Tractor traffic and nitrogen fertilization affect the herbage production of the red clover/grass sward

In recent years, soil compaction caused by agricultural machinery has been recognized as one of the main factors that can lower yields of perennial crops. At the same time changes in grassland fertilization intensity are observed. The objectives of the present study were to evaluate the interaction of different N fertilization rates and the intensity of tractor passes. This study was conducted as a field experiment located on a sandy loam soil classified as Mollic Fluvisol in Mydlniki near Krakow, Poland over a four-year period (2009-2012). The field experiment was established with four replications in a split-plot design with fertilization as the main plot and tractor traffic as the subplot. Three levels of N fertilization: untreated (control), 80 and 160 kg ha(-1) N, and four levels of tractor traffic intensity were applied using the following numbers of tractor passes: no passes (control), two passes, four passes and six passes. Nitrogen (N) fertilization significantly affected the percentage of only three species -Trifolium pratense, Lolium perenne and Festuca pratensis in the sward. T. pratense was negatively affected by with higher N rates but two grasses, L. perenne and F. pratensis, showed the opposite response. The tractor traffic significantly affected T. pratense and L. perenne occurrence in the swards. The percentage of T. pratense in the sward was reduced by tractor traffic, and this space was occupied by L. perenne. Changes in the botanical composition reflected in the herbage production. All investigated grass species increased biomass productivity in direct proportion to N rates. Only T. pratense was negatively affected by nitrogen fertilization and since 2011 it started to disappear from the swards treated with higher nitrogen rates. For all investigated components of the mixture no interaction between treatments, tractor traffic and nitrogen fertilization was observed.