SOIL MINERAL NITROGEN CONTENT AFTER UNDERSOWN CROPS IN AUTUMN AND SPRING IN THE CONVENTIONAL AND ORGANIC FARMING SYSTEM

The paper presents results of studies carried out from 2006 to 2009 which aimed to determine the effect of spring-or autumn-incorporated undersown crops on the mineral nitrogen content in the soil profile determined in autumn and in spring in conventional and organic farming systems. The field experiments were conducted at the Zawady Experimental Farm owned by the University of Natural Sciences and Humanities in Siedlce. The following treatments were examined: factor 1 the control (no undersown crops), an undersown crop with its biomass ploughed down in autumn (Persian clover, Westerwold ryegrass), and an undersown crop with its biomass used as a spring-in-corporated mulch (Persian clover, Westerwold ryegrass); factor 2 farming system: conventional and organic. Ammonium and nitrate nitrogen contents were determined in two soil layers (0-30 and 30-60 cm) twice, i.e. in autumn and spring. The results showed that the undersown crops reduced the amount of mineral nitrogen in soil in autumn and an increase was observed in spring. The highest soil concentrations of ammonium and nitrate ions were determined in spring following Persian clover incorporation. The autumn and spring-determined soil contents of mineral nitrogen were significantly higher in the conventional in comparison to organic farming system. Mulching of the soil surface with undersown crops over late autumn and winter, Westerwold ryegrass in particular, significantly reduced the mineral nitrogen content in soil in spring compared with the autumn-incorporated undersown crops.