Effect of soil compaction and N fertilization on soil pore characteristics and physical quality of sandy loam soil under red clover/grass sward

• Tractor multiple passes affected all pores with diameter above 0.5 μm in diameter.
• The soil compaction applied increased available water retention.
• The soil compaction reduced biological productivity by insufficient soil aeration.
• The mineral fertilization applied did not significantly affect any physical parameters of soil.

During the 20th century grassland production systems were intensified with higher rates of nitrogen usage and increasing heavy vehicular activity. This tendency is reflected in the higher soil degradation risk. However, in recent years European agri-environmental policies have promoted low-input grassland management practices. These changes in fertilization intensity may interact with the effects of machine traffic in affecting soil physical properties. The objective of this study, was to investigate the influence of soil compaction caused by tractor traffic and N fertilization levels on a soil pore system under a clover/grass mixture during the period from 2010 to 2012. This experiment was established in a split-plot design with fertilization as a main plot and soil compaction as a subplot. The N fertilizer treatments used were: untreated control (N0), 80 kg N ha−1 (N80) and 160 kg N ha−1 (N160). Four compaction treatments were applied using the following number of tractor passes: untreated control (P0), two passes (P2), four passes (P4) and six passes (P6). Undisturbed soil samples were collected in 2010–2012 in order to determine the water retention parameters and morphometric characterization of soil pores. The soil water characteristic curve was determined using pressure plates. The soil macropore system was also characterized using image analysis on the sections of soil samples hardened with polyester resin.The mineral fertilization did not significantly affect any physical parameters of soil at the trial. The intensive wheeling resulted in a higher value of bulk density and penetration resistance and lower values of total porosity. The soil compaction has distinctly influenced the soil water retention characteristics in the high matric potential range, which corresponds mainly with large pores (transmission pores and fissures) and storage pores. The result of changes in soil porosity was to increase the plant available water capacity. On the other hand, the relative field capacity indicated that in compacted soil under grassland plants, the biological activity was limited by insufficient soil aeration. This conclusion is in agreement with the results in plant production, which showed decrease in root and above ground biomass as the result of compaction.