EFFECT OF LAND PROTECTION ON THE CONTENT OF MINERAL NITROGEN IN SOIL

Soil temporarily excluded from agricultural production is exposed to several factors which deteriorate its chemical, physical and biological properties. To preserve the productive potential of uncropped soil, it is recommended that the soil surface be turfed. In the spring of 1996, a field experiment was set up in Knopin (municipality Dobre Miasto, the Province of Warmia and Mazury), where the effect of the type of a plant cover or its lack (bare fallow) on the physical and chemical characteristics of soil was investigated. One of the goals of the study reported below, which was conducted between 2000 and 2007, was to determine how the way uncropped farmland was maintained affected the content of nitrogen in soil. The experiment comprised five treatments: bare fallow, traditional fallow, a fallow field cropped with eastern galega (Galega orientalis Lam), or a mixture of eastern galega (Galega orientalis Lam.) and smooth brome (Bromus inermis), and finally a fallow field seeded with smooth brome (Bromus inermis). The set-aside land was only sampled for soil, while the biomass which grew on it every year was left in the field. Soil for analyses of physical and chemical characteristics was sampled from the 0-25 cm topmost layer, four times a year (April, July, September and November). During the research, there was only one agritechnical treatment performed, such as the mechanical weeding of the bare fallow field. The best way to maintain land excluded from agricultural production for many years proved to be by seeding it with eastern galega or its mixture with smooth brome. A far less beneficial solution was to grow smooth brome alone. When there was no grass on a field to synthetise free nitrogen, the content of nitrogen mineral forms in soil decreased. However, the depeletion of nitrogen from turfed soil was distinctly lower than from the soil kept as a bare fallow. Consistent eradication of weeds most probably ensured much better conditions for organic matter mineralisation. Hence, the concentration of nitrates in soil under a bare fallow field was the highest. The absence of a plant cover, as many authors maintain, exclude the possibility of the uptake of mineral nitrogen forms, which is conducive to their leaching into the deeper layers of a soil profile, a consequence which mostly affects nitrates. The study reported underneath supports this opinion. In the light of our results we can conclude that farmers should be discouraged from keeping set-aside land as a bare fallow.