was because N losses with the yield of potatoes are 1.7-2.0 times less than its losses with barley and even 3.0-4.5 times less than N losses with perennial grass. Efficient application of fertilizers results in decreased N-NO 3 and TP concentrations contained in soil water (r 2 ¼ 0.27-0.56 and r 2 ¼ 0.18-0.57 respectively). Compared to the field of perennial grass, the arable land contained even 9 times higher N-NO 3 concentrations and 2.6 times lower TP concentrations in the whole soil profile. However, when migrating into deeper soil layers in arable land, N-NO 3 concentrations decrease, while in the field of perennial grass the lowest N-NO 3 concentrations occur in the root zone of plants. TP concentrations tend to decrease in deeper soil layers in arable land as well as in the field of perennial grass.The selection of proper land use in studied soils is a particularly important factor. In order to reduce N-NO 3 leaching, the area of perennial grass might be increased, however this may result in higher TP concentrations. Moreover, it is precarious to leave tilled land for the period abundant in water (winter and spring).

Most natural streams were made into drains when they were converted into recipients of subsurface tile systems. During the process the variety of biotic and abiotic conditions within and around the streams was totally destroyed, and the new watercourses (open drain recipients) were deprived of the greater part of the ecological importance that the former streams had possessed in the landscape. The data collected over the last decade in Lithuania about drain bed deformations, sediment accumulation, vegetation cover successions, intentional afforestation of slopes, beaver expansion and succeeding water quality changes, revealed some self-perpetuating natural alterations which raise the ecological significance of open drains as refuges and corridors in the landscape for vegetation (the succession of herbage and appearance of woody vegetation on slopes), and the barriers protecting surface runoff from diffuse agricultural pollution (beaver impoundments). On the other hand, these alterations are useful in terms of maintenance of the functioning of those drains as recipients because they result in steadier (roughly parabolashaped) cross-section profiles, less threat of bed silting up and no need for the mowing of slopes (because of the growth of woody vegetation). This paper presents and discusses the study results highlighting the above-mentioned bidirectional effect of self-renaturalization, and considers the preconditions RÉ SUMÉ La plupart des écoulements naturels ont été canalisés dans des drains lors de leur conversion en collecteurs des flux de drainage souterrain. A cette occasion, la variété des conditions biotiques et abiotiques internes et externes aux écoulements a été totalement détruite, et les nouvelles voies d'eau (collecteurs à ciel ouvert) ont été privées de l'essentiel du contenu écologique des ruissellements naturels. Les données rassemblées depuis dix ans en Lithuanie sur les déformations des collecteurs, l'accumulation des sédiments, les successions du couvert végétal, le reboisement intentionnel des pentes, la prolifération des castors et les changements de qualité de l'eau, ont révélé quelques unes des altérations autoentretenues qui posent la question du poids écologique des drains à ciel ouvert comme refuge et couloirs de végétaux dans le paysage (succession des herbes et apparence de végétation boisée sur les pentes) et barrières protégeant le ruissellement de surface de la pollution agricole diffuse (barrages de castors). D'un autre côté, ces altérations sont utiles en termes de maintenance des drains collecteurs parce qu'elles conduisent à des profils en travers plus stables (forme grossière de parabole), une moindre menace pour l'envasement du fonds et un moindre besoin de tondre les pentes (présence de végétation boisée). Cet article présente et discute les résultats d'étude qui mettent en évidence les impacts bidirectionnels de la re-naturalisation

Abstract:The study presents the critical evaluation of existing drainage systems from legal, economical, environmental and technical viewpoints. Nearly 80% of agricultural land in Lithuania drained by underground drainage systems covers around 3 million hectares. The prevailing large scale drainage systems with a complex of engineering structures such as conducting ditches, drains and collectors, local roads, bridges or farm road-crossings, dikes, dams, water reservoirs, pumping stations for irrigation and for drainage need today an efficient management solution in a new economic situation. The detailed analysis of legal and economic instruments adopted in transferring the management responsibilities of drainage systems to users has been carried out. The study resulted in a number of practical contributions towards the amendments in the Law on Land Reclamation in Lithuania and the establishment of a system of indicators that would allow rationalisation of subsidy allocations for the maintenance and improvement of the drainage systems. These subsidies from the state budget make at the moment the largest share among the investment sources. The financial and in kind contribution of drainage users is permanently increasing as are the allocations of the structural funds for public projects. The EU pre-accession programme SAPARD started in 2000 has supported some investments in rural areas. Unfortunately, it did not support the drainage infrastructure properly. A critical review of previous measures allowed suggesting new actions within the framework of the actual support from EU structural funds.